Signatures of an Encounter Between the G2 Cloud and a Jet from Sgr A*. (arXiv:1303.3969v1 [astro-ph.HE]):
The recent discovery of the G2 cloud of dense, ionized gas on a trajectory
toward Sgr A*, the black hole at the dynamical center of the Galaxy, offers a
unique opportunity to observe an accretion event onto a massive black hole as
well as to probe its immediate environment. Simulations and models predict
increased X-ray and radio variability resulting from increased accretion driven
by drag on an atmosphere of hot, X-ray emitting gas surrounding Sgr A*. Here,
we present X-ray and radio light curves of the emission resulting from the
potential encounter of the G2 cloud with a relativistic jet from Sgr A*. This
interaction would violently shock a portion of the G2 cloud to temperatures
~10^8 K resulting in bright X-ray emission from the dense, shocked gas as it
adiabatically expands. The 2-10 keV luminosity may reach ~10 times the
quiescent X-ray flux of Sgr A*. Approximately 3 solar luminosity is emitted
above 10 keV at the peak of the light curve, with significant softening of the
spectrum occurring as the gas subsequently cools. Observations with NuSTAR
would therefore be able to confirm such an event as well as determine the cloud
speed. At radio wavelengths, the associated synchrotron radio emission may
reach levels of a few Jy, although this is more uncertain due to the efficiency
of converting the shock luminosity to that of electrons accelerated to
relativistic energies.
Saturday, April 6, 2013
The space density of magnetic cataclysmic variables. (arXiv:1303.4270v1 [astro-ph.SR])
The space density of magnetic cataclysmic variables. (arXiv:1303.4270v1 [astro-ph.SR]):
We use the complete, X-ray flux-limited ROSAT Bright Survey (RBS) to measure
the space density of magnetic cataclysmic variables (mCVs). The survey provides
complete optical identification of all sources with count rate >0.2/s over half
the sky ($|b|>30^\circ$), and detected 6 intermediate polars (IPs) and 24
polars. If we assume that the 30 mCVs included in the RBS are representative of
the intrinsic population, the space density of mCVs is $8^{+4}_{-2} \times
10^{-7}\,{\rmpc^{-3}}$. Considering polars and IPs separately, we find
$\rho_{polar}=5^{+3}_{-2} \times 10^{-7}\,{\rm pc^{-3}}$ and
$\rho_{IP}=3^{+2}_{-1} \times 10^{-7}\,{\rm pc^{-3}}$. Allowing for a 50%
high-state duty cycle for polars (and assuming that these systems are below the
RBS detection limit during their low states) doubles our estimate of
$\rho_{polar}$ and brings the total space density of mCVs to $1.3^{+0.6}_{-0.4}
\times 10^{-6}\,{\rm pc^{-3}}$. We also place upper limits on the sizes of
faint (but persistent) mCV populations that might have escaped detection in the
RBS. Although the large uncertainties in the $\rho$ estimates prevent us from
drawing strong conclusions, we discuss the implications of our results for the
evolutionary relationship between IPs and polars, the fraction of CVs with
strongly magnetic white dwarfs (WDs), and for the contribution of mCVs to
Galactic populations of hard X-ray sources at $L_X \ga 10^{31} {\rm erg/s}$.
Our space density estimates are consistent with the very simple model where
long-period IPs evolve into polars and account for the whole short-period polar
population. We find that the fraction of WDs that are strongly magnetic is not
significantly higher for CV primaries than for isolated WDs. Finally, the space
density of IPs is sufficiently high to explain the bright, hard X-ray source
population in the Galactic Centre.
We use the complete, X-ray flux-limited ROSAT Bright Survey (RBS) to measure
the space density of magnetic cataclysmic variables (mCVs). The survey provides
complete optical identification of all sources with count rate >0.2/s over half
the sky ($|b|>30^\circ$), and detected 6 intermediate polars (IPs) and 24
polars. If we assume that the 30 mCVs included in the RBS are representative of
the intrinsic population, the space density of mCVs is $8^{+4}_{-2} \times
10^{-7}\,{\rmpc^{-3}}$. Considering polars and IPs separately, we find
$\rho_{polar}=5^{+3}_{-2} \times 10^{-7}\,{\rm pc^{-3}}$ and
$\rho_{IP}=3^{+2}_{-1} \times 10^{-7}\,{\rm pc^{-3}}$. Allowing for a 50%
high-state duty cycle for polars (and assuming that these systems are below the
RBS detection limit during their low states) doubles our estimate of
$\rho_{polar}$ and brings the total space density of mCVs to $1.3^{+0.6}_{-0.4}
\times 10^{-6}\,{\rm pc^{-3}}$. We also place upper limits on the sizes of
faint (but persistent) mCV populations that might have escaped detection in the
RBS. Although the large uncertainties in the $\rho$ estimates prevent us from
drawing strong conclusions, we discuss the implications of our results for the
evolutionary relationship between IPs and polars, the fraction of CVs with
strongly magnetic white dwarfs (WDs), and for the contribution of mCVs to
Galactic populations of hard X-ray sources at $L_X \ga 10^{31} {\rm erg/s}$.
Our space density estimates are consistent with the very simple model where
long-period IPs evolve into polars and account for the whole short-period polar
population. We find that the fraction of WDs that are strongly magnetic is not
significantly higher for CV primaries than for isolated WDs. Finally, the space
density of IPs is sufficiently high to explain the bright, hard X-ray source
population in the Galactic Centre.
X-ray exploration of the outskirts of the nearby Centaurus cluster using Suzaku and Chandra. (arXiv:1303.4240v1 [astro-ph.CO])
X-ray exploration of the outskirts of the nearby Centaurus cluster using Suzaku and Chandra. (arXiv:1303.4240v1 [astro-ph.CO]):
We present Suzaku observations of the Centaurus cluster out to 0.95r200,
taken along a strip to the north west. We have also used congruent Chandra
observations of the outskirts to resolve point sources down to a threshold flux
around 7 times lower than that achievable with just Suzaku data, considerably
reducing the systematic uncertainties in the cosmic X-ray background emission
in the outskirts. We find that the temperature decreases by a factor of 2 from
the peak temperature to the outskirts. The entropy profile demonstrates a
central excess (within 0.5r200) over the baseline entropy profile predicted by
simulations of purely gravitational hierarchical structure formation. In the
outskirts the entropy profile is in reasonable agreement with the baseline
entropy profile from Voit et al., but lies slightly below it. We find that the
pressure profile agrees with the universal pressure profile of Arnaud et al.
but lies slightly above it in the outskirts. The excess pressure and decrement
in entropy in the outskirts appear to be the result of an excess in the
measured gas density, possible due to gas clumping biasing the density
measurements high. The gas mass fraction rises and reaches the mean cosmic
baryon fraction at the largest radius studied. The clumping corrected gas mass
fraction agrees with the expected hot gas fraction and with the simulations of
Young et al. We further the analysis of Walker et al. which studied the shapes
of the entropy profiles of the clusters so far explored in the outskirts with
Suzaku. When scaled by the self similar entropy the Suzaku entropy profiles
demonstrate a central excess over the baseline entropy profile, and are
consistent with it at around r500 . However outside r500 the entropy profiles
tend to lie below the baseline entropy profile.
We present Suzaku observations of the Centaurus cluster out to 0.95r200,
taken along a strip to the north west. We have also used congruent Chandra
observations of the outskirts to resolve point sources down to a threshold flux
around 7 times lower than that achievable with just Suzaku data, considerably
reducing the systematic uncertainties in the cosmic X-ray background emission
in the outskirts. We find that the temperature decreases by a factor of 2 from
the peak temperature to the outskirts. The entropy profile demonstrates a
central excess (within 0.5r200) over the baseline entropy profile predicted by
simulations of purely gravitational hierarchical structure formation. In the
outskirts the entropy profile is in reasonable agreement with the baseline
entropy profile from Voit et al., but lies slightly below it. We find that the
pressure profile agrees with the universal pressure profile of Arnaud et al.
but lies slightly above it in the outskirts. The excess pressure and decrement
in entropy in the outskirts appear to be the result of an excess in the
measured gas density, possible due to gas clumping biasing the density
measurements high. The gas mass fraction rises and reaches the mean cosmic
baryon fraction at the largest radius studied. The clumping corrected gas mass
fraction agrees with the expected hot gas fraction and with the simulations of
Young et al. We further the analysis of Walker et al. which studied the shapes
of the entropy profiles of the clusters so far explored in the outskirts with
Suzaku. When scaled by the self similar entropy the Suzaku entropy profiles
demonstrate a central excess over the baseline entropy profile, and are
consistent with it at around r500 . However outside r500 the entropy profiles
tend to lie below the baseline entropy profile.
The IMACS Cluster Building Survey. III. The Star Formation Histories of Field Galaxies. (arXiv:1303.3916v1 [astro-ph.CO])
The IMACS Cluster Building Survey. III. The Star Formation Histories of Field Galaxies. (arXiv:1303.3916v1 [astro-ph.CO]):
Using data from the IMACS Cluster Building Survey and from nearby galaxy
surveys, we examine the evolution of the rate of star formation in field
galaxies from $ z = 0.60$ to the present. Fitting the luminosity function to a
standard Schechter form, we find a rapid evolution of $M_B^*$ consistent with
that found in other deep surveys; at the present epoch $M_B^*$ is evolving at
the rate of $0.38 Gyr^{-1}$, several times faster than the predictions of
simple models for the evolution of old, coeval galaxies. The evolution of the
distribution of specific star formation rates (SSFR) is also too rapid to
explain by such models. We demonstrate that starbursts cannot, even in
principle, explain the evolution of the SSFR distribution. However, the rapid
evolution of both $M_B^*$ and the SSFR distribution can be explained if some
fraction of galaxies have star formation rates characterized by both short rise
and fall times and by an epoch of peak star formation more recent than the
majority of galaxies. Although galaxies of every stellar mass up to
$1.4\times10^{11}\Msun$ show a range of epochs of peak star formation, the
fraction of "younger" galaxies falls from about 40% at a mass of
$4\times10^{10}\Msun$ to zero at a mass of $1.4\times10^{11}\Msun$. The
incidence of younger galaxies appears to be insensitive to the density of the
local environment; but does depend on group membership: relatively isolated
galaxies are much more likely to be young than are group members.
Using data from the IMACS Cluster Building Survey and from nearby galaxy
surveys, we examine the evolution of the rate of star formation in field
galaxies from $ z = 0.60$ to the present. Fitting the luminosity function to a
standard Schechter form, we find a rapid evolution of $M_B^*$ consistent with
that found in other deep surveys; at the present epoch $M_B^*$ is evolving at
the rate of $0.38 Gyr^{-1}$, several times faster than the predictions of
simple models for the evolution of old, coeval galaxies. The evolution of the
distribution of specific star formation rates (SSFR) is also too rapid to
explain by such models. We demonstrate that starbursts cannot, even in
principle, explain the evolution of the SSFR distribution. However, the rapid
evolution of both $M_B^*$ and the SSFR distribution can be explained if some
fraction of galaxies have star formation rates characterized by both short rise
and fall times and by an epoch of peak star formation more recent than the
majority of galaxies. Although galaxies of every stellar mass up to
$1.4\times10^{11}\Msun$ show a range of epochs of peak star formation, the
fraction of "younger" galaxies falls from about 40% at a mass of
$4\times10^{10}\Msun$ to zero at a mass of $1.4\times10^{11}\Msun$. The
incidence of younger galaxies appears to be insensitive to the density of the
local environment; but does depend on group membership: relatively isolated
galaxies are much more likely to be young than are group members.
Winds, Clumps, and Interacting Cosmic Rays in M82. (arXiv:1303.4305v1 [astro-ph.HE])
Winds, Clumps, and Interacting Cosmic Rays in M82. (arXiv:1303.4305v1 [astro-ph.HE]):
We construct a family of models for the evolution of energetic particles in
the starburst galaxy M82 and compare them to observations to test the
calorimeter assumption that all cosmic ray energy is radiated in the starburst
region. Assuming constant cosmic ray acceleration efficiency with Milky Way
parameters, we calculate the cosmic-ray proton and primary and secondary
electron/positron populations as a function of energy. Cosmic rays are injected
with Galactic energy distributions and electron-to-proton ratio via type II
supernovae at the observed rate of 0.07/yr. From the cosmic ray spectra, we
predict the radio synchrotron and \gamma-ray spectra. To more accurately model
the radio spectrum, we incorporate a multiphase interstellar medium in the
starburst region of M82. Our model interstellar medium is highly fragmented
with compact dense molecular clouds and dense photoionized gas, both embedded
in a hot, low density medium in overall pressure equilibrium. The spectra
predicted by this one-zone model are compared to the observed radio and
\gamma-ray spectra of M82. Chi-squared tests are used with radio and \gamma-ray
observations and a range of model predictions to find the best-fit parameters.
The best-fit model yields constraints on key parameters in the starburst zone
of M82, including a magnetic field strength of ~250 \mu G and a wind advection
speed in the range of 300-700 km/s. We find that M82 is a good electron
calorimeter but not an ideal cosmic-ray proton calorimeter and discuss the
implications of our results for the astrophysics of the far infrared-radio
correlation in starburst galaxies.
We construct a family of models for the evolution of energetic particles in
the starburst galaxy M82 and compare them to observations to test the
calorimeter assumption that all cosmic ray energy is radiated in the starburst
region. Assuming constant cosmic ray acceleration efficiency with Milky Way
parameters, we calculate the cosmic-ray proton and primary and secondary
electron/positron populations as a function of energy. Cosmic rays are injected
with Galactic energy distributions and electron-to-proton ratio via type II
supernovae at the observed rate of 0.07/yr. From the cosmic ray spectra, we
predict the radio synchrotron and \gamma-ray spectra. To more accurately model
the radio spectrum, we incorporate a multiphase interstellar medium in the
starburst region of M82. Our model interstellar medium is highly fragmented
with compact dense molecular clouds and dense photoionized gas, both embedded
in a hot, low density medium in overall pressure equilibrium. The spectra
predicted by this one-zone model are compared to the observed radio and
\gamma-ray spectra of M82. Chi-squared tests are used with radio and \gamma-ray
observations and a range of model predictions to find the best-fit parameters.
The best-fit model yields constraints on key parameters in the starburst zone
of M82, including a magnetic field strength of ~250 \mu G and a wind advection
speed in the range of 300-700 km/s. We find that M82 is a good electron
calorimeter but not an ideal cosmic-ray proton calorimeter and discuss the
implications of our results for the astrophysics of the far infrared-radio
correlation in starburst galaxies.
The hard X-ray spectrum of NGC 1365: scattered light, not black hole spin. (arXiv:1303.4309v1 [astro-ph.HE])
The hard X-ray spectrum of NGC 1365: scattered light, not black hole spin. (arXiv:1303.4309v1 [astro-ph.HE]):
Active Galactic Nuclei (AGN) show excess X-ray emission above 10 keV compared
with extrapolation of spectra from lower energies. Risaliti et al. have
recently attempted to model the hard X-ray excess in the type 1.8 AGN NGC 1365,
concluding that the hard excess most likely arises from Compton-scattered
reflection of X-rays from an inner accretion disk close to the black hole.
Their analysis disfavored a model in which the hard excess arises from a high
column density of circumnuclear gas partially covering a primary X-ray source,
despite such components being required in the NGC 1365 data below 10 keV. Using
a Monte Carlo radiative transfer approach, we demonstrate that this conclusion
is invalidated by (i) use of slab absorption models, which have unrealistic
transmission spectra for partial covering gas, (ii) neglect of the effect of
Compton scattering on transmitted spectra and (iii) inadequate modeling of the
expected spectrum of scattered X-rays. The scattered spectrum is geometry
dependent and, for high global covering factors, may dominate above 10 keV. We
further show that, in models of circumnuclear gas, the suppression of the
observed hard X-ray flux by reprocessing may be no larger than required by the
`light bending' model invoked for inner disk reflection, and the expected
emission line strengths lie within the observed range. We conclude that the
time-invariant `red wing' in X-ray spectra is probably caused by continuum
transmitted through and scattered from circumnuclear gas, not by highly
redshifted line emission, and that measurement of black hole spin is not
possible.
Active Galactic Nuclei (AGN) show excess X-ray emission above 10 keV compared
with extrapolation of spectra from lower energies. Risaliti et al. have
recently attempted to model the hard X-ray excess in the type 1.8 AGN NGC 1365,
concluding that the hard excess most likely arises from Compton-scattered
reflection of X-rays from an inner accretion disk close to the black hole.
Their analysis disfavored a model in which the hard excess arises from a high
column density of circumnuclear gas partially covering a primary X-ray source,
despite such components being required in the NGC 1365 data below 10 keV. Using
a Monte Carlo radiative transfer approach, we demonstrate that this conclusion
is invalidated by (i) use of slab absorption models, which have unrealistic
transmission spectra for partial covering gas, (ii) neglect of the effect of
Compton scattering on transmitted spectra and (iii) inadequate modeling of the
expected spectrum of scattered X-rays. The scattered spectrum is geometry
dependent and, for high global covering factors, may dominate above 10 keV. We
further show that, in models of circumnuclear gas, the suppression of the
observed hard X-ray flux by reprocessing may be no larger than required by the
`light bending' model invoked for inner disk reflection, and the expected
emission line strengths lie within the observed range. We conclude that the
time-invariant `red wing' in X-ray spectra is probably caused by continuum
transmitted through and scattered from circumnuclear gas, not by highly
redshifted line emission, and that measurement of black hole spin is not
possible.
Monday, March 25, 2013
Planck 2013 results. XX. Cosmology from Sunyaev-Zeldovich cluster counts. (arXiv:1303.5080v1 [astro-ph.CO])
Planck 2013 results. XX. Cosmology from Sunyaev-Zeldovich cluster counts. (arXiv:1303.5080v1 [astro-ph.CO]):
We present constraints on cosmological parameters using number counts as a
function of redshift for a sub-sample of 189 galaxy clusters from the Planck SZ
(PSZ) catalogue. The PSZ is selected through the signature of the
Sunyaev--Zeldovich (SZ) effect, and the sub-sample used here has a
signal-to-noise threshold of seven, with each object confirmed as a cluster and
all but one with a redshift estimate. We discuss the calculation of the
expected cluster counts as a function of cosmological parameters, the
completeness of the sample, and the likelihood construction method. Using a
relation between mass M and SZ signal Y based on comparison to X-ray
measurements, we derive constraints on the power spectrum amplitude sigma_8 and
matter density parameter \Omega_m in a flat LCDM model. We test the robustness
of our estimates and find that possible biases in the Y-M relation and the halo
mass function appear larger than the statistical uncertainties from the cluster
sample. Assuming a bias between the X-ray determined mass and the true mass of
20%, motivated by comparison of the observed mass scaling relations to those
from a set of numerical simulations, we find that
sigma_8(Omega_m/0.27)^0.3=0.78+-0.01, with one-dimensional ranges
sigma_8=0.77+-0.02 and Omega_m=0.29+-0.02. The values of the cosmological
parameters are degenerate with the mass bias, and it is found that the larger
values of sigma_8 and Omega_m preferred by the Planck's measurements of the
primary CMB anisotropies can be accommodated by a mass bias of about 45%.
Alternatively, consistency with the primary CMB constraints can be achieved by
inclusion of processes that suppress power on small scales, such as a component
of massive neutrinos. We place our results in the context of other
determinations of cosmological parameters, and discuss issues that need to be
resolved in order to make progress in this field.
We present constraints on cosmological parameters using number counts as a
function of redshift for a sub-sample of 189 galaxy clusters from the Planck SZ
(PSZ) catalogue. The PSZ is selected through the signature of the
Sunyaev--Zeldovich (SZ) effect, and the sub-sample used here has a
signal-to-noise threshold of seven, with each object confirmed as a cluster and
all but one with a redshift estimate. We discuss the calculation of the
expected cluster counts as a function of cosmological parameters, the
completeness of the sample, and the likelihood construction method. Using a
relation between mass M and SZ signal Y based on comparison to X-ray
measurements, we derive constraints on the power spectrum amplitude sigma_8 and
matter density parameter \Omega_m in a flat LCDM model. We test the robustness
of our estimates and find that possible biases in the Y-M relation and the halo
mass function appear larger than the statistical uncertainties from the cluster
sample. Assuming a bias between the X-ray determined mass and the true mass of
20%, motivated by comparison of the observed mass scaling relations to those
from a set of numerical simulations, we find that
sigma_8(Omega_m/0.27)^0.3=0.78+-0.01, with one-dimensional ranges
sigma_8=0.77+-0.02 and Omega_m=0.29+-0.02. The values of the cosmological
parameters are degenerate with the mass bias, and it is found that the larger
values of sigma_8 and Omega_m preferred by the Planck's measurements of the
primary CMB anisotropies can be accommodated by a mass bias of about 45%.
Alternatively, consistency with the primary CMB constraints can be achieved by
inclusion of processes that suppress power on small scales, such as a component
of massive neutrinos. We place our results in the context of other
determinations of cosmological parameters, and discuss issues that need to be
resolved in order to make progress in this field.
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