Testing the rotating hot spot model using X-ray burst oscillations from 4U 1636-536. (arXiv:1303.0248v1 [astro-ph.HE]):
Precise and accurate measurements of neutron star masses and radii would
provide valuable information about the still uncertain properties of cold
matter at supranuclear densities. One promising approach to making such
measurements involves analysis of the X-ray flux oscillations often seen during
thermonuclear (type 1) X-ray bursts. These oscillations are almost certainly
produced by emission from hotter regions on the stellar surface modulated by
the rotation of the star. One consequence of the rotation is that the
oscillation should appear earlier at higher photon energies than at lower
energies. Ford (1999) found compelling evidence for such a hard lead in the
tail oscillations of one type 1 burst from Aql X-1. We have therefore analyzed
individually the oscillations observed in the tails of the four type 1 bursts
from 4U 1636-536 that, when averaged, provided the strongest evidence for a
soft lead in the analysis by Muno et al. (2003). We have also analyzed the
oscillation observed during the superburst from this star. We find that the
data from these five bursts, treated both individually and jointly, are fully
consistent with a rotating hot spot model. Unfortunately, the uncertainties in
these data are too large to provide interesting constraints on the mass and
radius of this star.
Showing posts with label Accretion. Show all posts
Showing posts with label Accretion. Show all posts
Sunday, March 10, 2013
Sunday, February 17, 2013
A Monte Carlo Code for Relativistic Radiation Transport Around Kerr Black Holes. (arXiv:1302.3214v1 [astro-ph.HE])
A Monte Carlo Code for Relativistic Radiation Transport Around Kerr Black Holes. (arXiv:1302.3214v1 [astro-ph.HE]):
We present a new code for radiation transport around Kerr black holes,
including arbitrary emission and absorption terms, as well as electron
scattering and polarization. The code is particularly useful for analyzing
accretion flows made up of optically thick disks and optically thin coronae. We
give a detailed description of the methods employed in the code, and also
present results from a number of numerical tests to assess its accuracy and
convergence.
We present a new code for radiation transport around Kerr black holes,
including arbitrary emission and absorption terms, as well as electron
scattering and polarization. The code is particularly useful for analyzing
accretion flows made up of optically thick disks and optically thin coronae. We
give a detailed description of the methods employed in the code, and also
present results from a number of numerical tests to assess its accuracy and
convergence.
X-ray diagnostics of chemical composition of the accretion disk and donor star in ultra-compact X-ray binaries. (arXiv:1302.2070v1 [astro-ph.HE])
X-ray diagnostics of chemical composition of the accretion disk and donor star in ultra-compact X-ray binaries. (arXiv:1302.2070v1 [astro-ph.HE]):
Non-solar composition of the donor star in ultra-compact X-ray binaries may
have a pronounced effect on the fluorescent lines appearing in their spectra
due to reprocessing of primary radiation by the accretion disk and the white
dwarf surface. We show that the most dramatic and easily observable consequence
of the anomalous C/O abundance, is the significant, by more than an order of
magnitude, attenuation of the Ka line of iron. It is caused by screening of the
presence of iron by oxygen - in the C/O dominated material the main interaction
process for a E ~ 7keV photon is absorption by oxygen rather than by iron,
contrary to the solar composition case. Ionization of oxygen at high mass
accretion rates adds a luminosity dependence to this behavior - the iron line
is significantly suppressed only at low luminosity, log(LX) less than 37-37.5,
and should recover its nominal strength at higher luminosity. The increase of
the EW of the Ka lines of carbon and oxygen, on the other hand, saturates at
rather moderate values. Screening by He is less important, due to its low
ionization threshold and because in the accretion disk it is mostly ionized.
Consequently, in the case of the He-rich donor, the iron line strength remains
close to its nominal value, determined by the iron abundance in the accretion
disk. This opens the possibility of constraining the nature of donor stars in
UCXBs by means of X-ray spectroscopy with moderate energy resolution.
Non-solar composition of the donor star in ultra-compact X-ray binaries may
have a pronounced effect on the fluorescent lines appearing in their spectra
due to reprocessing of primary radiation by the accretion disk and the white
dwarf surface. We show that the most dramatic and easily observable consequence
of the anomalous C/O abundance, is the significant, by more than an order of
magnitude, attenuation of the Ka line of iron. It is caused by screening of the
presence of iron by oxygen - in the C/O dominated material the main interaction
process for a E ~ 7keV photon is absorption by oxygen rather than by iron,
contrary to the solar composition case. Ionization of oxygen at high mass
accretion rates adds a luminosity dependence to this behavior - the iron line
is significantly suppressed only at low luminosity, log(LX) less than 37-37.5,
and should recover its nominal strength at higher luminosity. The increase of
the EW of the Ka lines of carbon and oxygen, on the other hand, saturates at
rather moderate values. Screening by He is less important, due to its low
ionization threshold and because in the accretion disk it is mostly ionized.
Consequently, in the case of the He-rich donor, the iron line strength remains
close to its nominal value, determined by the iron abundance in the accretion
disk. This opens the possibility of constraining the nature of donor stars in
UCXBs by means of X-ray spectroscopy with moderate energy resolution.
Monday, February 11, 2013
Nuclear Activity is more prevalent in Star-Forming Galaxies. (arXiv:1302.1202v1 [astro-ph.CO])
Nuclear Activity is more prevalent in Star-Forming Galaxies. (arXiv:1302.1202v1 [astro-ph.CO]):
We explore the question of whether low and moderate luminosity Active
Galactic Nuclei (AGNs) are preferentially found in galaxies that are undergoing
a transition from active star formation to quiescence. This notion has been
suggested by studies of the UV-to-optical colors of AGN hosts, which find them
to be common among galaxies in the so-called "Green Valley", a region of galaxy
color space believed to be composed mostly of galaxies undergoing
star-formation quenching. Combining the deepest current X-ray and Herschel.
PACS far-infrared (FIR) observations of the two Chandra Deep Fields (CDFs) with
redshifts, stellar masses and rest-frame photometry derived from the extensive
and uniform multi-wavelength data in these fields, we compare the rest-frame
U-V color distributions and SFR distributions of AGNs and carefully constructed
samples of inactive control galaxies. The UV-to-optical colors of AGNs are
consistent with equally massive inactive galaxies at redshifts out to z~2, but
we show that such colors are poor tracers of star formation. While the FIR
distributions of both star-forming AGNs and star-forming inactive galaxies are
statistically similar, we show that AGNs are preferentially found in
star-forming host galaxies, or, in other words, AGNs are less likely to be
found in weakly star-forming or quenched galaxies. We postulate that, among
X-ray selected AGNs of low and moderate accretion luminosities, the supply of
cold gas primarily determines the accretion rate distribution of the nuclear
black holes.
We explore the question of whether low and moderate luminosity Active
Galactic Nuclei (AGNs) are preferentially found in galaxies that are undergoing
a transition from active star formation to quiescence. This notion has been
suggested by studies of the UV-to-optical colors of AGN hosts, which find them
to be common among galaxies in the so-called "Green Valley", a region of galaxy
color space believed to be composed mostly of galaxies undergoing
star-formation quenching. Combining the deepest current X-ray and Herschel.
PACS far-infrared (FIR) observations of the two Chandra Deep Fields (CDFs) with
redshifts, stellar masses and rest-frame photometry derived from the extensive
and uniform multi-wavelength data in these fields, we compare the rest-frame
U-V color distributions and SFR distributions of AGNs and carefully constructed
samples of inactive control galaxies. The UV-to-optical colors of AGNs are
consistent with equally massive inactive galaxies at redshifts out to z~2, but
we show that such colors are poor tracers of star formation. While the FIR
distributions of both star-forming AGNs and star-forming inactive galaxies are
statistically similar, we show that AGNs are preferentially found in
star-forming host galaxies, or, in other words, AGNs are less likely to be
found in weakly star-forming or quenched galaxies. We postulate that, among
X-ray selected AGNs of low and moderate accretion luminosities, the supply of
cold gas primarily determines the accretion rate distribution of the nuclear
black holes.
The Evolution of Active Galactic Nuclei in Warm Dark Matter Cosmology. (arXiv:1302.2000v1 [astro-ph.CO])
The Evolution of Active Galactic Nuclei in Warm Dark Matter Cosmology. (arXiv:1302.2000v1 [astro-ph.CO]):
Recent measurements of the abundance of AGN with low-luminosities (L_X< 10^44
erg/s in the 2-10 keV energy band) at high redshifts z>4 provide a serious
challenge for Cold Dark Matter (CDM) models based on interaction-driven fueling
of AGN. Using a semi-analytic model of galaxy formation we investigate how such
observations fit in a Warm Dark Matter (WDM) scenario of galaxy formation, and
compare the results with those obtained in the standard CDM scenario with
different efficiencies for the stellar feedback. Taking on our previous
exploration of galaxy formation in WDM cosmology, we assume as a reference case
a spectrum which is suppressed - compared to the standard CDM case - below a
cut-off scale ~ 0.2$ Mpc corresponding (for thermal relic WDM particles) to a
mass m_X=0.75 keV. We run our fiducial semi-analytic model with such a WDM
spectrum to derive AGN luminosity functions from z~6 to the present over a wide
range of luminosities (10^43< L_X/erg/s < 10^46 in the 2-10 keV X-ray band), to
compare with recent observations and with the results in the CDM case. When
compared with the standard CDM case, the luminosity distributions we obtain
assuming a WDM spectrum are characterized by a similar behaviour at low
redshift, and by a flatter slope at faint magnitudes for z>3, which provide an
excellent fit to present observations. We discuss how such a result compares
with CDM models with maximized feedback efficiency, and how future deep AGN
surveys will allow for a better discrimination between feedback and
cosmological effects on the evolution of AGN in interaction-driven models for
AGN fueling.
Recent measurements of the abundance of AGN with low-luminosities (L_X< 10^44
erg/s in the 2-10 keV energy band) at high redshifts z>4 provide a serious
challenge for Cold Dark Matter (CDM) models based on interaction-driven fueling
of AGN. Using a semi-analytic model of galaxy formation we investigate how such
observations fit in a Warm Dark Matter (WDM) scenario of galaxy formation, and
compare the results with those obtained in the standard CDM scenario with
different efficiencies for the stellar feedback. Taking on our previous
exploration of galaxy formation in WDM cosmology, we assume as a reference case
a spectrum which is suppressed - compared to the standard CDM case - below a
cut-off scale ~ 0.2$ Mpc corresponding (for thermal relic WDM particles) to a
mass m_X=0.75 keV. We run our fiducial semi-analytic model with such a WDM
spectrum to derive AGN luminosity functions from z~6 to the present over a wide
range of luminosities (10^43< L_X/erg/s < 10^46 in the 2-10 keV X-ray band), to
compare with recent observations and with the results in the CDM case. When
compared with the standard CDM case, the luminosity distributions we obtain
assuming a WDM spectrum are characterized by a similar behaviour at low
redshift, and by a flatter slope at faint magnitudes for z>3, which provide an
excellent fit to present observations. We discuss how such a result compares
with CDM models with maximized feedback efficiency, and how future deep AGN
surveys will allow for a better discrimination between feedback and
cosmological effects on the evolution of AGN in interaction-driven models for
AGN fueling.
Tuesday, February 5, 2013
XMM-Newton/RGS Detection of the Missing Interstellar OVII Kalpha Absorption Line in the Spectrum of Cyg X-2. (arXiv:1302.0076v1 [astro-ph.HE])
XMM-Newton/RGS Detection of the Missing Interstellar OVII Kalpha Absorption Line in the Spectrum of Cyg X-2. (arXiv:1302.0076v1 [astro-ph.HE]):
The hot interstellar medium is an important part of the Galactic ecosystem
and can be effectively characterized through X-ray absorption line
spectroscopy. However, in a study of the hot medium using the accreting neutron
star X-ray binary, Cyg X-2, as a background light source, a mystery came about
when the putatively strong OVII Kalpha line was not detected in Chandra grating
observations, while other normally weaker lines such as OVII Kbeta as well as
OVI and OVIII Kalpha are clearly present (Yao et al. 2009). We have
investigated the grating spectra of Cyg X-2 from 10 XMM-Newton observations, in
search of the missing line. We detect it consistently in nine of these
observations, but the line is absent in the remaining one observation or is
inconsistent with the detection in others at a 4sigma confidence level. This
absence of the line resembles that seen in the Chandra observations. Similarly,
the OVI Kalpha line is found to disappear occasionally, but not in concert with
the variation of the OVII Kalpha line. All these variations are most likely due
to the presence of changing OVII and OVI Kalpha emission lines of Cyg X-2,
which are blurred together with the absorption ones in the X-ray spectra. A
re-examination of the Chandra grating data indeed shows evidence for a narrow
emission line slightly off the OVI Kalpha absorption line. We further show that
narrow NV emission lines with varying centroids and fluxes are present in
far-UV spectra from the Cosmic Origins Spectrograph aboard the Hubble Space
Telescope. These results provide new constraints on the accretion around the
neutron star and on the X-ray-heating of the stellar companion. The
understanding of these physical processes is also important to the fidelity of
using such local X-ray binaries for interstellar absorption line spectroscopy.
The hot interstellar medium is an important part of the Galactic ecosystem
and can be effectively characterized through X-ray absorption line
spectroscopy. However, in a study of the hot medium using the accreting neutron
star X-ray binary, Cyg X-2, as a background light source, a mystery came about
when the putatively strong OVII Kalpha line was not detected in Chandra grating
observations, while other normally weaker lines such as OVII Kbeta as well as
OVI and OVIII Kalpha are clearly present (Yao et al. 2009). We have
investigated the grating spectra of Cyg X-2 from 10 XMM-Newton observations, in
search of the missing line. We detect it consistently in nine of these
observations, but the line is absent in the remaining one observation or is
inconsistent with the detection in others at a 4sigma confidence level. This
absence of the line resembles that seen in the Chandra observations. Similarly,
the OVI Kalpha line is found to disappear occasionally, but not in concert with
the variation of the OVII Kalpha line. All these variations are most likely due
to the presence of changing OVII and OVI Kalpha emission lines of Cyg X-2,
which are blurred together with the absorption ones in the X-ray spectra. A
re-examination of the Chandra grating data indeed shows evidence for a narrow
emission line slightly off the OVI Kalpha absorption line. We further show that
narrow NV emission lines with varying centroids and fluxes are present in
far-UV spectra from the Cosmic Origins Spectrograph aboard the Hubble Space
Telescope. These results provide new constraints on the accretion around the
neutron star and on the X-ray-heating of the stellar companion. The
understanding of these physical processes is also important to the fidelity of
using such local X-ray binaries for interstellar absorption line spectroscopy.
Thursday, January 31, 2013
Ultra-fast outflows (aka UFOs) from AGNs and QSOs. (arXiv:1301.7199v1 [astro-ph.HE])
Ultra-fast outflows (aka UFOs) from AGNs and QSOs. (arXiv:1301.7199v1 [astro-ph.HE]):
During the last decade, strong observational evidence has been accumulated
for the existence of massive, high velocity winds/outflows (aka Ultra Fast
Outflows, UFOs) in nearby AGNs and in more distant quasars. Here we briefly
review some of the most recent developments in this field and discuss the
relevance of UFOs for both understanding the physics of accretion disk winds in
AGNs, and for quantifying the global amount of AGN feedback on the surrounding
medium.
During the last decade, strong observational evidence has been accumulated
for the existence of massive, high velocity winds/outflows (aka Ultra Fast
Outflows, UFOs) in nearby AGNs and in more distant quasars. Here we briefly
review some of the most recent developments in this field and discuss the
relevance of UFOs for both understanding the physics of accretion disk winds in
AGNs, and for quantifying the global amount of AGN feedback on the surrounding
medium.
Wednesday, January 23, 2013
Super-spinning compact objects generated by thick accretion disks. (arXiv:1212.5848v1 [gr-qc])
Super-spinning compact objects generated by thick accretion disks. (arXiv:1212.5848v1 [gr-qc]):
If astrophysical black hole candidates are the Kerr black holes predicted by
General Relativity, the value of their spin parameter must be subject to the
{\it theoretical bound} $|a_*| \le 1$. In this work, we consider the
possibility that these objects are either non-Kerr black holes in an
alternative theory of gravity or exotic compact objects in General Relativity.
Such a possibility is not in contradiction with current data and it can be
tested with future observational facilities. We study the accretion process
when their accretion disk is geometrically thick with a simple version of the
Polish doughnut model. The picture of the accretion process may be
qualitatively different from the one around a Kerr black hole. The inner edge
of the accretion disk may not have the typical cusp on the equatorial plane any
more, but there may be two cusps, respectively above and below the equatorial
plane. We discuss the evolution of the spin parameter as a consequence of the
accretion process and we estimate the maximum value of the spin parameter of
these objects as a function of their deformation. Lastly, we compare our
results with the current estimates of the mean radiative efficiency of AGNs. We
find the observational bound $|a_*| \lesssim 1.3$ for the spin parameter of the
super-massive black hole candidates at the centers of galaxies, which we argue
to be almost independent of the exact nature of these objects.
If astrophysical black hole candidates are the Kerr black holes predicted by
General Relativity, the value of their spin parameter must be subject to the
{\it theoretical bound} $|a_*| \le 1$. In this work, we consider the
possibility that these objects are either non-Kerr black holes in an
alternative theory of gravity or exotic compact objects in General Relativity.
Such a possibility is not in contradiction with current data and it can be
tested with future observational facilities. We study the accretion process
when their accretion disk is geometrically thick with a simple version of the
Polish doughnut model. The picture of the accretion process may be
qualitatively different from the one around a Kerr black hole. The inner edge
of the accretion disk may not have the typical cusp on the equatorial plane any
more, but there may be two cusps, respectively above and below the equatorial
plane. We discuss the evolution of the spin parameter as a consequence of the
accretion process and we estimate the maximum value of the spin parameter of
these objects as a function of their deformation. Lastly, we compare our
results with the current estimates of the mean radiative efficiency of AGNs. We
find the observational bound $|a_*| \lesssim 1.3$ for the spin parameter of the
super-massive black hole candidates at the centers of galaxies, which we argue
to be almost independent of the exact nature of these objects.
The relationship between X-ray emission and accretion in X-ray selected AGNs. (arXiv:1212.5693v1 [astro-ph.HE])
The relationship between X-ray emission and accretion in X-ray selected AGNs. (arXiv:1212.5693v1 [astro-ph.HE]):
We study the link between the X-ray emission in radio-quiet AGNs and the
accretion rate on the central Supermassive Black-Hole (SMBH) using a
well-defined and statistically complete sample of 70 type1 AGNs extracted from
the XMM-Newton Bright Serendipitous survey (XBS). To this end, we search and
quantify the statistical correlations between the main parameters that
characterize the X-ray emission (i.e. the X-ray spectral slope and the X-ray
loudness), and the accretion rate, both absolute and relative to the Eddington
limit (Eddington ratio). Here, we summarize and discuss the main statistical
correlations found and their possible implications on current disk-corona
models.
We study the link between the X-ray emission in radio-quiet AGNs and the
accretion rate on the central Supermassive Black-Hole (SMBH) using a
well-defined and statistically complete sample of 70 type1 AGNs extracted from
the XMM-Newton Bright Serendipitous survey (XBS). To this end, we search and
quantify the statistical correlations between the main parameters that
characterize the X-ray emission (i.e. the X-ray spectral slope and the X-ray
loudness), and the accretion rate, both absolute and relative to the Eddington
limit (Eddington ratio). Here, we summarize and discuss the main statistical
correlations found and their possible implications on current disk-corona
models.
Super-Eddington accreting massive black holes as long-lived cosmological standards. (arXiv:1301.4225v1 [astro-ph.CO])
Super-Eddington accreting massive black holes as long-lived cosmological standards. (arXiv:1301.4225v1 [astro-ph.CO]):
Super-Eddington accreting massive black holes (SEAMBHs) reach saturated
luminosities above a certain accretion rate due to photon trapping and
advection in slim accretion disks. We show that these SEAMBHs could provide a
new tool for estimating cosmological distances if they are properly identified
by hard X-ray observations, in particular by the slope of their 2-10 keV
continuum. To verify this idea we obtained black hole mass estimates and X-ray
data for a sample of 60 narrow line Seyfert 1 galaxies that we consider to be
the most promising SEAMBH candidates. We demonstrate that the distances derived
by the new method for the objects in the sample get closer to the standard
luminosity distances as the hard X-ray continuum gets steeper. The results
allow us to analyze the requirements for using the method in future samples of
active black holes and to demonstrate that the expected uncertainty, given
large enough samples, can make them into a useful, new cosmological ruler.
Super-Eddington accreting massive black holes (SEAMBHs) reach saturated
luminosities above a certain accretion rate due to photon trapping and
advection in slim accretion disks. We show that these SEAMBHs could provide a
new tool for estimating cosmological distances if they are properly identified
by hard X-ray observations, in particular by the slope of their 2-10 keV
continuum. To verify this idea we obtained black hole mass estimates and X-ray
data for a sample of 60 narrow line Seyfert 1 galaxies that we consider to be
the most promising SEAMBH candidates. We demonstrate that the distances derived
by the new method for the objects in the sample get closer to the standard
luminosity distances as the hard X-ray continuum gets steeper. The results
allow us to analyze the requirements for using the method in future samples of
active black holes and to demonstrate that the expected uncertainty, given
large enough samples, can make them into a useful, new cosmological ruler.
Sunday, January 20, 2013
Chaotic cold accretion onto black holes. (arXiv:1301.3130v1 [astro-ph.CO])
Chaotic cold accretion onto black holes. (arXiv:1301.3130v1 [astro-ph.CO]):
Using 3D AMR simulations, linking the 50 kpc to the sub-pc scales over the
course of 40 Myr, we systematically relax the classic Bondi assumptions in a
typical galaxy hosting a SMBH. In the realistic scenario, where the hot gas is
cooling, while heated and stirred on large scales, the accretion rate is
boosted up to two orders of magnitude compared with the Bondi prediction. The
cause is the nonlinear growth of thermal instabilities, leading to the
condensation of cold clouds and filaments when t_cool/t_ff < 10. Subsonic
turbulence of just over 100 km/s (M > 0.2) induces the formation of thermal
instabilities, even in the absence of heating, while in the transonic regime
turbulent dissipation inhibits their growth (t_turb/t_cool < 1). When heating
restores global thermodynamic balance, the formation of the multiphase medium
is violent, and the mode of accretion is fully cold and chaotic. The recurrent
collisions, shearing and tidal motions between clouds, filaments and the
central torus cause a significant reduction of angular momentum, boosting
accretion. On sub-pc scales the clouds are channelled to the very centre via a
funnel. A good approximation to the accretion rate is the cooling rate, which
can be used as subgrid model, physically reproducing the boost factor of 100
required by cosmological simulations, while accounting for fluctuations.
Chaotic cold accretion may be common in many systems, such as hot galactic
halos, groups, and clusters, generating high-velocity clouds and strong
variations of the AGN luminosity, jet orientation, and spin. In this mode, the
black hole can quickly react to the state of the entire host galaxy, leading to
efficient self-regulated feedback and the symbiotic Magorrian relation. During
phases of overheating, the hot mode becomes the single channel of accretion
(with a different cuspy temperature profile), though strongly suppressed by
turbulence.
Using 3D AMR simulations, linking the 50 kpc to the sub-pc scales over the
course of 40 Myr, we systematically relax the classic Bondi assumptions in a
typical galaxy hosting a SMBH. In the realistic scenario, where the hot gas is
cooling, while heated and stirred on large scales, the accretion rate is
boosted up to two orders of magnitude compared with the Bondi prediction. The
cause is the nonlinear growth of thermal instabilities, leading to the
condensation of cold clouds and filaments when t_cool/t_ff < 10. Subsonic
turbulence of just over 100 km/s (M > 0.2) induces the formation of thermal
instabilities, even in the absence of heating, while in the transonic regime
turbulent dissipation inhibits their growth (t_turb/t_cool < 1). When heating
restores global thermodynamic balance, the formation of the multiphase medium
is violent, and the mode of accretion is fully cold and chaotic. The recurrent
collisions, shearing and tidal motions between clouds, filaments and the
central torus cause a significant reduction of angular momentum, boosting
accretion. On sub-pc scales the clouds are channelled to the very centre via a
funnel. A good approximation to the accretion rate is the cooling rate, which
can be used as subgrid model, physically reproducing the boost factor of 100
required by cosmological simulations, while accounting for fluctuations.
Chaotic cold accretion may be common in many systems, such as hot galactic
halos, groups, and clusters, generating high-velocity clouds and strong
variations of the AGN luminosity, jet orientation, and spin. In this mode, the
black hole can quickly react to the state of the entire host galaxy, leading to
efficient self-regulated feedback and the symbiotic Magorrian relation. During
phases of overheating, the hot mode becomes the single channel of accretion
(with a different cuspy temperature profile), though strongly suppressed by
turbulence.
Wednesday, December 19, 2012
Constraining the Accretion Flow in Sgr A* by General Relativistic Dynamical and Polarized Radiative Modeling. (arXiv:1212.4149v1 [astro-ph.HE])
Constraining the Accretion Flow in Sgr A* by General Relativistic Dynamical and Polarized Radiative Modeling. (arXiv:1212.4149v1 [astro-ph.HE]):
We briefly summarize the method of simulating Sgr A* polarized sub-mm spectra
from the accretion flow and fitting the observed spectrum. The dynamical flow
model is based on three-dimensional general relativistic magneto hydrodynamic
simulations. Fully self-consistent radiative transfer of polarized
cyclo-synchrotron emission is performed. We compile a mean sub-mm spectrum of
Sgr A* and fit it with the mean simulated spectra. We estimate the ranges of
inclination angle theta=42-75deg, mass accretion rate
Mdot=(1.4-7.0)*10^{-8}Msun/yr, and electron temperature Te=(3-4)*10^{10}K at
6M. We discuss multiple caveats in dynamical modeling, which must be resolved
to make further progress.
We briefly summarize the method of simulating Sgr A* polarized sub-mm spectra
from the accretion flow and fitting the observed spectrum. The dynamical flow
model is based on three-dimensional general relativistic magneto hydrodynamic
simulations. Fully self-consistent radiative transfer of polarized
cyclo-synchrotron emission is performed. We compile a mean sub-mm spectrum of
Sgr A* and fit it with the mean simulated spectra. We estimate the ranges of
inclination angle theta=42-75deg, mass accretion rate
Mdot=(1.4-7.0)*10^{-8}Msun/yr, and electron temperature Te=(3-4)*10^{10}K at
6M. We discuss multiple caveats in dynamical modeling, which must be resolved
to make further progress.
Monday, December 17, 2012
Cosmological Evolution of Supermassive Black Holes: Mass Functions and Spins. (arXiv:1212.2187v1 [astro-ph.CO])
Cosmological Evolution of Supermassive Black Holes: Mass Functions and Spins. (arXiv:1212.2187v1 [astro-ph.CO]):
We derive the mass function of supermassive black holes (SMBHs) over the
redshift range 0<z<2, using the latest deep luminosity and mass functions of
field galaxies. Applying this mass function, combined with the bolometric
luminosity function of active galactic nuclei (AGNs), into the the continuity
equation of SMBH number density, we explicitly obtain the mass-dependent
cosmological evolution of the radiative efficiency for accretion. We suggest
that the accretion history of SMBHs and their spins evolve in two distinct
regimes: an early phase of prolonged accretion, plausibly driven by major
mergers, during which the black hole spins up, then switching to a period of
random, episodic accretion, governed by minor mergers and internal secular
processes, during which the hole spins down. The transition epoch depends on
mass, mirroring other evidence for "cosmic downsizing" in the AGN population.
We derive the mass function of supermassive black holes (SMBHs) over the
redshift range 0<z<2, using the latest deep luminosity and mass functions of
field galaxies. Applying this mass function, combined with the bolometric
luminosity function of active galactic nuclei (AGNs), into the the continuity
equation of SMBH number density, we explicitly obtain the mass-dependent
cosmological evolution of the radiative efficiency for accretion. We suggest
that the accretion history of SMBHs and their spins evolve in two distinct
regimes: an early phase of prolonged accretion, plausibly driven by major
mergers, during which the black hole spins up, then switching to a period of
random, episodic accretion, governed by minor mergers and internal secular
processes, during which the hole spins down. The transition epoch depends on
mass, mirroring other evidence for "cosmic downsizing" in the AGN population.
Wednesday, December 12, 2012
The origin of the lag spectra observed in AGN: Reverberation and the propagation of X-ray source fluctuations. (arXiv:1212.2213v1 [astro-ph.HE])
The origin of the lag spectra observed in AGN: Reverberation and the propagation of X-ray source fluctuations. (arXiv:1212.2213v1 [astro-ph.HE]):
The X-ray emission from active galactic nuclei (AGN) is highly variable.
Measurements of time lags (characterised by lag spectra) between variability in
the light curves in energy bands corresponding to directly observed continuum
emission from the corona around the black hole and to X-rays reflected from the
accretion disc adds a further dimension to studies of the structure and
energetics of these systems. We seek to understand these measurements in terms
of the physical parameters of the X-ray source (its location, extent, etc.)
through the calculation of theoretical lag spectra for a range of source
parameters in general relativistic ray tracing simulations, combined with
knowledge of the observed variability of the X-ray emission from AGN. Due to
the proximity of the emission to the central black hole, Shapiro delays are
important and the effects of general relativity should be considered when
interpreting the lags as the light travel time between the source and
reflector. We show that it is important to consider dilution of the lag by the
contribution of both the primary and reflected spectral components to the
observed energy bands. We find that the observed lag spectrum of the narrow
line Seyfert 1 galaxy 1H 0707-495 implies an X-ray source extending radially
outwards to around 35rg and at a height of around 2rg above the plane of the
accretion disc, consistent with the constraints obtained independently by
considering the emissivity profile of the accretion disc. By investigating the
influence of the propagation of X-ray luminosity fluctuations through the
source region we find it is possible to reproduce the shape of the low
frequency part of the lag spectrum (where the hard 'primary' band lags behind
the soft 'reflected' band) as the effect of luminosity fluctuations originating
in the centre of the X-ray source, close to the black hole, and propagating
outwards.
The X-ray emission from active galactic nuclei (AGN) is highly variable.
Measurements of time lags (characterised by lag spectra) between variability in
the light curves in energy bands corresponding to directly observed continuum
emission from the corona around the black hole and to X-rays reflected from the
accretion disc adds a further dimension to studies of the structure and
energetics of these systems. We seek to understand these measurements in terms
of the physical parameters of the X-ray source (its location, extent, etc.)
through the calculation of theoretical lag spectra for a range of source
parameters in general relativistic ray tracing simulations, combined with
knowledge of the observed variability of the X-ray emission from AGN. Due to
the proximity of the emission to the central black hole, Shapiro delays are
important and the effects of general relativity should be considered when
interpreting the lags as the light travel time between the source and
reflector. We show that it is important to consider dilution of the lag by the
contribution of both the primary and reflected spectral components to the
observed energy bands. We find that the observed lag spectrum of the narrow
line Seyfert 1 galaxy 1H 0707-495 implies an X-ray source extending radially
outwards to around 35rg and at a height of around 2rg above the plane of the
accretion disc, consistent with the constraints obtained independently by
considering the emissivity profile of the accretion disc. By investigating the
influence of the propagation of X-ray luminosity fluctuations through the
source region we find it is possible to reproduce the shape of the low
frequency part of the lag spectrum (where the hard 'primary' band lags behind
the soft 'reflected' band) as the effect of luminosity fluctuations originating
in the centre of the X-ray source, close to the black hole, and propagating
outwards.
Accretion disks around black holes in modified strong gravity. (arXiv:1212.2640v1 [astro-ph.CO])
Accretion disks around black holes in modified strong gravity. (arXiv:1212.2640v1 [astro-ph.CO]):
Stellar-mass black holes offer what is perhaps the best scenario to test
theories of gravity in the strong-field regime. In particular, f(R) theories,
which have been widely discuss in a cosmological context, can be constrained
through realistic astrophysical models of phenomena around black holes. We aim
at building radiative models of thin accretion disks for both Schwarzschild and
Kerr black holes in f(R) gravity. We study particle motion in
f(R)-Schwarzschild and Kerr space-times. We present the spectral energy
distribution of the accretion disk around constant Ricci scalar f(R) black
holes, and constrain specific f(R) prescriptions using features of these
systems. A precise determination of both the spin and accretion rate onto black
holes along with X-ray observations of their thermal spectrum might allow to
identify deviations of gravity from General Relativity. We use recent data on
the high-mass X-ray binary Cygnus X-1 to restrict the values of the parameters
of a class of f(R) models.
Stellar-mass black holes offer what is perhaps the best scenario to test
theories of gravity in the strong-field regime. In particular, f(R) theories,
which have been widely discuss in a cosmological context, can be constrained
through realistic astrophysical models of phenomena around black holes. We aim
at building radiative models of thin accretion disks for both Schwarzschild and
Kerr black holes in f(R) gravity. We study particle motion in
f(R)-Schwarzschild and Kerr space-times. We present the spectral energy
distribution of the accretion disk around constant Ricci scalar f(R) black
holes, and constrain specific f(R) prescriptions using features of these
systems. A precise determination of both the spin and accretion rate onto black
holes along with X-ray observations of their thermal spectrum might allow to
identify deviations of gravity from General Relativity. We use recent data on
the high-mass X-ray binary Cygnus X-1 to restrict the values of the parameters
of a class of f(R) models.
Saturday, December 1, 2012
Alignment of Magnetized Accretion Disks and Relativistic Jets with Spinning Black Holes. (arXiv:1211.3651v1 [astro-ph.CO])
Alignment of Magnetized Accretion Disks and Relativistic Jets with Spinning Black Holes. (arXiv:1211.3651v1 [astro-ph.CO]):
Accreting black holes (BHs) produce intense radiation and powerful
relativistic jets, which are affected by the BH's spin magnitude and direction.
While thin disks might align with the BH spin axis via the Bardeen-Petterson
effect, this does not apply to jet systems with thick disks. We used fully
three-dimensional general relativistic magnetohydrodynamical simulations to
study accreting BHs with various BH spin vectors and disk thicknesses with
magnetic flux reaching saturation. Our simulations reveal a "magneto-spin
alignment" mechanism that causes magnetized disks and jets to align with the BH
spin near BHs and further away to reorient with the outer disk. This mechanism
has implications for the evolution of BH mass and spin, BH feedback on host
galaxies, and resolved BH images for SgrA* and M87.
Accreting black holes (BHs) produce intense radiation and powerful
relativistic jets, which are affected by the BH's spin magnitude and direction.
While thin disks might align with the BH spin axis via the Bardeen-Petterson
effect, this does not apply to jet systems with thick disks. We used fully
three-dimensional general relativistic magnetohydrodynamical simulations to
study accreting BHs with various BH spin vectors and disk thicknesses with
magnetic flux reaching saturation. Our simulations reveal a "magneto-spin
alignment" mechanism that causes magnetized disks and jets to align with the BH
spin near BHs and further away to reorient with the outer disk. This mechanism
has implications for the evolution of BH mass and spin, BH feedback on host
galaxies, and resolved BH images for SgrA* and M87.
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.
Friday, November 9, 2012
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.
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.
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.
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