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.
Showing posts with label BHspin. Show all posts
Showing posts with label BHspin. Show all posts
Saturday, April 6, 2013
Sunday, March 10, 2013
Black Hole Spin via Continuum Fitting and the Role of Spin in Powering Transient Jets. (arXiv:1303.1583v1 [astro-ph.HE])
Black Hole Spin via Continuum Fitting and the Role of Spin in Powering Transient Jets. (arXiv:1303.1583v1 [astro-ph.HE]):
The spins of ten stellar black holes have been measured using the
continuum-fitting method. These black holes are located in two distinct classes
of X-ray binary systems, one that is persistently X-ray bright and another that
is transient. Both the persistent and transient black holes remain for long
periods in a state where their spectra are dominated by a thermal accretion
disk component. The spin of a black hole of known mass and distance can be
measured by fitting this thermal continuum spectrum to the thin-disk model of
Novikov and Thorne; the key fit parameter is the radius of the inner edge of
the black hole's accretion disk. Strong observational and theoretical evidence
links the inner-disk radius to the radius of the innermost stable circular
orbit, which is trivially related to the dimensionless spin parameter a* (|a*|
\le 1). The ten spins that have so far been measured by this continuum-fitting
method range widely from a* \approx 0 to a* > 0.95. The robustness of the
method is demonstrated by the dozens or hundreds of independent and consistent
measurements of spin that have been obtained for several black holes, and
through careful consideration of many sources of systematic error. Among the
results discussed is a sharp dichotomy between the transient and persistent
black holes; the latter have higher spins and larger masses. Also discussed is
recently discovered evidence in the transient sources for a correlation between
the power of ballistic jets and black hole spin.
The spins of ten stellar black holes have been measured using the
continuum-fitting method. These black holes are located in two distinct classes
of X-ray binary systems, one that is persistently X-ray bright and another that
is transient. Both the persistent and transient black holes remain for long
periods in a state where their spectra are dominated by a thermal accretion
disk component. The spin of a black hole of known mass and distance can be
measured by fitting this thermal continuum spectrum to the thin-disk model of
Novikov and Thorne; the key fit parameter is the radius of the inner edge of
the black hole's accretion disk. Strong observational and theoretical evidence
links the inner-disk radius to the radius of the innermost stable circular
orbit, which is trivially related to the dimensionless spin parameter a* (|a*|
\le 1). The ten spins that have so far been measured by this continuum-fitting
method range widely from a* \approx 0 to a* > 0.95. The robustness of the
method is demonstrated by the dozens or hundreds of independent and consistent
measurements of spin that have been obtained for several black holes, and
through careful consideration of many sources of systematic error. Among the
results discussed is a sharp dichotomy between the transient and persistent
black holes; the latter have higher spins and larger masses. Also discussed is
recently discovered evidence in the transient sources for a correlation between
the power of ballistic jets and black hole spin.
A rapidly spinning supermassive black hole at the centre of NGC 1365. (arXiv:1302.7002v1 [astro-ph.HE])
A rapidly spinning supermassive black hole at the centre of NGC 1365. (arXiv:1302.7002v1 [astro-ph.HE]):
Broad X-ray emission lines from neutral and partially ionized iron observed
in active galaxies have been interpreted as fluorescence produced by the
reflection of hard X-rays off the inner edge of an accretion disk. In this
model, line broadening and distortion result from rapid rotation and
relativistic effects near the black hole, the line shape being sensitive to its
spin. Alternative models in which the distortions result from absorption by
intervening structures provide an equally good description of the data, and
there has been no general agreement on which is correct. Recent claims that the
black hole (2E6 solar masses) at the centre of the galaxy NGC 1365 is rotating
at close to its maximum possible speed rest on the assumption of relativistic
reflection. Here we report X-ray observations of NGC 1365 that reveal the
relativistic disk features through broadened Fe line emission and an associated
Compton scattering excess of 10-30 keV. Using temporal and spectral analyses,
we disentangle continuum changes due to time-variable absorption from
reflection, which we find arises from a region within 2.5 gravitational radii
of the rapidly spinning black hole. Absorption-dominated models that do not
include relativistic disk reflection can be ruled out both statistically and on
physical grounds.
Broad X-ray emission lines from neutral and partially ionized iron observed
in active galaxies have been interpreted as fluorescence produced by the
reflection of hard X-rays off the inner edge of an accretion disk. In this
model, line broadening and distortion result from rapid rotation and
relativistic effects near the black hole, the line shape being sensitive to its
spin. Alternative models in which the distortions result from absorption by
intervening structures provide an equally good description of the data, and
there has been no general agreement on which is correct. Recent claims that the
black hole (2E6 solar masses) at the centre of the galaxy NGC 1365 is rotating
at close to its maximum possible speed rest on the assumption of relativistic
reflection. Here we report X-ray observations of NGC 1365 that reveal the
relativistic disk features through broadened Fe line emission and an associated
Compton scattering excess of 10-30 keV. Using temporal and spectral analyses,
we disentangle continuum changes due to time-variable absorption from
reflection, which we find arises from a region within 2.5 gravitational radii
of the rapidly spinning black hole. Absorption-dominated models that do not
include relativistic disk reflection can be ruled out both statistically and on
physical grounds.
Sunday, February 17, 2013
Does black hole spin play a key role in the FSRQ/BL Lac dichotomy?. (arXiv:1302.3442v1 [astro-ph.HE])
Does black hole spin play a key role in the FSRQ/BL Lac dichotomy?. (arXiv:1302.3442v1 [astro-ph.HE]):
It is believed that jets emerging from blazars (Flat Spectrum Radio Quasars
(FSRQs) and BL Lacs) are almost aligned to the line-of-sight. BL Lacs usually
exhibit lower luminosity and harder power law spectra at gamma-ray energies
than FSRQs. It was argued previously that the difference in accretion rates is
mainly responsible for the large observed luminosity mismatch between them.
However, when intrinsic luminosities are derived by correcting for beaming
effects, this mismatch is significantly reduced. We show that spin plays an
important role to reveal the dichotomy of luminosity distributions between BL
Lacs and FSRQs, suggesting BL Lacs to be low luminous and slow rotators
compared to FSRQs.
It is believed that jets emerging from blazars (Flat Spectrum Radio Quasars
(FSRQs) and BL Lacs) are almost aligned to the line-of-sight. BL Lacs usually
exhibit lower luminosity and harder power law spectra at gamma-ray energies
than FSRQs. It was argued previously that the difference in accretion rates is
mainly responsible for the large observed luminosity mismatch between them.
However, when intrinsic luminosities are derived by correcting for beaming
effects, this mismatch is significantly reduced. We show that spin plays an
important role to reveal the dichotomy of luminosity distributions between BL
Lacs and FSRQs, suggesting BL Lacs to be low luminous and slow rotators
compared to FSRQs.
Observational constraints on the powering mechanism of transient relativistic jets. (arXiv:1301.6771v1 [astro-ph.HE])
Observational constraints on the powering mechanism of transient relativistic jets. (arXiv:1301.6771v1 [astro-ph.HE]):
We revisit the paradigm of the dependency of jet power on black hole spin in
accreting black hole systems. In a previous paper we showed that the luminosity
of compact jets continuously launched due to accretion onto black holes in
X-ray binaries (analogous to those that dominate the kinetic feedback from AGN)
do not appear to correlate with reported black hole spin measurements. It is
therefore unclear whether extraction of the black hole spin energy is the main
driver powering compact jets from accreting black holes. Occasionally, black
hole X-ray binaries produce discrete, transient (ballistic) jets for a brief
time over accretion state changes. Here, we quantify the dependence of the
power of these transient jets (adopting two methods to infer the jet power) on
black hole spin, making use of all the available data in the current
literature, which includes 12 BHs with both measured spin parameters and radio
flares over the state transition. In several sources, regular, well-sampled
radio monitoring has shown that the peak radio flux differs dramatically
depending on the outburst (up to a factor of 1000) whereas the total power
required to energise the flare may only differ by a factor ~< 4 between
outbursts. The peak flux is determined by the total energy in the flare and the
time over which it is radiated (which can vary considerably between outbursts).
Using a Bayesian fitting routine we rule out a statistically significant
positive correlation between transient jet power measured using these methods,
and current estimates of black hole spin. Even when selecting subsamples of the
data that disregard some methods of black hole spin measurement or jet power
measurement, no correlation is found in all cases.
We revisit the paradigm of the dependency of jet power on black hole spin in
accreting black hole systems. In a previous paper we showed that the luminosity
of compact jets continuously launched due to accretion onto black holes in
X-ray binaries (analogous to those that dominate the kinetic feedback from AGN)
do not appear to correlate with reported black hole spin measurements. It is
therefore unclear whether extraction of the black hole spin energy is the main
driver powering compact jets from accreting black holes. Occasionally, black
hole X-ray binaries produce discrete, transient (ballistic) jets for a brief
time over accretion state changes. Here, we quantify the dependence of the
power of these transient jets (adopting two methods to infer the jet power) on
black hole spin, making use of all the available data in the current
literature, which includes 12 BHs with both measured spin parameters and radio
flares over the state transition. In several sources, regular, well-sampled
radio monitoring has shown that the peak radio flux differs dramatically
depending on the outburst (up to a factor of 1000) whereas the total power
required to energise the flare may only differ by a factor ~< 4 between
outbursts. The peak flux is determined by the total energy in the flare and the
time over which it is radiated (which can vary considerably between outbursts).
Using a Bayesian fitting routine we rule out a statistically significant
positive correlation between transient jet power measured using these methods,
and current estimates of black hole spin. Even when selecting subsamples of the
data that disregard some methods of black hole spin measurement or jet power
measurement, no correlation is found in all cases.
Measuring Black Hole Spin using X-ray Reflection Spectroscopy. (arXiv:1302.3260v1 [astro-ph.HE])
Measuring Black Hole Spin using X-ray Reflection Spectroscopy. (arXiv:1302.3260v1 [astro-ph.HE]):
I review the current status of X-ray reflection (a.k.a. broad iron line)
based black hole spin measurements. This is a powerful technique that allows us
to measure robust black hole spins across the mass range, from the stellar-mass
black holes in X-ray binaries to the supermassive black holes in active
galactic nuclei. After describing the basic assumptions of this approach, I lay
out the detailed methodology focusing on "best practices" that have been found
necessary to obtain robust results. Reflecting my own biases, this review is
slanted towards a discussion of supermassive black hole (SMBH) spin in active
galactic nuclei (AGN). Pulling together all of the available XMM-Newton and
Suzaku results from the literature that satisfy objective quality control
criteria, it is clear that a large fraction of SMBHs are rapidly-spinning,
although there are tentative hints of a more slowly spinning population at high
(M>5*10^7Msun) and low (M<2*10^6Msun) mass. I also engage in a brief review of
the spins of stellar-mass black holes in X-ray binaries. In general,
reflection-based and continuum-fitting based spin measures are in agreement,
although there remain two objects (GROJ1655-40 and 4U1543-475) for which that
is not true. I end this review by discussing the exciting frontier of
relativistic reverberation, particularly the discovery of broad iron line
reverberation in XMM-Newton data for the Seyfert galaxies NGC4151, NGC7314 and
MCG-5-23-16. As well as confirming the basic paradigm of relativistic disk
reflection, this detection of reverberation demonstrates that future large-area
X-ray observatories such as LOFT will make tremendous progress in studies of
strong gravity using relativistic reverberation in AGN.
I review the current status of X-ray reflection (a.k.a. broad iron line)
based black hole spin measurements. This is a powerful technique that allows us
to measure robust black hole spins across the mass range, from the stellar-mass
black holes in X-ray binaries to the supermassive black holes in active
galactic nuclei. After describing the basic assumptions of this approach, I lay
out the detailed methodology focusing on "best practices" that have been found
necessary to obtain robust results. Reflecting my own biases, this review is
slanted towards a discussion of supermassive black hole (SMBH) spin in active
galactic nuclei (AGN). Pulling together all of the available XMM-Newton and
Suzaku results from the literature that satisfy objective quality control
criteria, it is clear that a large fraction of SMBHs are rapidly-spinning,
although there are tentative hints of a more slowly spinning population at high
(M>5*10^7Msun) and low (M<2*10^6Msun) mass. I also engage in a brief review of
the spins of stellar-mass black holes in X-ray binaries. In general,
reflection-based and continuum-fitting based spin measures are in agreement,
although there remain two objects (GROJ1655-40 and 4U1543-475) for which that
is not true. I end this review by discussing the exciting frontier of
relativistic reverberation, particularly the discovery of broad iron line
reverberation in XMM-Newton data for the Seyfert galaxies NGC4151, NGC7314 and
MCG-5-23-16. As well as confirming the basic paradigm of relativistic disk
reflection, this detection of reverberation demonstrates that future large-area
X-ray observatories such as LOFT will make tremendous progress in studies of
strong gravity using relativistic reverberation in AGN.
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.
Irradiation of an Accretion Disc by a Jet: General Properties and Implications for Spin Measurements of Black Holes. (arXiv:1301.4922v1 [astro-ph.HE])
Irradiation of an Accretion Disc by a Jet: General Properties and Implications for Spin Measurements of Black Holes. (arXiv:1301.4922v1 [astro-ph.HE]):
X-ray irradiation of the accretion disc leads to strong reflection features,
which are then broadened and distorted by relativistic effects. We present a
detailed, general relativistic approach to model this irradiation for different
geometries of the primary X-ray source. These geometries include the standard
point source on the rotational axis as well as more jet-like sources, which are
radially elongated and accelerating. Incorporating this code in the relline
model for relativistic line emission, the line shape for any configuration can
be predicted. We study how different irradiation geometries affect the
determination of the spin of the black hole. Broad emission lines are produced
only for compact irradiating sources situated close to the black hole. This is
the only case where the black hole spin can be unambiguously determined. In all
other cases the line shape is narrower, which could either be explained by a
low spin or an elongated source. We conclude that for all those cases and
independent of the quality of the data, no unique solution for the spin value
exists.
X-ray irradiation of the accretion disc leads to strong reflection features,
which are then broadened and distorted by relativistic effects. We present a
detailed, general relativistic approach to model this irradiation for different
geometries of the primary X-ray source. These geometries include the standard
point source on the rotational axis as well as more jet-like sources, which are
radially elongated and accelerating. Incorporating this code in the relline
model for relativistic line emission, the line shape for any configuration can
be predicted. We study how different irradiation geometries affect the
determination of the spin of the black hole. Broad emission lines are produced
only for compact irradiating sources situated close to the black hole. This is
the only case where the black hole spin can be unambiguously determined. In all
other cases the line shape is narrower, which could either be explained by a
low spin or an elongated source. We conclude that for all those cases and
independent of the quality of the data, no unique solution for the spin value
exists.
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.
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.
Jet Power and Black Hole Spin: Testing an Empirical Relationship and Using it to Predict the Spins of Six Black Holes. (arXiv:1211.5379v1 [astro-ph.HE])
Jet Power and Black Hole Spin: Testing an Empirical Relationship and Using it to Predict the Spins of Six Black Holes. (arXiv:1211.5379v1 [astro-ph.HE]):
Using 5 GHz radio luminosity at light-curve maximum as a proxy for jet power
and black-hole spin measurements obtained via the continuum-fitting method,
Narayan & McClintock (2012) presented the first direct evidence for a
relationship between jet power and black hole spin for four transient
black-hole binaries. We test and confirm their empirical relationship using a
fifth source, H1743-322, whose spin was recently measured. We show that this
relationship is consistent with Fe-line spin measurements provided that the
black hole spin axis is assumed to be aligned with the binary angular momentum
axis. We also show that, during a major outburst of a black hole transient, the
system reasonably approximates an X-ray standard candle. We further show, using
the standard synchrotron bubble model, that the radio luminosity at light-curve
maximum is a good proxy for jet kinetic energy. Thus, the observed tight
correlation between radio power and black hole spin indicates a strong
underlying link between mechanical jet power and black hole spin. Using the
fitted correlation between radio power and spin for the above five calibration
sources, we predict the spins of six other black holes in X-ray/radio transient
systems with low-mass companions. Remarkably, these predicted spins are all
relatively low, especially when compared to the high measured spins of black
holes in persistent, wind-fed systems with massive companions.
Using 5 GHz radio luminosity at light-curve maximum as a proxy for jet power
and black-hole spin measurements obtained via the continuum-fitting method,
Narayan & McClintock (2012) presented the first direct evidence for a
relationship between jet power and black hole spin for four transient
black-hole binaries. We test and confirm their empirical relationship using a
fifth source, H1743-322, whose spin was recently measured. We show that this
relationship is consistent with Fe-line spin measurements provided that the
black hole spin axis is assumed to be aligned with the binary angular momentum
axis. We also show that, during a major outburst of a black hole transient, the
system reasonably approximates an X-ray standard candle. We further show, using
the standard synchrotron bubble model, that the radio luminosity at light-curve
maximum is a good proxy for jet kinetic energy. Thus, the observed tight
correlation between radio power and black hole spin indicates a strong
underlying link between mechanical jet power and black hole spin. Using the
fitted correlation between radio power and spin for the above five calibration
sources, we predict the spins of six other black holes in X-ray/radio transient
systems with low-mass companions. Remarkably, these predicted spins are all
relatively low, especially when compared to the high measured spins of black
holes in persistent, wind-fed systems with massive companions.
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
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.
Thursday, October 25, 2012
Jet Launching Structure Resolved Near the Supermassive Black Hole in M87. (arXiv:1210.6132v1 [astro-ph.HE])
Jet Launching Structure Resolved Near the Supermassive Black Hole in M87. (arXiv:1210.6132v1 [astro-ph.HE]):
Approximately 10% of active galactic nuclei exhibit relativistic jets, which
are powered by accretion of matter onto super massive black holes. While the
measured width profiles of such jets on large scales agree with theories of
magnetic collimation, predicted structure on accretion disk scales at the jet
launch point has not been detected. We report radio interferometry observations
at 1.3mm wavelength of the elliptical galaxy M87 that spatially resolve the
base of the jet in this source. The derived size of 5.5 +/- 0.4 Schwarzschild
radii is significantly smaller than the innermost edge of a retrograde
accretion disk, suggesting that the M87 jet is powered by an accretion disk in
a prograde orbit around a spinning black hole.
Approximately 10% of active galactic nuclei exhibit relativistic jets, which
are powered by accretion of matter onto super massive black holes. While the
measured width profiles of such jets on large scales agree with theories of
magnetic collimation, predicted structure on accretion disk scales at the jet
launch point has not been detected. We report radio interferometry observations
at 1.3mm wavelength of the elliptical galaxy M87 that spatially resolve the
base of the jet in this source. The derived size of 5.5 +/- 0.4 Schwarzschild
radii is significantly smaller than the innermost edge of a retrograde
accretion disk, suggesting that the M87 jet is powered by an accretion disk in
a prograde orbit around a spinning black hole.
Thursday, October 18, 2012
Suzaku observations of 'bare' active galactic nuclei. (arXiv:1210.4593v1 [astro-ph.HE])
Suzaku observations of 'bare' active galactic nuclei. (arXiv:1210.4593v1 [astro-ph.HE]):
We present a X-ray spectral analysis of a large sample of 25 'bare' active
galactic nuclei, sources with little or no complicating intrinsic absorption,
observed with Suzaku. Our work focuses on studying the potential contribution
from relativistic disc reflection, and examining the implications of this
interpretation for the intrinsic spectral complexities frequently displayed by
AGN in the X-ray bandpass. During the analysis, we take the unique approach of
attempting to simultaneously undertake a systematic analysis of the whole
sample, as well as a detailed treatment of each individual source, and find
that disc reflection has the required flexibility to successfully reproduce the
broadband spectrum observed for all of the sources considered. Where possible,
we use the reflected emission to place constraints on the black hole spin for
this sample of sources. Our analysis suggests a general preference for rapidly
rotating black holes, which if taken at face value is most consistent with the
scenario in which SMBH growth is dominated by prolonged, ordered accretion.
However, there may be observational biases towards AGN with high spin in the
compiled sample, limiting our ability to draw strong conclusions for the
general population at this stage. Finally, contrary to popular belief, our
analysis also implies that the dichotomy between radio loud/radio quiet AGN is
not solely related to black hole spin.
We present a X-ray spectral analysis of a large sample of 25 'bare' active
galactic nuclei, sources with little or no complicating intrinsic absorption,
observed with Suzaku. Our work focuses on studying the potential contribution
from relativistic disc reflection, and examining the implications of this
interpretation for the intrinsic spectral complexities frequently displayed by
AGN in the X-ray bandpass. During the analysis, we take the unique approach of
attempting to simultaneously undertake a systematic analysis of the whole
sample, as well as a detailed treatment of each individual source, and find
that disc reflection has the required flexibility to successfully reproduce the
broadband spectrum observed for all of the sources considered. Where possible,
we use the reflected emission to place constraints on the black hole spin for
this sample of sources. Our analysis suggests a general preference for rapidly
rotating black holes, which if taken at face value is most consistent with the
scenario in which SMBH growth is dominated by prolonged, ordered accretion.
However, there may be observational biases towards AGN with high spin in the
compiled sample, limiting our ability to draw strong conclusions for the
general population at this stage. Finally, contrary to popular belief, our
analysis also implies that the dichotomy between radio loud/radio quiet AGN is
not solely related to black hole spin.
Monday, October 15, 2012
A general relativistic model of accretion disks with coronae surrounding Kerr black holes. (arXiv:1210.2662v1 [astro-ph.HE])
A general relativistic model of accretion disks with coronae surrounding Kerr black holes. (arXiv:1210.2662v1 [astro-ph.HE]):
We calculate the structure of a standard accretion disk with corona
surrounding a massive Kerr black hole in general relativistic frame, in which
the corona is assumed to be heated by the reconnection of the strongly buoyant
magnetic fields generated in the cold accretion disk. The emergent spectra of
the accretion disk-corona systems are calculated by using the relativistic
ray-tracing method. We propose a new method to calculate the emergent
Comptonized spectra from the coronae. The spectra of the disk-corona systems
with a modified $\alpha$-magnetic stress show that both the hard X-ray spectral
index and the hard X-ray bolometric correction factor $L_{\rm bol}/L_{\rm
X,2-10keV}$ increase with the dimensionless mass accretion rate, which are
qualitatively consistent with the observations of active galactic nuclei
(AGNs). The fraction of the power dissipated in the corona decreases with
increasing black hole spin parameter $a$, which leads to lower electron
temperatures of the coronas for rapidly spinning black holes. The X-ray
emission from the coronas surrounding rapidly spinning black holes becomes weak
and soft. The ratio of the X-ray luminosity to the optical/UV luminosity
increases with the viewing angle, while the spectral shape in the X-ray band is
insensitive with the viewing angle. We find that the spectral index in the
infrared waveband depends on the mass accretion rate and the black hole spin
$a$, which deviates from $f_\nu\propto\nu^{1/3}$ expected by the standard thin
disk model.
We calculate the structure of a standard accretion disk with corona
surrounding a massive Kerr black hole in general relativistic frame, in which
the corona is assumed to be heated by the reconnection of the strongly buoyant
magnetic fields generated in the cold accretion disk. The emergent spectra of
the accretion disk-corona systems are calculated by using the relativistic
ray-tracing method. We propose a new method to calculate the emergent
Comptonized spectra from the coronae. The spectra of the disk-corona systems
with a modified $\alpha$-magnetic stress show that both the hard X-ray spectral
index and the hard X-ray bolometric correction factor $L_{\rm bol}/L_{\rm
X,2-10keV}$ increase with the dimensionless mass accretion rate, which are
qualitatively consistent with the observations of active galactic nuclei
(AGNs). The fraction of the power dissipated in the corona decreases with
increasing black hole spin parameter $a$, which leads to lower electron
temperatures of the coronas for rapidly spinning black holes. The X-ray
emission from the coronas surrounding rapidly spinning black holes becomes weak
and soft. The ratio of the X-ray luminosity to the optical/UV luminosity
increases with the viewing angle, while the spectral shape in the X-ray band is
insensitive with the viewing angle. We find that the spectral index in the
infrared waveband depends on the mass accretion rate and the black hole spin
$a$, which deviates from $f_\nu\propto\nu^{1/3}$ expected by the standard thin
disk model.
Friday, October 12, 2012
Constraints on Compton-thick winds from black hole accretion disks: can we see the inner disk?. (arXiv:1210.3029v1 [astro-ph.HE])
Constraints on Compton-thick winds from black hole accretion disks: can we see the inner disk?. (arXiv:1210.3029v1 [astro-ph.HE]):
Strong evidence is emerging that winds can be driven from the central regions
of accretion disks in both active galactic nuclei (AGN) and Galactic black hole
binaries (GBHBs). Direct evidence for highly-ionized, Compton-thin inner-disk
winds comes from observations of blueshifted (v~0.05-0.1c) iron-K X-ray
absorption lines. However, it has been suggested that the inner regions of
black hole accretion disks can also drive Compton-thick winds --- such winds
would enshroud the inner disk, preventing us from seeing direct signatures of
the accretion disk (i.e. the photospheric thermal emission, or the
Doppler/gravitationally broadened iron K-alpha line). Here, we show that,
provided the source is sub-Eddington, the well-established wind driving
mechanisms fail to launch a Compton-thick wind from the inner disk. For the
accelerated region of the wind to be Compton-thick, the momentum carried in the
wind must exceed the available photon momentum by a factor of at least
2/lambda, where lambda is the Eddington ratio of the source, ruling out
radiative acceleration unless the source is very close to the Eddington limit.
Compton-thick winds also carry large mass-fluxes, and a consideration of the
connections between the wind and the disk show this to be incompatible with
magneto-centrifugal driving. Finally, thermal driving of the wind is ruled out
on the basis of the large Compton-radii that typify black hole systems. In the
absence of some new acceleration mechanism, we conclude that the inner regions
of sub-Eddington accretion disks around black holes are indeed naked.
Strong evidence is emerging that winds can be driven from the central regions
of accretion disks in both active galactic nuclei (AGN) and Galactic black hole
binaries (GBHBs). Direct evidence for highly-ionized, Compton-thin inner-disk
winds comes from observations of blueshifted (v~0.05-0.1c) iron-K X-ray
absorption lines. However, it has been suggested that the inner regions of
black hole accretion disks can also drive Compton-thick winds --- such winds
would enshroud the inner disk, preventing us from seeing direct signatures of
the accretion disk (i.e. the photospheric thermal emission, or the
Doppler/gravitationally broadened iron K-alpha line). Here, we show that,
provided the source is sub-Eddington, the well-established wind driving
mechanisms fail to launch a Compton-thick wind from the inner disk. For the
accelerated region of the wind to be Compton-thick, the momentum carried in the
wind must exceed the available photon momentum by a factor of at least
2/lambda, where lambda is the Eddington ratio of the source, ruling out
radiative acceleration unless the source is very close to the Eddington limit.
Compton-thick winds also carry large mass-fluxes, and a consideration of the
connections between the wind and the disk show this to be incompatible with
magneto-centrifugal driving. Finally, thermal driving of the wind is ruled out
on the basis of the large Compton-radii that typify black hole systems. In the
absence of some new acceleration mechanism, we conclude that the inner regions
of sub-Eddington accretion disks around black holes are indeed naked.
Friday, October 5, 2012
A Metric for Testing the Nature of Black Holes. (arXiv:1210.0483v1 [gr-qc])
A Metric for Testing the Nature of Black Holes. (arXiv:1210.0483v1 [gr-qc]):
In general relativity, astrophysical black holes are uniquely described by
the Kerr metric. Observational tests of the Kerr nature of these compact
objects and, hence, of general relativity, require a metric that encompasses a
broader class of black holes as possible alternatives to the usual Kerr black
holes. Several such Kerr-like metrics have been constructed to date, which
depend on a set of free parameters and which reduce smoothly to the Kerr metric
if all deviations vanish. Many of these metrics, however, are valid only for
small values of the spin or small perturbations of the Kerr metric or contain
regions of space where they are unphysical hampering their ability to properly
model the accretions flows of black holes. In this paper, I describe a
Kerr-like black hole metric that is regular everywhere outside of the event
horizon for black holes with arbitrary spins even for large deviations from the
Kerr metric. This metric, therefore, provides an ideal framework for tests of
the nature of black holes with observations of the emission from their
accretion flows, and I give several examples of such tests across the
electromagnetic spectrum with current and near-future instruments.
Note: This paper notes how broadened Fe-K lines could test the black hole 'No-Hair' Theorem, with predictions of line shapes.
In general relativity, astrophysical black holes are uniquely described by
the Kerr metric. Observational tests of the Kerr nature of these compact
objects and, hence, of general relativity, require a metric that encompasses a
broader class of black holes as possible alternatives to the usual Kerr black
holes. Several such Kerr-like metrics have been constructed to date, which
depend on a set of free parameters and which reduce smoothly to the Kerr metric
if all deviations vanish. Many of these metrics, however, are valid only for
small values of the spin or small perturbations of the Kerr metric or contain
regions of space where they are unphysical hampering their ability to properly
model the accretions flows of black holes. In this paper, I describe a
Kerr-like black hole metric that is regular everywhere outside of the event
horizon for black holes with arbitrary spins even for large deviations from the
Kerr metric. This metric, therefore, provides an ideal framework for tests of
the nature of black holes with observations of the emission from their
accretion flows, and I give several examples of such tests across the
electromagnetic spectrum with current and near-future instruments.
Note: This paper notes how broadened Fe-K lines could test the black hole 'No-Hair' Theorem, with predictions of line shapes.
The evolution of active galactic nuclei and their spins. (arXiv:1210.1025v1 [astro-ph.HE])
The evolution of active galactic nuclei and their spins. (arXiv:1210.1025v1 [astro-ph.HE]):
Massive black holes (MBHs) in contrast to stellar mass black holes are
expected to substantially change their properties over their lifetime. MBH
masses increase by several order of magnitude over the Hubble time, as
illustrated by Soltan's argument. MBH spins also must evolve through the series
of accretion and mergers events that grow the MBH's masses. We present a simple
model that traces the joint evolution of MBH masses and spins across cosmic
time. Our model includes MBH-MBH mergers, merger-driven gas accretion,
stochastic fueling of MBHs through molecular cloud capture, and a basic
implementation of accretion of recycled gas. This approach aims at improving
the modeling of low-redshift MBHs and AGN, whose properties can be more easily
estimated observationally. Despite the simplicity of the model, it captures
well the global evolution of the MBH population from z\sim6 to today. Under our
assumptions, we find that the typical spin and radiative efficiency of MBHs
decrease with cosmic time because of the higher incidence of stochastic
processes in gas-rich galaxies and MBH-MBH mergers in gas-poor galaxies. At z=0
the spin distribution in gas-poor galaxies peaks at spins 0.4-0.8, and it is
not strongly mass dependent. MBHs in gas-rich galaxies have a more complex
evolution, with low-mass MBHs at low redshift having low spins, and spins
increasing at larger masses and redshifts. We also find that at z>1 MBH spins
are on average highest in high luminosity AGN, while at lower redshifts these
differences disappear.
Massive black holes (MBHs) in contrast to stellar mass black holes are
expected to substantially change their properties over their lifetime. MBH
masses increase by several order of magnitude over the Hubble time, as
illustrated by Soltan's argument. MBH spins also must evolve through the series
of accretion and mergers events that grow the MBH's masses. We present a simple
model that traces the joint evolution of MBH masses and spins across cosmic
time. Our model includes MBH-MBH mergers, merger-driven gas accretion,
stochastic fueling of MBHs through molecular cloud capture, and a basic
implementation of accretion of recycled gas. This approach aims at improving
the modeling of low-redshift MBHs and AGN, whose properties can be more easily
estimated observationally. Despite the simplicity of the model, it captures
well the global evolution of the MBH population from z\sim6 to today. Under our
assumptions, we find that the typical spin and radiative efficiency of MBHs
decrease with cosmic time because of the higher incidence of stochastic
processes in gas-rich galaxies and MBH-MBH mergers in gas-poor galaxies. At z=0
the spin distribution in gas-poor galaxies peaks at spins 0.4-0.8, and it is
not strongly mass dependent. MBHs in gas-rich galaxies have a more complex
evolution, with low-mass MBHs at low redshift having low spins, and spins
increasing at larger masses and redshifts. We also find that at z>1 MBH spins
are on average highest in high luminosity AGN, while at lower redshifts these
differences disappear.
The Closest Look at 1H0707-495: X-ray Reverberation Lags with 1.3 Ms of Data. (arXiv:1210.1465v1 [astro-ph.HE])
The Closest Look at 1H0707-495: X-ray Reverberation Lags with 1.3 Ms of Data. (arXiv:1210.1465v1 [astro-ph.HE]):
Reverberation lags in AGN were first discovered in the NLS1 galaxy,
1H0707-495. We present a follow-up analysis using 1.3 Ms of data, which allows
for the closest ever look at the reverberation signature of this remarkable
source. We confirm previous findings of a hard lag of ~100 seconds at
frequencies v ~ [0.5 - 4] e-4 Hz, and a soft lag of ~30 seconds at higher
frequencies, v ~ [0.6 - 3] e-3 Hz. These two frequency domains clearly show
different energy dependences in their lag spectra. We also find evidence for a
signature from the broad Fe K line in the high frequency lag spectrum. We use
Monte Carlo simulations to show how the lag and coherence measurements respond
to the addition of Poisson noise and to dilution by other components. With our
better understanding of these effects on the lag, we show that the lag-energy
spectra can be modelled with a scenario in which low frequency hard lags are
produced by a compact corona responding to accretion rate fluctuations
propagating through an optically thick accretion disc, and the high frequency
soft lags are produced by short light-travel delay associated with reflection
of coronal power-law photons off the disc.
Reverberation lags in AGN were first discovered in the NLS1 galaxy,
1H0707-495. We present a follow-up analysis using 1.3 Ms of data, which allows
for the closest ever look at the reverberation signature of this remarkable
source. We confirm previous findings of a hard lag of ~100 seconds at
frequencies v ~ [0.5 - 4] e-4 Hz, and a soft lag of ~30 seconds at higher
frequencies, v ~ [0.6 - 3] e-3 Hz. These two frequency domains clearly show
different energy dependences in their lag spectra. We also find evidence for a
signature from the broad Fe K line in the high frequency lag spectrum. We use
Monte Carlo simulations to show how the lag and coherence measurements respond
to the addition of Poisson noise and to dilution by other components. With our
better understanding of these effects on the lag, we show that the lag-energy
spectra can be modelled with a scenario in which low frequency hard lags are
produced by a compact corona responding to accretion rate fluctuations
propagating through an optically thick accretion disc, and the high frequency
soft lags are produced by short light-travel delay associated with reflection
of coronal power-law photons off the disc.
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