Kelvin-Helmholtz instabilities at the sloshing cold fronts in the Virgo cluster as a measure for the effective ICM viscosity. (arXiv:1211.4874v1 [astro-ph.CO]):
Sloshing cold fronts (CFs) arise from minor merger triggered gas sloshing.
Their detailed structure depends on the properties of the intra-cluster medium
(ICM): hydrodynamical simulations predict the CFs to be distorted by
Kelvin-Helmholtz instabilities (KHIs), but aligned magnetic fields, viscosity,
or thermal conduction can suppress the KHIs. Thus, observing the detailed
structure of sloshing CFs can be used to constrain these ICM properties. Both
smooth and distorted sloshing CFs have been observed, indicating that the KHI
is suppressed in some clusters, but not in all. Consequently, we need to
address at least some sloshing clusters individually before drawing general
conclusions about the ICM properties. We present the first detailed attempt to
constrain the ICM properties in a specific cluster from the structure of its
sloshing CF. Proximity and brightness make the Virgo cluster an ideal target.
We combine observations and Virgo-specific hydrodynamical sloshing simulations.
Here we focus on a Spitzer-like temperature dependent viscosity as a mechanism
to suppress the KHI, but discuss the alternative mechanisms in detail. We
identify the CF at 90 kpc north and north-east of the Virgo center as the best
location in the cluster to observe a possible KHI suppression. For viscosities
$\gtrsim$ 10% of the Spitzer value KHIs at this CF are suppressed. We describe
in detail the observable signatures at low and high viscosities, i.e. in the
presence or absence of KHIs. We find indications for a low ICM viscosity in
archival XMM-Newton data and demonstrate the detectability of the predicted
features in deep Chandra observations.
Thursday, November 22, 2012
On the orientation and magnitude of the black hole spin in galactic nuclei. (arXiv:1211.4871v1 [astro-ph.CO])
On the orientation and magnitude of the black hole spin in galactic nuclei. (arXiv:1211.4871v1 [astro-ph.CO]):
Massive black holes in galactic nuclei vary their mass M and spin vector J
due to accretion. In this study we relax, for the first time, the assumption
that accretion can be either chaotic, i.e. when the accretion episodes are
randomly and isotropically oriented, or coherent, i.e. when they occur all in a
preferred plane. Instead, we consider different degrees of anisotropy in the
fueling, never confining to accretion events on a fixed direction. We follow
the black hole growth evolving contemporarily mass, spin modulus a and spin
direction. We discover the occurrence of two regimes. An early phase (M <~ 10
million solar masses) in which rapid alignment of the black hole spin direction
to the disk angular momentum in each single episode leads to erratic changes in
the black hole spin orientation and at the same time to large spins (a ~ 0.8).
A second phase starts when the black hole mass increases above >~ 10 million
solar masses and the accretion disks carry less mass and angular momentum
relatively to the hole. In the absence of a preferential direction the black
holes tend to spin-down in this phase. However, when a modest degree of
anisotropy in the fueling process (still far from being coherent) is present,
the black hole spin can increase up to a ~ 1 for very massive black holes (M >~
100 million solar masses), and its direction is stable over the many accretion
cycles. We discuss the implications that our results have in the realm of the
observations of black hole spin and jet orientations.
Massive black holes in galactic nuclei vary their mass M and spin vector J
due to accretion. In this study we relax, for the first time, the assumption
that accretion can be either chaotic, i.e. when the accretion episodes are
randomly and isotropically oriented, or coherent, i.e. when they occur all in a
preferred plane. Instead, we consider different degrees of anisotropy in the
fueling, never confining to accretion events on a fixed direction. We follow
the black hole growth evolving contemporarily mass, spin modulus a and spin
direction. We discover the occurrence of two regimes. An early phase (M <~ 10
million solar masses) in which rapid alignment of the black hole spin direction
to the disk angular momentum in each single episode leads to erratic changes in
the black hole spin orientation and at the same time to large spins (a ~ 0.8).
A second phase starts when the black hole mass increases above >~ 10 million
solar masses and the accretion disks carry less mass and angular momentum
relatively to the hole. In the absence of a preferential direction the black
holes tend to spin-down in this phase. However, when a modest degree of
anisotropy in the fueling process (still far from being coherent) is present,
the black hole spin can increase up to a ~ 1 for very massive black holes (M >~
100 million solar masses), and its direction is stable over the many accretion
cycles. We discuss the implications that our results have in the realm of the
observations of black hole spin and jet orientations.
Monday, November 12, 2012
Probing General Relativity with Accreting Black Holes. (arXiv:1211.2146v1 [astro-ph.HE])
Probing General Relativity with Accreting Black Holes. (arXiv:1211.2146v1 [astro-ph.HE]):
Most of the X-ray emission from luminous accreting black holes emerges from
within 20 gravitational radii. The effective emission radius is several times
smaller if the black hole is rapidly spinning. General Relativistic effects can
then be very important. Large spacetime curvature causes strong lightbending
and large gravitational redshifts. The hard X-ray, power-law-emitting corona
irradiates the accretion disc generating an X-ray reflection component. Atomic
features in the reflection spectrum allow gravitational redshifts to be
measured. Time delays between observed variations in the power-law and the
reflection spectrum (reverberation) enable the physical scale of the reflecting
region to be determined. The relative strength of the reflection and power-law
continuum depends on light bending. All of these observed effects enable the
immediate environment of the black hole where the effects of General Relativity
are on display to be probed and explored.
Most of the X-ray emission from luminous accreting black holes emerges from
within 20 gravitational radii. The effective emission radius is several times
smaller if the black hole is rapidly spinning. General Relativistic effects can
then be very important. Large spacetime curvature causes strong lightbending
and large gravitational redshifts. The hard X-ray, power-law-emitting corona
irradiates the accretion disc generating an X-ray reflection component. Atomic
features in the reflection spectrum allow gravitational redshifts to be
measured. Time delays between observed variations in the power-law and the
reflection spectrum (reverberation) enable the physical scale of the reflecting
region to be determined. The relative strength of the reflection and power-law
continuum depends on light bending. All of these observed effects enable the
immediate environment of the black hole where the effects of General Relativity
are on display to be probed and explored.
Present-day Galactic Evolution: Low-metallicity, Warm, Ionized Gas Inflow Associated with High-Velocity Cloud Complex A. (arXiv:1211.1973v1 [astro-ph.GA])
Present-day Galactic Evolution: Low-metallicity, Warm, Ionized Gas Inflow Associated with High-Velocity Cloud Complex A. (arXiv:1211.1973v1 [astro-ph.GA]):
The high-velocity cloud (HVC) Complex A is a probe of the physical conditions
in the Galactic halo. The kinematics, morphology, distance, and metallicity of
Complex A indicate that it represents new material that is accreting onto the
Galaxy. We present Wisconsin H-alpha Mapper (WHAM) kinematically resolved
observations of Complex A over the velocity range of -250 to -50 km/s in the
local standard of rest reference frame. These observations include the first
full H-alpha intensity map of Complex A across (l, b) = (124, 18) to (171, 53)
and deep targeted observations in H-alpha, [S II]6716, [N II]6584, and [O
I]6300 towards regions with high H I column densities, background quasars, and
stars. The H-alpha data imply that the masses of neutral and ionized material
in the cloud are similar, both being greater than a million solar masses. We
find that the Bland-Hawthorn & Maloney (1999, 2001) model for the intensity of
the ionizing radiation near the Milky Way is consistent with the known distance
of the high-latitude part of Complex A and an assumed cloud geometry that puts
the lower-latitude parts of the cloud at a distance of 7 to 8 kpc. This
compatibility implies a 5% ionizing photon escape fraction from the Galactic
disk. We also provide the nitrogen and sulfur upper abundance solutions for a
series of temperatures, metallicities, and cloud configurations for purely
photoionized gas; these solutions are consistent with the sub-solar abundances
found by previous studies, especially for temperatures above 10,000 K or for
gas with a high fraction of singly-ionized nitrogen and sulfur.
The high-velocity cloud (HVC) Complex A is a probe of the physical conditions
in the Galactic halo. The kinematics, morphology, distance, and metallicity of
Complex A indicate that it represents new material that is accreting onto the
Galaxy. We present Wisconsin H-alpha Mapper (WHAM) kinematically resolved
observations of Complex A over the velocity range of -250 to -50 km/s in the
local standard of rest reference frame. These observations include the first
full H-alpha intensity map of Complex A across (l, b) = (124, 18) to (171, 53)
and deep targeted observations in H-alpha, [S II]6716, [N II]6584, and [O
I]6300 towards regions with high H I column densities, background quasars, and
stars. The H-alpha data imply that the masses of neutral and ionized material
in the cloud are similar, both being greater than a million solar masses. We
find that the Bland-Hawthorn & Maloney (1999, 2001) model for the intensity of
the ionizing radiation near the Milky Way is consistent with the known distance
of the high-latitude part of Complex A and an assumed cloud geometry that puts
the lower-latitude parts of the cloud at a distance of 7 to 8 kpc. This
compatibility implies a 5% ionizing photon escape fraction from the Galactic
disk. We also provide the nitrogen and sulfur upper abundance solutions for a
series of temperatures, metallicities, and cloud configurations for purely
photoionized gas; these solutions are consistent with the sub-solar abundances
found by previous studies, especially for temperatures above 10,000 K or for
gas with a high fraction of singly-ionized nitrogen and sulfur.
Testing the space-time geometry around black hole candidates with the analysis of the broad K$\alpha$ iron line. (arXiv:1211.2513v1 [gr-qc])
Testing the space-time geometry around black hole candidates with the analysis of the broad K$\alpha$ iron line. (arXiv:1211.2513v1 [gr-qc]):
Astrophysical black hole candidates are thought to be the Kerr black holes
predicted by General Relativity, but there is not yet a clear evidence that the
geometry of the space-time around these objects is really described by the Kerr
metric. In order to confirm the Kerr black hole hypothesis, we have to observe
strong gravity features and check they are in agreement with the ones predicted
by General Relativity. In this paper, I study what kind of information can be
extracted by analyzing the broad K$\alpha$ iron line, which is often seen in
the X-ray spectrum of both stellar-mass and super-massive black hole candidates
and whose shape is supposed to be strongly affected by the space-time geometry.
I extend previous studies in the literature. It turns out that there is a
strong degeneracy between the spin parameter and the deformation parameter;
that is, the line emitted around a Kerr black hole with a certain spin can be
very similar to the one coming from the space-time around a non-Kerr object
with a quite different spin. As in this paper I include the effect of the
disk's inclination angle, which is also a fit parameter, this degeneracy is
much stronger than the one found in previous studies. Despite that, the
analysis of the broad K$\alpha$ iron line is potentially more powerful than the
continuum-fitting method, as it can put a bound on possible deviations from the
Kerr geometry independently of the value of the spin parameter and without
additional measurements.
Astrophysical black hole candidates are thought to be the Kerr black holes
predicted by General Relativity, but there is not yet a clear evidence that the
geometry of the space-time around these objects is really described by the Kerr
metric. In order to confirm the Kerr black hole hypothesis, we have to observe
strong gravity features and check they are in agreement with the ones predicted
by General Relativity. In this paper, I study what kind of information can be
extracted by analyzing the broad K$\alpha$ iron line, which is often seen in
the X-ray spectrum of both stellar-mass and super-massive black hole candidates
and whose shape is supposed to be strongly affected by the space-time geometry.
I extend previous studies in the literature. It turns out that there is a
strong degeneracy between the spin parameter and the deformation parameter;
that is, the line emitted around a Kerr black hole with a certain spin can be
very similar to the one coming from the space-time around a non-Kerr object
with a quite different spin. As in this paper I include the effect of the
disk's inclination angle, which is also a fit parameter, this degeneracy is
much stronger than the one found in previous studies. Despite that, the
analysis of the broad K$\alpha$ iron line is potentially more powerful than the
continuum-fitting method, as it can put a bound on possible deviations from the
Kerr geometry independently of the value of the spin parameter and without
additional measurements.
Inclination-Dependent AGN Flux Profiles From Strong Lensing of the Kerr Space-Time. (arXiv:1211.2510v1 [astro-ph.HE])
Inclination-Dependent AGN Flux Profiles From Strong Lensing of the Kerr Space-Time. (arXiv:1211.2510v1 [astro-ph.HE]):
Recent quasar microlensing observations have constrained the X-ray emission
sizes of quasars to be about 10 gravitational radii, one order of magnitude
smaller than the optical emission sizes. Using a new ray-tracing code for the
Kerr space-time, we find that the observed X-ray flux is strongly influenced by
the gravity field of the central black hole, even for observers at moderate
inclination angles. We calculate inclination-dependent flux profiles of active
galactic nuclei in the optical and X-ray bands by combining the Kerr lensing
and projection effects for future references. We further study the dependence
of the X-ray-to-optical flux ratio on the inclination angle caused by
differential lensing distortion of the X-ray and optical emission, assuming
several corona geometries. The strong lensing X-ray-to-optical magnification
ratio can change by a factor of ~10 for normal quasars in some cases, and
another factor of ~10 for broad absorption line quasars (BALs) and obscured
quasars. Comparing our results with the observed distributions in normal and
broad absorption line quasars, we find that the inclination angle dependence of
the magnification ratios can change the X-ray-to-optical flux ratio
distributions significantly. In particular, the mean value of the spectrum
slope parameter $\alpha_{ox},$ $0.3838\log F_{2 keV}/F_{2500 {\AA}}$, can
differ by ~0.1-0.2 between normal and broad absorption line quasars, depending
on corona geometries, suggesting larger intrinsic absorptions in BALs.
Recent quasar microlensing observations have constrained the X-ray emission
sizes of quasars to be about 10 gravitational radii, one order of magnitude
smaller than the optical emission sizes. Using a new ray-tracing code for the
Kerr space-time, we find that the observed X-ray flux is strongly influenced by
the gravity field of the central black hole, even for observers at moderate
inclination angles. We calculate inclination-dependent flux profiles of active
galactic nuclei in the optical and X-ray bands by combining the Kerr lensing
and projection effects for future references. We further study the dependence
of the X-ray-to-optical flux ratio on the inclination angle caused by
differential lensing distortion of the X-ray and optical emission, assuming
several corona geometries. The strong lensing X-ray-to-optical magnification
ratio can change by a factor of ~10 for normal quasars in some cases, and
another factor of ~10 for broad absorption line quasars (BALs) and obscured
quasars. Comparing our results with the observed distributions in normal and
broad absorption line quasars, we find that the inclination angle dependence of
the magnification ratios can change the X-ray-to-optical flux ratio
distributions significantly. In particular, the mean value of the spectrum
slope parameter $\alpha_{ox},$ $0.3838\log F_{2 keV}/F_{2500 {\AA}}$, can
differ by ~0.1-0.2 between normal and broad absorption line quasars, depending
on corona geometries, suggesting larger intrinsic absorptions in BALs.
Disentangling Resonant Scattering and Gas Motions in Galaxy Cluster Emission Line Profiles. (arXiv:1211.2375v1 [astro-ph.CO])
Disentangling Resonant Scattering and Gas Motions in Galaxy Cluster Emission Line Profiles. (arXiv:1211.2375v1 [astro-ph.CO]):
Future high spectral resolution telescopes will enable us to place tight
constraints on turbulence in the intra-cluster medium through the line widths
of strong emission lines. At the same time, these bright lines are the most
prone to be optically thick. This requires us to separate the effects of
resonant scattering from turbulence, both of which could broaden the lines. How
this can be achieved has yet not been quantitatively addressed. In this paper,
we propose a flexible new parametrization for the line profile, which allows
these effects to be distinguished. The model has only 3 free parameters, which
we calibrate with Monte-Carlo radiative transfer simulations. We provide
fitting functions and tables that allow the results of these calculations to be
easily incorporated into a fast spectral fitting package. In a mock spectral
fit, we explicitly show that this parameterization allows us to correctly
estimate the turbulent amplitude and metallicity of a cluster such as Perseus,
which would otherwise give significantly biased results. We also show how the
physical origin of the line shape can be understood analytically.
Future high spectral resolution telescopes will enable us to place tight
constraints on turbulence in the intra-cluster medium through the line widths
of strong emission lines. At the same time, these bright lines are the most
prone to be optically thick. This requires us to separate the effects of
resonant scattering from turbulence, both of which could broaden the lines. How
this can be achieved has yet not been quantitatively addressed. In this paper,
we propose a flexible new parametrization for the line profile, which allows
these effects to be distinguished. The model has only 3 free parameters, which
we calibrate with Monte-Carlo radiative transfer simulations. We provide
fitting functions and tables that allow the results of these calculations to be
easily incorporated into a fast spectral fitting package. In a mock spectral
fit, we explicitly show that this parameterization allows us to correctly
estimate the turbulent amplitude and metallicity of a cluster such as Perseus,
which would otherwise give significantly biased results. We also show how the
physical origin of the line shape can be understood analytically.
Friday, November 9, 2012
Cluster science from ROSAT to eROSITA. (arXiv:1210.5132v1 [astro-ph.CO])
Cluster science from ROSAT to eROSITA. (arXiv:1210.5132v1 [astro-ph.CO]):
Galaxy clusters are one of the important cosmological probes to test the
consistency of the observable structure and evolution of our Universe with the
predictions of specific cosmological models. We use results from our analysis
of the X-ray flux-limited REFLEX cluster sample from the ROSAT All-Sky Survey
to illustrate the constraints on cosmological parameters that can be achieved
with this approach. The upcoming eROSITA project of the Spektrum-Roentgen-Gamma
mission will increase these capabilities by two orders of magnitude and
importantly also increase the redshift range of such studies. We use the
projected instrument performance to make predictions on the scope of the
eROSITA survey and the potential of its exploitation.
Galaxy clusters are one of the important cosmological probes to test the
consistency of the observable structure and evolution of our Universe with the
predictions of specific cosmological models. We use results from our analysis
of the X-ray flux-limited REFLEX cluster sample from the ROSAT All-Sky Survey
to illustrate the constraints on cosmological parameters that can be achieved
with this approach. The upcoming eROSITA project of the Spektrum-Roentgen-Gamma
mission will increase these capabilities by two orders of magnitude and
importantly also increase the redshift range of such studies. We use the
projected instrument performance to make predictions on the scope of the
eROSITA survey and the potential of its exploitation.
Statistics and implications of substructure detected in a representative sample of X-ray clusters. (arXiv:1210.5130v1 [astro-ph.CO])
Statistics and implications of substructure detected in a representative sample of X-ray clusters. (arXiv:1210.5130v1 [astro-ph.CO]):
We present a morphological study of 35 X-ray luminous galaxy clusters at
0.15<z<0.3, selected in a similar manner to the Local Cluster Substructure
Survey (LoCuSS), for which deep XMM-Newton observations are available. We
characterise the structure of the X-ray surface brightness distribution of each
cluster by measuring both their power ratios and centroid shift, and thus rank
the clusters by the degree of substructure. These complementary probes give a
consistent description of the cluster morphologies with some well understood
exceptions. We find a remarkably tight correlation of regular morphology with
the occurrence of cool cores in clusters. We also compare our measurements of
X-ray morphology with measurements of the luminosity gap statistics and
ellipticity of the brightest cluster galaxy (BCG). We check how our new X-ray
morphological analysis maps onto cluster scaling relations, finding that (i)
clusters with relatively undisturbed X-ray morphologies are on average more
luminous at fixed X-ray temperature than those with disturbed morphologies, and
(ii) disturbed clusters have larger X-ray masses than regular clusters for a
given temperature in the M-T relation. We also show that the scatter in the
ratio of X-ray and weak lensing based cluster mass measurements is larger for
disturbed clusters than for those of more regular morphology. Overall, our
results demonstrate the feasibility of assembling a self-consistent picture of
the physical structure of clusters from X-ray and optical data, and the
potential to apply this in the measurement of cosmological cluster scaling
relations.
We present a morphological study of 35 X-ray luminous galaxy clusters at
0.15<z<0.3, selected in a similar manner to the Local Cluster Substructure
Survey (LoCuSS), for which deep XMM-Newton observations are available. We
characterise the structure of the X-ray surface brightness distribution of each
cluster by measuring both their power ratios and centroid shift, and thus rank
the clusters by the degree of substructure. These complementary probes give a
consistent description of the cluster morphologies with some well understood
exceptions. We find a remarkably tight correlation of regular morphology with
the occurrence of cool cores in clusters. We also compare our measurements of
X-ray morphology with measurements of the luminosity gap statistics and
ellipticity of the brightest cluster galaxy (BCG). We check how our new X-ray
morphological analysis maps onto cluster scaling relations, finding that (i)
clusters with relatively undisturbed X-ray morphologies are on average more
luminous at fixed X-ray temperature than those with disturbed morphologies, and
(ii) disturbed clusters have larger X-ray masses than regular clusters for a
given temperature in the M-T relation. We also show that the scatter in the
ratio of X-ray and weak lensing based cluster mass measurements is larger for
disturbed clusters than for those of more regular morphology. Overall, our
results demonstrate the feasibility of assembling a self-consistent picture of
the physical structure of clusters from X-ray and optical data, and the
potential to apply this in the measurement of cosmological cluster scaling
relations.
Plerionic Supernova Remnants. (arXiv:1210.5406v1 [astro-ph.GA])
Plerionic Supernova Remnants. (arXiv:1210.5406v1 [astro-ph.GA]):
Plerions represent ideal laboratories for the search for neutron stars, the
study of their relativistic winds, and their interaction with their surrounding
supernova ejecta and/or the interstellar medium. As well, they are widely
believed to represent efficient engines for particle acceleration up to the
knee of the cosmic ray spectrum (at about 1E15 eV). Multi-wavelength
observations from the radio to the highest TeV energies, combined with
modelling, have opened a new window to study these objects, and particularly
shed light on their intrinsic properties, diversity, and evolution.
High-resolution X-ray observations are further revealing the structure and
sites for shock acceleration. The missing shells in the majority of these
objects remain puzzling, and the presence of plerions around highly magnetized
neutron stars is still questionable. I review the current status and statistics
of observations of plerionic supernova remnants (SNRs), highlighting combined
radio and X-ray observations of a growing class of atypical, non Crab-like,
plerionic SNRs in our Galaxy. I will also briefly describe the latest
developments to our high-energy SNRs catalogue recently released to the
community, and finally highlight the key questions to be addressed in this
field with future high-energy missions, including Astro-H in the very near
future.
Plerions represent ideal laboratories for the search for neutron stars, the
study of their relativistic winds, and their interaction with their surrounding
supernova ejecta and/or the interstellar medium. As well, they are widely
believed to represent efficient engines for particle acceleration up to the
knee of the cosmic ray spectrum (at about 1E15 eV). Multi-wavelength
observations from the radio to the highest TeV energies, combined with
modelling, have opened a new window to study these objects, and particularly
shed light on their intrinsic properties, diversity, and evolution.
High-resolution X-ray observations are further revealing the structure and
sites for shock acceleration. The missing shells in the majority of these
objects remain puzzling, and the presence of plerions around highly magnetized
neutron stars is still questionable. I review the current status and statistics
of observations of plerionic supernova remnants (SNRs), highlighting combined
radio and X-ray observations of a growing class of atypical, non Crab-like,
plerionic SNRs in our Galaxy. I will also briefly describe the latest
developments to our high-energy SNRs catalogue recently released to the
community, and finally highlight the key questions to be addressed in this
field with future high-energy missions, including Astro-H in the very near
future.
Chandra View of the Warm-Hot IGM toward 1ES 1553+113: Absorption Line Detections and Identifications (Paper I). (arXiv:1210.7177v1 [astro-ph.CO])
Chandra View of the Warm-Hot IGM toward 1ES 1553+113: Absorption Line Detections and Identifications (Paper I). (arXiv:1210.7177v1 [astro-ph.CO]):
About 30-40 percent of the expected number of baryons is still missing in the
local Universe (z \lesssim 0.4). They are predicted to be hiding in a web of
intergalactic gas at temperatures of about 10^5-10^7 K (the WHIM). Detecting
this matter has had limited success so far, because of its low-density and high
temperature, which makes it difficult to detect with current far-ultraviolet
and X-ray instrumentation.
Here we present the first results from our pilot 500 ks Chandra-LETG
observation of the soft X-ray brightest source in the z > 0.4 sky, the blazar
1ES 1553+113. We identify a total of 11 possible absorption lines, with
single-line statistical significances between 2.2-4.1 sigma. Six of these lines
are detected at high significance (3.6 < \sigma < 4.1), while the remaining
five are regarded as marginal detections in association with either other X-ray
lines detected at higher significance and/or FUV signposts. Three of these
lines are consistent with metal absorption at z~0. The remaining 8 lines may be
imprinted by intervening absorbers and are all consistent with being
high-ionization counterparts of FUV HI and/or OVI IGM signposts. In particular,
four of these eight absorption lines (4.1\sigma, 4.1\sigma, 3.8\sigma and
2.7\sigma), are identified as CV and CVI absorbers belonging to two WHIM
systems at z_X = 0.312 and z_X = 0.133, which also produce broad HI and OVI
absorption in the FUV. The true statistical significances of these two X-ray
absorption systems, after properly accounting for the number of redshift
trials, are 5.8\sigma and 3.8\sigma.
About 30-40 percent of the expected number of baryons is still missing in the
local Universe (z \lesssim 0.4). They are predicted to be hiding in a web of
intergalactic gas at temperatures of about 10^5-10^7 K (the WHIM). Detecting
this matter has had limited success so far, because of its low-density and high
temperature, which makes it difficult to detect with current far-ultraviolet
and X-ray instrumentation.
Here we present the first results from our pilot 500 ks Chandra-LETG
observation of the soft X-ray brightest source in the z > 0.4 sky, the blazar
1ES 1553+113. We identify a total of 11 possible absorption lines, with
single-line statistical significances between 2.2-4.1 sigma. Six of these lines
are detected at high significance (3.6 < \sigma < 4.1), while the remaining
five are regarded as marginal detections in association with either other X-ray
lines detected at higher significance and/or FUV signposts. Three of these
lines are consistent with metal absorption at z~0. The remaining 8 lines may be
imprinted by intervening absorbers and are all consistent with being
high-ionization counterparts of FUV HI and/or OVI IGM signposts. In particular,
four of these eight absorption lines (4.1\sigma, 4.1\sigma, 3.8\sigma and
2.7\sigma), are identified as CV and CVI absorbers belonging to two WHIM
systems at z_X = 0.312 and z_X = 0.133, which also produce broad HI and OVI
absorption in the FUV. The true statistical significances of these two X-ray
absorption systems, after properly accounting for the number of redshift
trials, are 5.8\sigma and 3.8\sigma.
A Deep Chandra Observation of the AGN Outburst and Merger in Hickson Compact Group 62. (arXiv:1210.7079v1 [astro-ph.CO])
A Deep Chandra Observation of the AGN Outburst and Merger in Hickson Compact Group 62. (arXiv:1210.7079v1 [astro-ph.CO]):
We report on an analysis of new Chandra data of the galaxy group HCG 62, well
known for possessing cavities in its intragroup medium (IGM) that were inflated
by the radio lobes of its central active galactic nucleus (AGN). With the new
data, a factor of three deeper than previous Chandra data, we re-examine the
energetics of the cavities and determine new constraints on their contents. We
confirm that the ratio of radiative to mechanical power of the AGN outburst
that created the cavities is less than 10^-4, among the lowest of any known
cavity system, implying that the relativistic electrons in the lobes can supply
only a tiny fraction of the pressure required to support the cavities. This
finding implies additional pressure support in the lobes from heavy particles
(e.g., protons) or thermal gas. Using spectral fits to emission in the
cavities, we constrain any such volume-filling thermal gas to have a
temperature kT > 4.3 keV. For the first time, we detect X-ray emission from the
central AGN, with a luminosity of L(2-10 keV) = (1.1 +/- 0.4) x 10^39 erg s^-1
and properties typical of a low-luminosity AGN. Lastly, we report evidence for
a recent merger from the surface brightness, temperature, and metallicity
structure of the IGM.
We report on an analysis of new Chandra data of the galaxy group HCG 62, well
known for possessing cavities in its intragroup medium (IGM) that were inflated
by the radio lobes of its central active galactic nucleus (AGN). With the new
data, a factor of three deeper than previous Chandra data, we re-examine the
energetics of the cavities and determine new constraints on their contents. We
confirm that the ratio of radiative to mechanical power of the AGN outburst
that created the cavities is less than 10^-4, among the lowest of any known
cavity system, implying that the relativistic electrons in the lobes can supply
only a tiny fraction of the pressure required to support the cavities. This
finding implies additional pressure support in the lobes from heavy particles
(e.g., protons) or thermal gas. Using spectral fits to emission in the
cavities, we constrain any such volume-filling thermal gas to have a
temperature kT > 4.3 keV. For the first time, we detect X-ray emission from the
central AGN, with a luminosity of L(2-10 keV) = (1.1 +/- 0.4) x 10^39 erg s^-1
and properties typical of a low-luminosity AGN. Lastly, we report evidence for
a recent merger from the surface brightness, temperature, and metallicity
structure of the IGM.
Three-dimensional numerical investigations of the morphology of type Ia SNRs. (arXiv:1210.7790v1 [astro-ph.GA])
Three-dimensional numerical investigations of the morphology of type Ia SNRs. (arXiv:1210.7790v1 [astro-ph.GA]):
We explore the morphology of Type Ia supernova remnants (SNRs) using
three-dimensional hydrodynamics modeling and an exponential density profile.
Our model distinguishes ejecta from the interstellar medium (ISM), and tracks
the ionization age of shocked ejecta, both of which allow for additional
analysis of the simulated remnants. We also include the adiabatic index as a
free parameter, which affects the compressibility of the fluid and emulates the
efficiency of cosmic ray acceleration by shock fronts. In addition to
generating 3-D images of the simulations, we compute line-of-sight projections
through the remnants for comparison against observations of Tycho's SNR and SN
1006. We find that several features observed in these two remnants, such as the
separation between the fluid discontinuities and the presence of ejecta knots
ahead of the forward shock, can be generated by smooth ejecta without any
initial clumpiness. Our results are consistent with SN 1006 being dynamically
younger than Tycho's SNR, and more efficiently accelerating cosmic rays at its
forward shock. We conclude that clumpiness is not a necessary condition to
reproduce many observed features of Type Ia supernova remnants, particularly
the radial profiles and the fleecy emission from ejecta at the central region
of both remnants.
We explore the morphology of Type Ia supernova remnants (SNRs) using
three-dimensional hydrodynamics modeling and an exponential density profile.
Our model distinguishes ejecta from the interstellar medium (ISM), and tracks
the ionization age of shocked ejecta, both of which allow for additional
analysis of the simulated remnants. We also include the adiabatic index as a
free parameter, which affects the compressibility of the fluid and emulates the
efficiency of cosmic ray acceleration by shock fronts. In addition to
generating 3-D images of the simulations, we compute line-of-sight projections
through the remnants for comparison against observations of Tycho's SNR and SN
1006. We find that several features observed in these two remnants, such as the
separation between the fluid discontinuities and the presence of ejecta knots
ahead of the forward shock, can be generated by smooth ejecta without any
initial clumpiness. Our results are consistent with SN 1006 being dynamically
younger than Tycho's SNR, and more efficiently accelerating cosmic rays at its
forward shock. We conclude that clumpiness is not a necessary condition to
reproduce many observed features of Type Ia supernova remnants, particularly
the radial profiles and the fleecy emission from ejecta at the central region
of both remnants.
The X-ray flaring properties of Sgr A* during six years of monitoring with Swift. (arXiv:1210.7237v1 [astro-ph.HE])
The X-ray flaring properties of Sgr A* during six years of monitoring with Swift. (arXiv:1210.7237v1 [astro-ph.HE]):
Starting in 2006, Swift has been targeting a region of \sim 21'X21' around
Sagittarius A* (Sgr A*) with the onboard X-ray telescope. The short,
quasi-daily observations offer an unique view of the long-term X-ray behavior
of the supermassive black hole. We report on the data obtained between 2006
February and 2011 October, which encompasses 715 observations with a total
accumulated exposure time of \sim 0.8 Ms. A total of six confirmed X-ray flares
were detected with Swift, which all had an average 2-10 keV luminosity of Lx
(1-4)E35 erg/s (assuming a distance of 8 kpc). This more than doubles the
number of such bright X-ray flares observed from Sgr A*. The most luminous
X-ray flare seen with Swift may have reached a 2-10 keV peak intensity of Lx
6E35 erg/s, which would make it the brightest X-ray flare detected so far. One
of the Swift-detected flares was considerably softer than the other five,
indicating that flares of similar intensity can have different spectral
properties. An additional ten candidate X-ray flares were detected with an
estimated average intensity of Lx (0.7-1)E35 erg/s (2-10 keV). The Swift
campaign allows us to constrain the occurrence rate of bright (Lx > 1E35 erg/s)
X-ray flares to be ~0.2-0.5 per day, which is consistent with previous
estimates. This analysis of the occurrence rate and properties of the X-ray
flares seen with Swift offers an important calibration point to asses whether
the flaring behavior of Sgr A* changes as a result of its interaction with the
gas cloud that is projected to make a close passage in 2013.
Starting in 2006, Swift has been targeting a region of \sim 21'X21' around
Sagittarius A* (Sgr A*) with the onboard X-ray telescope. The short,
quasi-daily observations offer an unique view of the long-term X-ray behavior
of the supermassive black hole. We report on the data obtained between 2006
February and 2011 October, which encompasses 715 observations with a total
accumulated exposure time of \sim 0.8 Ms. A total of six confirmed X-ray flares
were detected with Swift, which all had an average 2-10 keV luminosity of Lx
(1-4)E35 erg/s (assuming a distance of 8 kpc). This more than doubles the
number of such bright X-ray flares observed from Sgr A*. The most luminous
X-ray flare seen with Swift may have reached a 2-10 keV peak intensity of Lx
6E35 erg/s, which would make it the brightest X-ray flare detected so far. One
of the Swift-detected flares was considerably softer than the other five,
indicating that flares of similar intensity can have different spectral
properties. An additional ten candidate X-ray flares were detected with an
estimated average intensity of Lx (0.7-1)E35 erg/s (2-10 keV). The Swift
campaign allows us to constrain the occurrence rate of bright (Lx > 1E35 erg/s)
X-ray flares to be ~0.2-0.5 per day, which is consistent with previous
estimates. This analysis of the occurrence rate and properties of the X-ray
flares seen with Swift offers an important calibration point to asses whether
the flaring behavior of Sgr A* changes as a result of its interaction with the
gas cloud that is projected to make a close passage in 2013.
A soft X-ray reverberation lag in ESO 113-G010. (arXiv:1210.7874v1 [astro-ph.HE])
A soft X-ray reverberation lag in ESO 113-G010. (arXiv:1210.7874v1 [astro-ph.HE]):
Reverberation lags have recently been discovered in a handful of nearby,
variable AGN. Here, we analyze a ~100 ksec archival XMM-Newton observation of
the highly variable AGN, ESO 113-G010 in order to search for lags between hard
(1.5 - 4.5 keV) and soft (0.3 - 0.9 keV) energy bands. At the lowest
frequencies available in the lightcurve, we find hard lags where the power-law
dominated hard band lags the soft band where the reflection fraction is high.
However, at higher frequencies in the range 2E-4 - 3E-4 Hz we find a soft lag
of 325 +/- 89 seconds at greater than the 3.5-sigma level. The general
evolution from hard to soft lags as the frequency increases is similar to other
AGN where soft lags have been detected. We interpret this soft lag as due to
reverberation, with the reflection component responding to variability in the
power-law. For a black hole mass of 7E6 M_solar this corresponds to a
light-crossing time of ~9 GM/c^3, however, dilution effects mean that the
intrinsic lag is likely longer than this. Based on recent black hole
mass-scaling for lag properties, the lag amplitude and frequency are more
consistent with a black hole a few times more massive than the best estimates,
though flux-dependent effects could easily add scatter this large.
Reverberation lags have recently been discovered in a handful of nearby,
variable AGN. Here, we analyze a ~100 ksec archival XMM-Newton observation of
the highly variable AGN, ESO 113-G010 in order to search for lags between hard
(1.5 - 4.5 keV) and soft (0.3 - 0.9 keV) energy bands. At the lowest
frequencies available in the lightcurve, we find hard lags where the power-law
dominated hard band lags the soft band where the reflection fraction is high.
However, at higher frequencies in the range 2E-4 - 3E-4 Hz we find a soft lag
of 325 +/- 89 seconds at greater than the 3.5-sigma level. The general
evolution from hard to soft lags as the frequency increases is similar to other
AGN where soft lags have been detected. We interpret this soft lag as due to
reverberation, with the reflection component responding to variability in the
power-law. For a black hole mass of 7E6 M_solar this corresponds to a
light-crossing time of ~9 GM/c^3, however, dilution effects mean that the
intrinsic lag is likely longer than this. Based on recent black hole
mass-scaling for lag properties, the lag amplitude and frequency are more
consistent with a black hole a few times more massive than the best estimates,
though flux-dependent effects could easily add scatter this large.
Formation of millisecond pulsars - NS initial mass and EOS constraints. (arXiv:1210.8331v1 [astro-ph.SR])
Formation of millisecond pulsars - NS initial mass and EOS constraints. (arXiv:1210.8331v1 [astro-ph.SR]):
Recent measurement of a high millisecond pulsar mass (PSR J1614-2230,
1.97+-0.04 Msun) compared with the low mass of PSR J0751+1807 (1.26+-0.14 Msun)
indicates a large span of masses of recycled pulsars and suggests a broad range
of neutron stars masses at birth. We aim at reconstructing the pre-accretion
masses for these pulsars while taking into account interaction of the magnetic
field with a thin accretion disk, magnetic field decay and relativistic 2D
solutions for stellar configurations for a set of equations of state. We
briefly discuss the evolutionary scenarios leading to the formation of these
neutron stars and study the influence of the equation of state.
Recent measurement of a high millisecond pulsar mass (PSR J1614-2230,
1.97+-0.04 Msun) compared with the low mass of PSR J0751+1807 (1.26+-0.14 Msun)
indicates a large span of masses of recycled pulsars and suggests a broad range
of neutron stars masses at birth. We aim at reconstructing the pre-accretion
masses for these pulsars while taking into account interaction of the magnetic
field with a thin accretion disk, magnetic field decay and relativistic 2D
solutions for stellar configurations for a set of equations of state. We
briefly discuss the evolutionary scenarios leading to the formation of these
neutron stars and study the influence of the equation of state.
Implications for compact stars of a soft nuclear equation of state from heavy-ion data. (arXiv:1211.0427v1 [astro-ph.HE])
Implications for compact stars of a soft nuclear equation of state from heavy-ion data. (arXiv:1211.0427v1 [astro-ph.HE]):
We study the implications on compact star properties of a soft nuclear
equation of state determined from kaon production at subthreshold energies in
heavy-ion collisions. On one hand, we apply these results to study radii and
moments of inertia of light neutron stars. Heavy-ion data provides constraints
on nuclear matter at densities relevant for those stars and, in particular, to
the density dependence of the symmetry energy of nuclear matter. On the other
hand, we derive a limit for the highest allowed neutron star mass of three
solar masses. For that purpose, we use the information on the nucleon potential
obtained from the analysis of the heavy-ion data combined with causality on the
nuclear equation of state.
We study the implications on compact star properties of a soft nuclear
equation of state determined from kaon production at subthreshold energies in
heavy-ion collisions. On one hand, we apply these results to study radii and
moments of inertia of light neutron stars. Heavy-ion data provides constraints
on nuclear matter at densities relevant for those stars and, in particular, to
the density dependence of the symmetry energy of nuclear matter. On the other
hand, we derive a limit for the highest allowed neutron star mass of three
solar masses. For that purpose, we use the information on the nucleon potential
obtained from the analysis of the heavy-ion data combined with causality on the
nuclear equation of state.
Large Synoptic Survey Telescope: Dark Energy Science Collaboration. (arXiv:1211.0310v1 [astro-ph.CO])
Large Synoptic Survey Telescope: Dark Energy Science Collaboration. (arXiv:1211.0310v1 [astro-ph.CO]):
This white paper describes the LSST Dark Energy Science Collaboration (DESC),
whose goal is the study of dark energy and related topics in fundamental
physics with data from the Large Synoptic Survey Telescope (LSST). It provides
an overview of dark energy science and describes the current and anticipated
state of the field. It makes the case for the DESC by laying out a robust
analytical framework for dark energy science that has been defined by its
members and the comprehensive three-year work plan they have developed for
implementing that framework. The analysis working groups cover five key probes
of dark energy: weak lensing, large scale structure, galaxy clusters, Type Ia
supernovae, and strong lensing. The computing working groups span cosmological
simulations, galaxy catalogs, photon simulations and a systematic software and
computational framework for LSST dark energy data analysis. The technical
working groups make the connection between dark energy science and the LSST
system. The working groups have close linkages, especially through the use of
the photon simulations to study the impact of instrument design and survey
strategy on analysis methodology and cosmological parameter estimation. The
white paper describes several high priority tasks identified by each of the 16
working groups. Over the next three years these tasks will help prepare for
LSST analysis, make synergistic connections with ongoing cosmological surveys
and provide the dark energy community with state of the art analysis tools.
Members of the community are invited to join the LSST DESC, according to the
membership policies described in the white paper. Applications to sign up for
associate membership may be made by submitting the Web form at
this http URL with a short statement
of the work they wish to pursue that is relevant to the LSST DESC.
This white paper describes the LSST Dark Energy Science Collaboration (DESC),
whose goal is the study of dark energy and related topics in fundamental
physics with data from the Large Synoptic Survey Telescope (LSST). It provides
an overview of dark energy science and describes the current and anticipated
state of the field. It makes the case for the DESC by laying out a robust
analytical framework for dark energy science that has been defined by its
members and the comprehensive three-year work plan they have developed for
implementing that framework. The analysis working groups cover five key probes
of dark energy: weak lensing, large scale structure, galaxy clusters, Type Ia
supernovae, and strong lensing. The computing working groups span cosmological
simulations, galaxy catalogs, photon simulations and a systematic software and
computational framework for LSST dark energy data analysis. The technical
working groups make the connection between dark energy science and the LSST
system. The working groups have close linkages, especially through the use of
the photon simulations to study the impact of instrument design and survey
strategy on analysis methodology and cosmological parameter estimation. The
white paper describes several high priority tasks identified by each of the 16
working groups. Over the next three years these tasks will help prepare for
LSST analysis, make synergistic connections with ongoing cosmological surveys
and provide the dark energy community with state of the art analysis tools.
Members of the community are invited to join the LSST DESC, according to the
membership policies described in the white paper. Applications to sign up for
associate membership may be made by submitting the Web form at
this http URL with a short statement
of the work they wish to pursue that is relevant to the LSST DESC.
Black hole mergers: do gas discs lead to spin alignment?. (arXiv:1211.0284v1 [astro-ph.CO])
Black hole mergers: do gas discs lead to spin alignment?. (arXiv:1211.0284v1 [astro-ph.CO]):
In this Letter we revisit arguments suggesting that the Bardeen-Petterson
effect can coalign the spins of a central supermassive black hole binary
accreting from a circumbinary (or circumnuclear) gas disc. We improve on
previous estimates by adding the dependence on system parameters, and noting
that the nonlinear nature of warp propagation in a thin viscous disc affects
alignment. This reduces the disc's ability to communicate the warp, and can
severely reduce the effectiveness of disc-assisted spin alignment. We test our
predictions with a Monte Carlo realization of random misalignments and
accretion rates and we find that the outcome depends strongly on the spin
magnitude. We estimate a generous upper limit to the probability of alignment
by making assumptions which favour it throughout. Even with these assumptions,
about 40% of black holes with $a \gtrsim 0.5$ do not have time to align with
the disc. If the residual misalignment is not small and it is maintained down
to the final coalescence phase this can give a powerful recoil velocity to the
merged hole. Highly spinning black holes are thus more likely of being subject
to strong recoils, the occurrence of which is currently debated.
In this Letter we revisit arguments suggesting that the Bardeen-Petterson
effect can coalign the spins of a central supermassive black hole binary
accreting from a circumbinary (or circumnuclear) gas disc. We improve on
previous estimates by adding the dependence on system parameters, and noting
that the nonlinear nature of warp propagation in a thin viscous disc affects
alignment. This reduces the disc's ability to communicate the warp, and can
severely reduce the effectiveness of disc-assisted spin alignment. We test our
predictions with a Monte Carlo realization of random misalignments and
accretion rates and we find that the outcome depends strongly on the spin
magnitude. We estimate a generous upper limit to the probability of alignment
by making assumptions which favour it throughout. Even with these assumptions,
about 40% of black holes with $a \gtrsim 0.5$ do not have time to align with
the disc. If the residual misalignment is not small and it is maintained down
to the final coalescence phase this can give a powerful recoil velocity to the
merged hole. Highly spinning black holes are thus more likely of being subject
to strong recoils, the occurrence of which is currently debated.
A low-scatter survey-based mass proxy for clusters of galaxies. (arXiv:1211.0790v1 [astro-ph.CO])
A low-scatter survey-based mass proxy for clusters of galaxies. (arXiv:1211.0790v1 [astro-ph.CO]):
Estimates of cosmological parameters using galaxy clusters have the scatter
in the observable at a given mass as a fundamental parameter. This work
computes the amplitude of the scatter for a newly introduced mass proxy, the
product of the cluster total luminosity times the mass-to-light ratio, usually
referred as stellar mass. The analysis of 12 galaxy clusters with excellent
total masses shows a tight correlation between the stellar mass, or stellar
fraction, and total mass within r500 with negligible intrinsic scatter: the 90%
upper limit is 0.06 dex, the posterior mean is 0.027 dex. This scatter is
similar to the one of best-determined mass proxies, such as Yx, i.e. the
product of X-ray temperature and gas mass. The size of the cluster sample used
to determine the intrinsic scatter is small, as in previous works proposing
low-scatter proxies because very accurate masses are needed to infer very small
values of intrinsic scatter. Three-quarters of the studied clusters have lgM
<~14 Msol, which is advantageous from a cosmological perspective because these
clusters are far more abundant than more massive clusters. At the difference of
other mass proxies such as Yx, stellar mass can be determined with survey data
up to at least z=0.9 using upcoming optical near-infrared surveys, such as DES
and Euclid, or even with currently available surveys, covering however smaller
solid angles. On the other end, the uncertainty about the predicted mass of a
single cluster is large, 0.21 to 0.32 dex, depending on cluster richness. This
is largely because the proxy itself has ~0.10 dex errors for clusters of lgM<~
14 Msol mass.
Estimates of cosmological parameters using galaxy clusters have the scatter
in the observable at a given mass as a fundamental parameter. This work
computes the amplitude of the scatter for a newly introduced mass proxy, the
product of the cluster total luminosity times the mass-to-light ratio, usually
referred as stellar mass. The analysis of 12 galaxy clusters with excellent
total masses shows a tight correlation between the stellar mass, or stellar
fraction, and total mass within r500 with negligible intrinsic scatter: the 90%
upper limit is 0.06 dex, the posterior mean is 0.027 dex. This scatter is
similar to the one of best-determined mass proxies, such as Yx, i.e. the
product of X-ray temperature and gas mass. The size of the cluster sample used
to determine the intrinsic scatter is small, as in previous works proposing
low-scatter proxies because very accurate masses are needed to infer very small
values of intrinsic scatter. Three-quarters of the studied clusters have lgM
<~14 Msol, which is advantageous from a cosmological perspective because these
clusters are far more abundant than more massive clusters. At the difference of
other mass proxies such as Yx, stellar mass can be determined with survey data
up to at least z=0.9 using upcoming optical near-infrared surveys, such as DES
and Euclid, or even with currently available surveys, covering however smaller
solid angles. On the other end, the uncertainty about the predicted mass of a
single cluster is large, 0.21 to 0.32 dex, depending on cluster richness. This
is largely because the proxy itself has ~0.10 dex errors for clusters of lgM<~
14 Msol mass.
On the Hot Gas Content of the Milky Way Halo. (arXiv:1211.0758v1 [astro-ph.CO])
On the Hot Gas Content of the Milky Way Halo. (arXiv:1211.0758v1 [astro-ph.CO]):
The Milky Way appears to be missing baryons, as the observed mass in stars
and gas is well below the cosmic mean. One possibility is that a substantial
fraction of the Galaxy's baryons are embedded within an extended,
million-degree hot halo, an idea supported indirectly by observations of warm
gas clouds in the halo and gas-free dwarf spheroidal satellites. X-ray
observations have established that hot gas does exist in our Galaxy beyond the
local hot bubble; however, it may be distributed in a hot disk configuration.
Moreover, recent investigations into the X-ray constraints have suggested that
any Galactic corona must be insignificant. Here we re-examine the observational
data, particularly in the X-ray and radio bands, in order to determine whether
it is possible for a substantial fraction of the Galaxy's baryons to exist in ~
10^6 K gas. In agreement with past studies, we find that a baryonically closed
halo is clearly ruled out if one assumes that the hot corona is distributed
with a cuspy NFW profile. However, if the hot corona of the galaxy is in an
extended, low-density distribution with a large central core, as expected for
an adiabatic gas in hydrostatic equilibrium, then it may contain up to 10^11
M_sun of material, possibly accounting for all of the missing Galactic baryons.
We briefly discuss some potential avenues for discriminating between a massive,
extended hot halo and a local hot disk.
The Milky Way appears to be missing baryons, as the observed mass in stars
and gas is well below the cosmic mean. One possibility is that a substantial
fraction of the Galaxy's baryons are embedded within an extended,
million-degree hot halo, an idea supported indirectly by observations of warm
gas clouds in the halo and gas-free dwarf spheroidal satellites. X-ray
observations have established that hot gas does exist in our Galaxy beyond the
local hot bubble; however, it may be distributed in a hot disk configuration.
Moreover, recent investigations into the X-ray constraints have suggested that
any Galactic corona must be insignificant. Here we re-examine the observational
data, particularly in the X-ray and radio bands, in order to determine whether
it is possible for a substantial fraction of the Galaxy's baryons to exist in ~
10^6 K gas. In agreement with past studies, we find that a baryonically closed
halo is clearly ruled out if one assumes that the hot corona is distributed
with a cuspy NFW profile. However, if the hot corona of the galaxy is in an
extended, low-density distribution with a large central core, as expected for
an adiabatic gas in hydrostatic equilibrium, then it may contain up to 10^11
M_sun of material, possibly accounting for all of the missing Galactic baryons.
We briefly discuss some potential avenues for discriminating between a massive,
extended hot halo and a local hot disk.
X-ray Spectroscopy of Clusters of Galaxies. (arXiv:1211.0679v1 [astro-ph.CO])
X-ray Spectroscopy of Clusters of Galaxies. (arXiv:1211.0679v1 [astro-ph.CO]):
Clusters of galaxies are the most massive objects in the Universe and precise
knowledge of their mass structure is important to understand the history of
structure formation and constrain still unknown types of dark contents of the
Universe. X-ray spectroscopy of galaxy clusters provides rich information about
the physical state of hot intracluster gas and the underlying potential
structure. In this paper, starting from the basic description of clusters under
equilibrium conditions, we review properties of clusters revealed primarily
through X-ray observations considering their thermal and dynamical evolutions.
The future prospects of cluster studies using upcoming X-ray missions are also
mentioned.
Clusters of galaxies are the most massive objects in the Universe and precise
knowledge of their mass structure is important to understand the history of
structure formation and constrain still unknown types of dark contents of the
Universe. X-ray spectroscopy of galaxy clusters provides rich information about
the physical state of hot intracluster gas and the underlying potential
structure. In this paper, starting from the basic description of clusters under
equilibrium conditions, we review properties of clusters revealed primarily
through X-ray observations considering their thermal and dynamical evolutions.
The future prospects of cluster studies using upcoming X-ray missions are also
mentioned.
Maximum mass of neutron stars and strange neutron-star cores. (arXiv:1211.1231v1 [astro-ph.SR])
Maximum mass of neutron stars and strange neutron-star cores. (arXiv:1211.1231v1 [astro-ph.SR]):
Recent measurement of mass of PSR J1614-2230 rules out most of existing
models of equation of state (EOS) of dense matter with high-density softening
due to hyperonization, based on the recent hyperon-nucleon and hyperon-hyperon
interactions, leading to a "hyperon puzzle".
We study a specific solution of "hyperon puzzle", consisting in replacing a
too soft hyperon core by a sufficiently stiff quark core. We construct an
analytic approximation fitting very well modern EOSs of 2SC and CFL color
superconducting phases of quark matter. This allows us for simulating continua
of sequences of first-order phase transitions from hadronic matter to the 2SC,
and then to the CFL state of color superconducting quark matter.
We obtain constraints in the parameter space of the EOS of superconducting
quark cores, resulting from M_max> 2 M_sol. We also derive constraints that
would result from significantly higher measured masses. For 2.4 M_sol required
stiffness of the CFL quark core should have been close to the causality limit,
the density jump at the phase transition being very small.
Condition M_max > 2 M_sol puts strong constraints on the EOSs of the 2SC and
CFL phases of quark matter. Density jumps at the phase transitions have to be
sufficiently small and sound speeds in quark matter - sufficiently large. A
strict condition of thermodynamic stability of quark phase results in the
maximum mass of hybrid stars similar to that of purely baryon stars. Therefore,
to get M_max>2 M_sol for stable hybrid stars, both sufficiently strong
additional hyperon repulsion at high density baryon matter and a sufficiently
stiff EOS of quark matter would be needed. However, it is likely that the high
density instability of quark matter (reconfinement) indicates actually the
inadequacy of the point-particle model of baryons in dense matter at very high
densities.
Recent measurement of mass of PSR J1614-2230 rules out most of existing
models of equation of state (EOS) of dense matter with high-density softening
due to hyperonization, based on the recent hyperon-nucleon and hyperon-hyperon
interactions, leading to a "hyperon puzzle".
We study a specific solution of "hyperon puzzle", consisting in replacing a
too soft hyperon core by a sufficiently stiff quark core. We construct an
analytic approximation fitting very well modern EOSs of 2SC and CFL color
superconducting phases of quark matter. This allows us for simulating continua
of sequences of first-order phase transitions from hadronic matter to the 2SC,
and then to the CFL state of color superconducting quark matter.
We obtain constraints in the parameter space of the EOS of superconducting
quark cores, resulting from M_max> 2 M_sol. We also derive constraints that
would result from significantly higher measured masses. For 2.4 M_sol required
stiffness of the CFL quark core should have been close to the causality limit,
the density jump at the phase transition being very small.
Condition M_max > 2 M_sol puts strong constraints on the EOSs of the 2SC and
CFL phases of quark matter. Density jumps at the phase transitions have to be
sufficiently small and sound speeds in quark matter - sufficiently large. A
strict condition of thermodynamic stability of quark phase results in the
maximum mass of hybrid stars similar to that of purely baryon stars. Therefore,
to get M_max>2 M_sol for stable hybrid stars, both sufficiently strong
additional hyperon repulsion at high density baryon matter and a sufficiently
stiff EOS of quark matter would be needed. However, it is likely that the high
density instability of quark matter (reconfinement) indicates actually the
inadequacy of the point-particle model of baryons in dense matter at very high
densities.
The Impact of Suzaku Measurements on Astroparticle Physics. (arXiv:1211.1115v1 [astro-ph.CO])
The Impact of Suzaku Measurements on Astroparticle Physics. (arXiv:1211.1115v1 [astro-ph.CO]):
Results from the Suzaku X-ray broad-band observations of clusters of galaxies
are summarized. Aiming at understanding the physics of gas heating/particle
acceleration and the cluster dynamical evolution, we search for non-thermal
hard X-ray emission from merging clusters, particularly A2163 and the Bullet
Cluster, based on the Suzaku and XMM-Newton/Chandra joint analyzes. The
observed hard X-ray emission is well represented by the single- or
multi-temperature thermal models including super-hot (kT~20 keV) gas. On the
other hand, no significant non-thermal hard X-ray emission has been detected.
Together with the presently available literature, the hard X-ray properties
have been studied for about 10 clusters with Suzaku. The present status on
Suzaku measurements of non-thermal X-ray emission and the cluster magnetic
field are summarized and compared with those from the RXTE, BeppoSAX, and Swift
satellites. The future prospects are briefly mentioned.
Results from the Suzaku X-ray broad-band observations of clusters of galaxies
are summarized. Aiming at understanding the physics of gas heating/particle
acceleration and the cluster dynamical evolution, we search for non-thermal
hard X-ray emission from merging clusters, particularly A2163 and the Bullet
Cluster, based on the Suzaku and XMM-Newton/Chandra joint analyzes. The
observed hard X-ray emission is well represented by the single- or
multi-temperature thermal models including super-hot (kT~20 keV) gas. On the
other hand, no significant non-thermal hard X-ray emission has been detected.
Together with the presently available literature, the hard X-ray properties
have been studied for about 10 clusters with Suzaku. The present status on
Suzaku measurements of non-thermal X-ray emission and the cluster magnetic
field are summarized and compared with those from the RXTE, BeppoSAX, and Swift
satellites. The future prospects are briefly mentioned.
X-ray properties of BzK-selected galaxies in the deepest X-ray fields. (arXiv:1211.1028v1 [astro-ph.GA])
X-ray properties of BzK-selected galaxies in the deepest X-ray fields. (arXiv:1211.1028v1 [astro-ph.GA]):
We investigate the X-ray properties of BzK-selected galaxies at z $\sim$ 2
using deep X-ray data in the Chandra Deep Field South and North (CDFS and
CDFN). Of these we directly detect in X-rays 49 sBzKs in CDFS and 32 sBzKs in
CDFN. Stacking the undetected sources also reveals a significant X-ray signal.
Investigating the X-ray detection rate and stacked flux versus the IR excess
parameter (i.e. SFRtotal/SFRUV,corr), we find no strong evidence for an
increased X-ray detection rate, or a harder X-ray spectrum in IR Excess sBzKs.
This is particularly the case when one accounts for the strong correlation
between the IR excess parameter and the bolometric IR luminosity (LIR), e.g.
when controlling for LIR, the IR Non-Excess sBzKs show a detection rate at
least as high. While both direct detections and stacking suggest that the AGN
fraction in sBzK galaxies is high, there is no clear evidence for widespread
Compton thick activity in either the sBzK population generally, or the IR
Excess sBzK subsample. The very hard X-ray signal obtained for the latter in
earlier work was most likely contaminated by a few hard X-ray sources now
directly detected in deeper X-ray data. The X-ray detection fraction of passive
BzK galaxies in our sample is if anything higher than that of sBZKs, so there
is no evidence for coeval black hole growth and star formation from X-ray
analysis of the BzK populations. Because increased AGN activity in the IR
excess population is not indicated by our X-ray analysis, it appears that the
bulk of the IR Excess sBzK population are luminous star-forming galaxies whose
SFRs are either overestimated at 24 microns, underestimated in the UV, or both.
This conclusion reinforces recent results from Herschel which show similar
effects.
We investigate the X-ray properties of BzK-selected galaxies at z $\sim$ 2
using deep X-ray data in the Chandra Deep Field South and North (CDFS and
CDFN). Of these we directly detect in X-rays 49 sBzKs in CDFS and 32 sBzKs in
CDFN. Stacking the undetected sources also reveals a significant X-ray signal.
Investigating the X-ray detection rate and stacked flux versus the IR excess
parameter (i.e. SFRtotal/SFRUV,corr), we find no strong evidence for an
increased X-ray detection rate, or a harder X-ray spectrum in IR Excess sBzKs.
This is particularly the case when one accounts for the strong correlation
between the IR excess parameter and the bolometric IR luminosity (LIR), e.g.
when controlling for LIR, the IR Non-Excess sBzKs show a detection rate at
least as high. While both direct detections and stacking suggest that the AGN
fraction in sBzK galaxies is high, there is no clear evidence for widespread
Compton thick activity in either the sBzK population generally, or the IR
Excess sBzK subsample. The very hard X-ray signal obtained for the latter in
earlier work was most likely contaminated by a few hard X-ray sources now
directly detected in deeper X-ray data. The X-ray detection fraction of passive
BzK galaxies in our sample is if anything higher than that of sBZKs, so there
is no evidence for coeval black hole growth and star formation from X-ray
analysis of the BzK populations. Because increased AGN activity in the IR
excess population is not indicated by our X-ray analysis, it appears that the
bulk of the IR Excess sBzK population are luminous star-forming galaxies whose
SFRs are either overestimated at 24 microns, underestimated in the UV, or both.
This conclusion reinforces recent results from Herschel which show similar
effects.
Sunyaev-Zel'dovich-Measured Pressure Profiles from the Bolocam X-ray/SZ Galaxy Cluster Sample. (arXiv:1211.1632v1 [astro-ph.CO])
Sunyaev-Zel'dovich-Measured Pressure Profiles from the Bolocam X-ray/SZ Galaxy Cluster Sample. (arXiv:1211.1632v1 [astro-ph.CO]):
We describe Sunyaev-Zel'dovich (SZ) effect measurements and analysis of the
intracluster medium (ICM) pressure profiles of a set of 45 massive galaxy
clusters imaged using Bolocam at the Caltech Submillimeter Observatory. We have
used masses determined from Chandra X-ray observations to scale each cluster's
profile by the overdensity radius R500 and the mass-and-redshift-dependent
normalization factor P500. We deproject the average pressure profile of our
sample into 13 logarithmically spaced radial bins between 0.07R500 and 3.5R500.
We find that a generalized Navarro, Frenk, and White (gNFW) profile describes
our data with sufficient goodness-of-fit and best-fit parameters (C500, alpha,
beta, gamma, P0 = 1.18, 0.86, 3.67, 0.67, 4.29). We also use the X-ray data to
define cool-core and disturbed subsamples of clusters, and we constrain the
average pressure profiles of each of these subsamples. We find that given the
precision of our data the average pressure profiles of disturbed and cool-core
clusters are consistent with one another at R>~0.15R500, with cool-core systems
showing indications of higher pressure at R<~0.15R500. In addition, for the
first time, we place simultaneous constraints on the mass scaling of cluster
pressure profiles, their ensemble mean profile, and their radius-dependent
intrinsic scatter between 0.1R500 and 2.0R500. The scatter among profiles is
minimized at radii between ~0.2R500 and ~0.5R500, with a value of ~20%. The
best-fit mass scaling has a power-law slope of 0.49, which is shallower than
the nominal prediction of 2/3 from self-similar hydrostatic equilibrium models.
These results for the intrinsic scatter and mass scaling are largely consistent
with previous analyses, most of which have relied heavily on X-ray derived
pressures of clusters at significantly lower masses and redshifts compared to
our sample.
We describe Sunyaev-Zel'dovich (SZ) effect measurements and analysis of the
intracluster medium (ICM) pressure profiles of a set of 45 massive galaxy
clusters imaged using Bolocam at the Caltech Submillimeter Observatory. We have
used masses determined from Chandra X-ray observations to scale each cluster's
profile by the overdensity radius R500 and the mass-and-redshift-dependent
normalization factor P500. We deproject the average pressure profile of our
sample into 13 logarithmically spaced radial bins between 0.07R500 and 3.5R500.
We find that a generalized Navarro, Frenk, and White (gNFW) profile describes
our data with sufficient goodness-of-fit and best-fit parameters (C500, alpha,
beta, gamma, P0 = 1.18, 0.86, 3.67, 0.67, 4.29). We also use the X-ray data to
define cool-core and disturbed subsamples of clusters, and we constrain the
average pressure profiles of each of these subsamples. We find that given the
precision of our data the average pressure profiles of disturbed and cool-core
clusters are consistent with one another at R>~0.15R500, with cool-core systems
showing indications of higher pressure at R<~0.15R500. In addition, for the
first time, we place simultaneous constraints on the mass scaling of cluster
pressure profiles, their ensemble mean profile, and their radius-dependent
intrinsic scatter between 0.1R500 and 2.0R500. The scatter among profiles is
minimized at radii between ~0.2R500 and ~0.5R500, with a value of ~20%. The
best-fit mass scaling has a power-law slope of 0.49, which is shallower than
the nominal prediction of 2/3 from self-similar hydrostatic equilibrium models.
These results for the intrinsic scatter and mass scaling are largely consistent
with previous analyses, most of which have relied heavily on X-ray derived
pressures of clusters at significantly lower masses and redshifts compared to
our sample.
Constraints of the compactness of the isolated neutron stars via X-ray phase-resolved spectroscopy. (arXiv:1211.1842v1 [astro-ph.SR])
Constraints of the compactness of the isolated neutron stars via X-ray phase-resolved spectroscopy. (arXiv:1211.1842v1 [astro-ph.SR]):
A model with a condensed iron surface and partially ionized hydrogen-thin
atmosphere allows us to fit simultaneously the observed general spectral shape
and the broad absorption feature (observed at 0.3 keV) in different spin phases
of the isolated neutron star RBS 1223. We constrain some physical properties of
the X-ray emitting areas, i.e. the temperatures (Tpole1 ~ 105eV, Tpole2 ~99eV),
magnetic field strengths Bpole1 ~ Bpole2 ~ 8.6x10^13G) at the poles, and their
distribution parameters (a1 ~ 0.61, a2 ~ 0.29, indicating an absence of strong
toroidal magnetic field component). In addition, we are able to place some
constraints on the geometry of the emerging X-ray emission and the
gravitational redshift (z ~0.16+0.03-0.01) of the isolated neutron star RBS
1223.
A model with a condensed iron surface and partially ionized hydrogen-thin
atmosphere allows us to fit simultaneously the observed general spectral shape
and the broad absorption feature (observed at 0.3 keV) in different spin phases
of the isolated neutron star RBS 1223. We constrain some physical properties of
the X-ray emitting areas, i.e. the temperatures (Tpole1 ~ 105eV, Tpole2 ~99eV),
magnetic field strengths Bpole1 ~ Bpole2 ~ 8.6x10^13G) at the poles, and their
distribution parameters (a1 ~ 0.61, a2 ~ 0.29, indicating an absence of strong
toroidal magnetic field component). In addition, we are able to place some
constraints on the geometry of the emerging X-ray emission and the
gravitational redshift (z ~0.16+0.03-0.01) of the isolated neutron star RBS
1223.
X-Ray Determination of the Variable Rate of Mass Accretion onto TW Hydrae. (arXiv:1211.1710v1 [astro-ph.SR])
X-Ray Determination of the Variable Rate of Mass Accretion onto TW Hydrae. (arXiv:1211.1710v1 [astro-ph.SR]):
Diagnostics of electron temperature (T_e), electron density (n_e), and
hydrogen column density (N_H) from the Chandra High Energy Transmission Grating
spectrum of He-like Ne IX in TW Hydrae (TW Hya), in conjunction with a
classical accretion model, allow us to infer the accretion rate onto the star
directly from measurements of the accreting material. The new method introduces
the use of the absorption of Ne IX lines as a measure of the column density of
the intervening, accreting material. On average, the derived mass accretion
rate for TW Hya is 1.5 x 10^{-9} M_{\odot} yr^{-1}, for a stellar magnetic
field strength of 600 Gauss and a filling factor of 3.5%. Three individual
Chandra exposures show statistically significant differences in the Ne IX line
ratios, indicating changes in N_H, T_e, and n_e by factors of 0.28, 1.6, and
1.3, respectively. In exposures separated by 2.7 days, the observations
reported here suggest a five-fold reduction in the accretion rate. This
powerful new technique promises to substantially improve our understanding of
the accretion process in young stars.
Diagnostics of electron temperature (T_e), electron density (n_e), and
hydrogen column density (N_H) from the Chandra High Energy Transmission Grating
spectrum of He-like Ne IX in TW Hydrae (TW Hya), in conjunction with a
classical accretion model, allow us to infer the accretion rate onto the star
directly from measurements of the accreting material. The new method introduces
the use of the absorption of Ne IX lines as a measure of the column density of
the intervening, accreting material. On average, the derived mass accretion
rate for TW Hya is 1.5 x 10^{-9} M_{\odot} yr^{-1}, for a stellar magnetic
field strength of 600 Gauss and a filling factor of 3.5%. Three individual
Chandra exposures show statistically significant differences in the Ne IX line
ratios, indicating changes in N_H, T_e, and n_e by factors of 0.28, 1.6, and
1.3, respectively. In exposures separated by 2.7 days, the observations
reported here suggest a five-fold reduction in the accretion rate. This
powerful new technique promises to substantially improve our understanding of
the accretion process in young stars.
Properties of gas clumps and gas clumping factor in the intra cluster medium. (arXiv:1211.1695v1 [astro-ph.CO])
Properties of gas clumps and gas clumping factor in the intra cluster medium. (arXiv:1211.1695v1 [astro-ph.CO]):
The spatial distribution of gas matter inside galaxy clusters is not
completely smooth, but may host gas clumps associated with substructures. These
overdense gas substructures are generally a source of unresolved bias of X-ray
observations towards high density gas, but their bright luminosity peaks may be
resolved sources within the ICM, that deep X-ray exposures may be (already)
capable to detect. In this paper we aim at investigating both features, using a
set of high-resolution cosmological simulations with ENZO. First, we monitor
how the bias by unresolved gas clumping may yield incorrect estimates of global
cluster parameters and affects the measurements of baryon fractions by X-ray
observations. We find that based on X-ray observations of narrow radial strips,
it is difficult to recover the real baryon fraction to better than 10 - 20
percent uncertainty. Second, we investigated the possibility of observing
bright X-ray clumps in the nearby Universe (z<=0.3). We produced simple mock
X-ray observations for several instruments (XMM, Suzaku and ROSAT) and
extracted the statistics of potentially detectable bright clumps. Some of the
brightest clumps predicted by simulations may already have been already
detected in X- ray images with a large field of view. However, their small
projected size makes it difficult to prove their existence based on X-ray
morphology only. Preheating, AGN feedback and cosmic rays are found to have
little impact on the statistical properties of gas clumps.
The spatial distribution of gas matter inside galaxy clusters is not
completely smooth, but may host gas clumps associated with substructures. These
overdense gas substructures are generally a source of unresolved bias of X-ray
observations towards high density gas, but their bright luminosity peaks may be
resolved sources within the ICM, that deep X-ray exposures may be (already)
capable to detect. In this paper we aim at investigating both features, using a
set of high-resolution cosmological simulations with ENZO. First, we monitor
how the bias by unresolved gas clumping may yield incorrect estimates of global
cluster parameters and affects the measurements of baryon fractions by X-ray
observations. We find that based on X-ray observations of narrow radial strips,
it is difficult to recover the real baryon fraction to better than 10 - 20
percent uncertainty. Second, we investigated the possibility of observing
bright X-ray clumps in the nearby Universe (z<=0.3). We produced simple mock
X-ray observations for several instruments (XMM, Suzaku and ROSAT) and
extracted the statistics of potentially detectable bright clumps. Some of the
brightest clumps predicted by simulations may already have been already
detected in X- ray images with a large field of view. However, their small
projected size makes it difficult to prove their existence based on X-ray
morphology only. Preheating, AGN feedback and cosmic rays are found to have
little impact on the statistical properties of gas clumps.
A Deep X-ray View of the Hot Halo in the Edge-on Spiral Galaxy NGC 891. (arXiv:1211.1669v1 [astro-ph.CO])
A Deep X-ray View of the Hot Halo in the Edge-on Spiral Galaxy NGC 891. (arXiv:1211.1669v1 [astro-ph.CO]):
NGC 891 is a nearby edge-on galaxy that is similar to the Milky Way and has a
hot X-ray emitting halo that could arise from accretion, a galactic fountain,
or a combination of the two. The metallicity of the gas can help distinguish
between these models, and here we report on results that use 138 ks of archival
Chandra data and 92 ks of new XMM-Newton data to measure the temperature and
metallicity of the hot halo of the galaxy. We find good fits for a thermal
model with kT ~ 0.2 keV and Z ~ 0.1 solar, and rule out solar metallicity to
more than 99% confidence. This result suggests accretion from the intergalactic
medium as the origin for the hot halo. However, it is also possible to fit a
two-temperature thermal model with solar metallicity where kT_1 = 0.1 keV and
kT_2 = 0.25 keV. A consideration of the cooling rate and scale height prefers
the single-temperature model. We also find that the cooling rate in the hot gas
cannot explain the massive HI halo in the steady state. In addition, a galactic
fountain model cannot eject enough mass to account for the HI halo, and we
speculate that the neutral halo may be gas from a prior outflow that has since
cooled.
NGC 891 is a nearby edge-on galaxy that is similar to the Milky Way and has a
hot X-ray emitting halo that could arise from accretion, a galactic fountain,
or a combination of the two. The metallicity of the gas can help distinguish
between these models, and here we report on results that use 138 ks of archival
Chandra data and 92 ks of new XMM-Newton data to measure the temperature and
metallicity of the hot halo of the galaxy. We find good fits for a thermal
model with kT ~ 0.2 keV and Z ~ 0.1 solar, and rule out solar metallicity to
more than 99% confidence. This result suggests accretion from the intergalactic
medium as the origin for the hot halo. However, it is also possible to fit a
two-temperature thermal model with solar metallicity where kT_1 = 0.1 keV and
kT_2 = 0.25 keV. A consideration of the cooling rate and scale height prefers
the single-temperature model. We also find that the cooling rate in the hot gas
cannot explain the massive HI halo in the steady state. In addition, a galactic
fountain model cannot eject enough mass to account for the HI halo, and we
speculate that the neutral halo may be gas from a prior outflow that has since
cooled.
Subscribe to:
Posts (Atom)