518 research outputs found
The spin dependence of the Blandford-Znajek effect
The interaction of large scale magnetic fields with the event horizon of
rotating black holes (the Blandford-Znajek [1977] mechanism) forms the basis
for some models of the most relativistic jets. We explore a scenario in which
the central inward "plunging" region of the accretion flow enhances the
trapping of large scale poloidal field on the black hole. The study is carried
out using a fully relativistic treatment in Kerr spacetime, with the focus
being to determine the spin dependence of the Blandford-Znajek effect. We find
that large scale magnetic fields are enhanced on the black hole compared to the
inner accretion flow and that the ease with which this occurs for lower
prograde black hole spin, produces a spin dependence in the Blandford-Znajek
effect that has attractive applications to recent observations. Among these is
the correlation between inferred accretion rate and nuclear jet power observed
by Allen et al. (2006) in X-ray luminous elliptical galaxies. If the black hole
rotation in these elliptical galaxies is in the prograde sense compared with
that of the inner accretion disk, we show that both the absolute value and the
uniformity of the implied jet-production efficiency can be explained by the
flux-trapping model. The basic scenario that emerges from this study is that a
range of intermediate values of black hole spins could be powering these AGN.
We also suggest that the jets in the most energetic radio-galaxies may be
powered by accretion onto {\it retrograde} rapidly-rotating black holes.Comment: ApJ accepte
A reflection origin for the soft and hard X-ray excess of Ark 120
Over the last few years several models have been proposed to interpret the
widespread soft excess observed in the X-ray spectra of type 1 active galactic
nuclei (AGN). In particular, reflection from the photoionized accretion disc
blurred by relativistic effects has proven to be successful in reproducing both
the spectral shape and the variability pattern of many sources. As a further
test to this scenario we present the analysis of a recent ~100 ks long Suzaku
observation of Arakelian 120, a prototypical 'bare' Seyfert 1 galaxy in which
no complex absorption system is expected to mimic a soft excess or mask the
intrinsic properties of this key component. We show that a reflection model
allowing for both warm/blurred and cold/distant reprocessing provides a
self-consistent and convincing interpretation of the broadband X-ray emission
of Ark 120, also characterized by a structured iron feature and a high-energy
hump. Although warm absorbers, winds/outflows and multiple Comptonizing regions
may play significant roles in sources with more spectral complexity, this case
study adds evidence to the presence of blurred disc reflection as a basic
component of the X-ray spectra of type 1 AGN.Comment: 11 pages, 6 figures, accepted for publication in MNRA
Re-examining the XMM-Newton Spectrum of the Black Hole Candidate XTE J1652-453
The XMM-Newton spectrum of the black hole candidate XTE J1652-453 shows a
broad and strong Fe K-alpha emission line, generally believed to originate from
reflection of the inner accretion disc. These data have been analysed by
Hiemstra et al. (2011) using a variety of phenomenological models. We
re-examine the spectrum with a self-consistent relativistic reflection model. A
narrow absorption line near 7.2 keV may be present, which if real is likely the
Fe XXVI absorption line arising from highly ionised, rapidly outflowing disc
wind. The blue shift of this feature corresponds to a velocity of about 11100
km/s, which is much larger than the typical values seen in stellar-mass black
holes. Given that we also find the source to have a low inclination (i < 32
degrees; close to face-on), we would therefore be seeing through the very base
of outflow. This could be a possible explanation for the unusually high
velocity. We use a reflection model combined with a relativistic convolution
kernel which allows for both prograde and retrograde black hole spin, and treat
the potential absorption feature with a physical model for a photo-ionised
plasma. In this manner, assuming the disc is not truncated, we could only
constrain the spin of the black hole in XTE J1652-453 to be less than ~ 0.5
Jc/GM^{2} at the 90% confidence limit.Comment: 8 pages, 8 figures, accepted for publication in MNRA
Where is the Radiation Edge in Magnetized Black Hole Accretion discs?
General Relativistic (GR) Magnetohydrodynamic (MHD) simulations of black hole
accretion find significant magnetic stresses near and inside the innermost
stable circular orbit (ISCO), suggesting that such flows could radiate in a
manner noticeably different from the prediction of the standard model, which
assumes that there are no stresses in that region. We provide estimates of how
phenomenologically interesting parameters like the ``radiation edge", the
innermost ring of the disc from which substantial thermal radiation escapes to
infinity, may be altered by stresses near the ISCO. These estimates are based
on data from a large number of three-dimensional GRMHD simulations combined
with GR ray-tracing. For slowly spinning black holes (), the radiation
edge lies well inside where the standard model predicts, particularly when the
system is viewed at high inclination. For more rapidly spinning black holes,
the contrast is smaller. At fixed total luminosity, the characteristic
temperature of the accretion flow increases between a factor of over
that predicted by the standard model, whilst at fixed mass accretion rate,
there is a corresponding enhancement of the accretion luminosity which may be
anywhere from tens of percent to order unity. When all these considerations are
combined, we find that, for fixed black hole mass, luminosity, and inclination
angle, our uncertainty in the characteristic temperature of the radiation
reaching distant observers due to uncertainty in dissipation profile (around a
factor of 3) is {\it greater} than the uncertainty due to a complete lack of
knowledge of the black hole's spin (around a factor of 2) and furthermore that
spin estimates based on the stress-free inner boundary condition provide an
upper limit to .Comment: 20 pages, 17 figures, accepted by MNRAS; major changes to original,
including entirely new sections discussing characteristic temperature of
black hole accretion flows and implications for measurements of black hole
spin, along with substantially expanded conclusio
An XMM-Newton view of the `bare' nucleus of Fairall 9
We present the spectral results from a 130 ks observation, obtained from the
X-ray Multi-Mirror Mission-Newton (XMM-Newton) observatory, of the type I
Seyfert galaxy Fairall 9. An X-ray hardness-ratio analysis of the light-curves,
reveals a `softer-when-brighter' behaviour which is typical for radio-quiet
type I Seyfert galaxies. Moreover, we analyse the high spectral-resolution data
of the reflection grating spectrometer and we did not find any significant
evidence supporting the presence of warm-absorber in the low X-ray energy part
of the source's spectrum. This means that the central nucleus of Fairall 9 is
`clean' and thus its X-ray spectral properties probe directly the physical
conditions of the central engine. The overall X-ray spectrum in the 0.5-10 keV
energy-range, derived from the EPIC data, can be modelled by a relativistically
blurred disc-reflection model. This spectral model yields for Fairall 9 an
intermediate black-hole best-fit spin parameter of
.Comment: Accepted for publication in MNRAS. The paper contains 11 figures and
1 tabl
Effects Of Circum-nuclear Disk Gas Evolution And The Spin Of Central Black Holes
Mass and spin are the only two parameters needed to completely characterize
black holes in General Relativity. However, the interaction between black holes
and their environment is where complexity lies, as the relevant physical
processes occur over a large range of scales. That is particularly relevant in
the case of super-massive black holes (SMBHs), hosted in galaxy centers, and
surrounded by swirling gas and various generations of stars. These compete with
the SMBH for gas consumption and affect both dynamics and thermodynamics of the
gas itself. How the behavior of such fiery environment influence the angular
momentum of the gas accreted onto SMBHs, and, hence, black-hole spins is
uncertain. We explore the interaction between SMBHs and their environment via
first 3D sub-parsec resolution simulations (ranging from 0.1 pc to 1 kpc
scales) that study the evolution of the SMBH spin by including the effects of
star formation, stellar feedback, radiative transfer, and metal pollution
according to the proper stellar yields and lifetimes. This approach is crucial
to investigate the impact of star formation processes and feedback effects on
the angular momentum of the material that could accrete on the central hole. We
find that star formation and feedback mechanisms can locally inject significant
amounts of entropy in the surrounding medium, and impact on the inflow
inclination angles and Eddington fractions. As a consequence, the resulting
trends show upper-intermediate equilibrium values for the spin parameter, a, of
about 0.6 - 0.9, corresponding to radiative efficiencies \epsilon = 9% - 15%.
These results suggest that star formation feedback taking place in the
circum-nuclear disk during the in-fall cannot induce alone very strong chaotic
trends in the gas flow, quite independently from the different numerical
parameters.Comment: Changes in title and updates in references. Inclusion of additional
runs and parameter studies. Conclusions unaffected. Accepte
Estimates of Black Hole Spin Properties of 55 Sources
Studies of black hole spin and other parameters as a function of redshift
provide information about the physical state and merger and accretion histories
of the systems. One way that black hole spin may be estimated is through
observations of extended radio sources. These sources, powered by outflows from
an AGN, allow the beam power and total outflow energy to be studied. In a broad
class of models, the beam power of the outflow is related to the spin of the
black hole. This relationship is used to estimate black hole spins for 55 radio
sources. The samples studied include 7 FRII quasars and 19 FRII radio galaxies
with redshifts between 0.056 and 1.79, and 29 radio sources associated with CD
galaxies with redshifts between 0.0035 and 0.291. The FRII sources studied have
estimated spin values of between about 0.2 and 1; there is a range of values at
a given redshift, and the values tend to increase with increasing redshift.
Results obtained for FRII quasars are very similar to those obtained for FRII
galaxies. A broader range of spin values are obtained for the sample of radio
sources associated with CD galaxies studied. The fraction of the spin energy
extracted per outflow event is estimated and ranges from about 0.03 to 0.5 for
FRII sources and 0.002 to about 1 for radio sources associated with CD
galaxies; the data are consistent with this fraction being independent of
redshift though the uncertainties are large. The results obtained are
consistent with those predicted by numerical simulations that track the merger
and accretion history of AGN, supporting the idea that, for AGN with powerful
large-scale outflows, beam power is directly related to black hole spin.Comment: 13 pages; to appear in MNRA
Constraints on the black hole spin in the quasar SDSS J094533.99+100950.1
The spin of the black hole is an important parameter which may be responsible
for the properties of the inflow and outflow of the material surrounding a
black hole. Broad band IR/optical/UV spectrum of the quasar SDSS
J094533.99+100950.1 is clearly disk-dominated, with the spectrum peaking up in
the observed frequency range. Therefore, disk fitting method usually used for
Galactic black holes can be used in this object to determine the black hole
spin. We develop the numerical code for computing disk properties, including
radius-dependent hardening factor, and we apply the ray-tracing method to
incorporate all general relativity effects in light propagation. We show that
the simple multicolor disk model gives a good fit, without any other component
required, and the disk extends down to the marginally stable orbit. The best
fit accretion rate is 0.13, well below the Eddington limit, and the black hole
spin is moderate, 0.3. The contour error for the fit combined with the
constraints for the black hole mass and the disk inclination gives a constraint
that the spin is lower than 0.8. We discuss the sources of possible systematic
errors in the parameter determinations
Multimessenger astronomy with pulsar timing and X-ray observations of massive black hole binaries
We demonstrate that very massive (>10^8\msun), cosmologically nearby (z<1)
black hole binaries (MBHBs), which are primary targets for ongoing and upcoming
pulsar timing arrays (PTAs), are particularly appealing multimessenger
carriers. According to current models for massive black hole formation and
evolution, the planned Square Kilometer Array (SKA) will collect gravitational
wave signals from thousands of such massive systems, being able to individually
resolve and locate in the sky several of them (maybe up to a hundred). By
employing a standard model for the evolution of MBHBs in circumbinary discs,
with the aid of dedicated numerical simulations, we characterize the gas-binary
interplay, identifying possible electromagnetic signatures of the PTA sources.
We concentrate our investigation on two particularly promising scenarios in the
high energy domain, namely, the detection of X-ray periodic variability and of
double broad K\alpha iron lines. Up to several hundreds of periodic X-ray
sources with a flux >10^-13 erg s^-1 cm^-2 will be in the reach of upcoming
X-ray observatories. Double relativistic K\alpha lines may be observable in a
handful of low redshift (z<0.3) sources by proposed deep X-ray probes, such as
Athena. (Abridged)Comment: 19 pages, 11 figures, submitted to MNRAS, minor revision of the
reference lis
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