527 research outputs found
What can we learn about quasars from alpha_OX measurements in galactic black hole binaries?
We draw a comparison between AGN and Galactic black hole binaries using a
uniform description of spectral energy distribution of these two classes of
accreting X-ray sources. We parametrize spectra of GBHs with an alpha_GBH
parameter which we define as a slope of a nominal power law function between 3
and 20 keV. We show that this parameter can be treated as an equivalent of the
X-ray loudness, alpha_OX, used to describe AGN spectra. We do not find linear
correlation between the alpha_GBH and disc flux (similar to that between
alpha_OX and optical/UV luminosity found in AGN). Instead, we show that
alpha_GBH follows a well defined pattern during a GBH outburst. We find that
alpha_GBH tend to cluster around 1, 1.5 and 2, which correspond to a hard, very
high/intermediate and soft spectral state, respectively. We conclude that
majority of the observed Type 1 radio quiet AGN are in a spectral state
corresponding to a very high/intermediate state of GBHs. The same conclusion is
valid for radio loud AGN. We also study variations of the spectral slopes
(alpha_GBH and the X-ray photon index, Gamma) as a function of disc and
Comptonization fluxes. We discuss these dependencies in the context of
correlations of alpha_OX and Gamma with the optical/UV and X-ray 2 keV fluxes
considered for AGN and quasars.Comment: 10 pages, 7 figures, accepted for publication in MNRA
Self-consistent computation of gamma-ray spectra due to proton-proton interactions in black hole systems
In the inner regions of an accretion disk around a black hole, relativistic
protons can interact with ambient matter to produce electrons, positrons and
-rays. The resultant steady state electron and positron particle
distributions are self-consistently computed taking into account Coulomb and
Compton cooling, pair production (due to annihilation)
and pair annihilation. While earlier works used the diffusion approximation to
obtain the particle distributions, here we solve a more general
integro-differential equation that correctly takes into account the large
change in particle energy that occur when the leptons Compton scatter off hard
X-rays. Thus this formalism can also be applied to the hard state of black hole
systems, where the dominant ambient photons are hard X-rays. The corresponding
photon energy spectrum is calculated and compared with broadband data of black
hole binaries in different spectral states. The results indicate that the
-ray spectra ( MeV) of both the soft and hard spectral states
and the entire hard X-ray/-ray spectrum of the ultra-soft state, could
be due to interactions. These results are consistent with the hypothesis
that there always exists in these systems a -ray spectral component due
to interactions which can contribute between 0.5 to 10% of the total
bolometric luminosty. The model predicts that {\it GLAST} would be able to
detect black hole binaries and provide evidence for the presence of non-thermal
protons which in turn would give insight into the energy dissipation process
and jet formation in these systems.Comment: Accepted for publication in MNRA
Spectral variability in Cygnus X-3
We model the broad-band X-ray spectrum of Cyg X-3 in all states displayed by
this source as observed by the Rossi X-ray Timing Explorer. From our models, we
derive for the first time unabsorbed spectral shapes and luminosities for the
full range of spectral states. We interpret the unabsorbed spectra in terms of
Comptonization by a hybrid electron distribution and strong Compton reflection.
We study the spectral evolution and compare with other black hole as well as
neutron star sources. We show that a neutron star accretor is not consistent
with the spectral evolution as a function of Ledd and especially not with the
transition to a hard state. Our results point to the compact object in Cyg X-3
being a massive, ~30 Msun black hole.Comment: 14 pages, 9 figures, accepted for publication in MNRA
Compton scattering as the explanation of the peculiar X-ray properties of Cyg X-3
We consider implications of a possible presence of a Thomson-thick,
low-temperature, plasma cloud surrounding the compact object in the binary
system Cyg X-3. The presence of such a cloud was earlier inferred from the
energy-independent orbital modulation of the X-ray flux and the lack of high
frequencies in its power spectra. Here, we study the effect of Compton
scattering by the cloud on the X-ray energy and power spectra, concentrating on
the hard spectral state. The process reduces the energy of the high-energy
break/cut-off in the energy spectra, which allows us to determine the Thomson
optical depth. This, together with the observed cut-off in the power spectrum,
determines the size of the plasma to be 2x10^9 cm. At this size, the cloud will
be in thermal equilibrium in the photon field of the X-ray source, which yields
the cloud temperature of 3 keV, which refines the determination of the Thomson
optical depth to 7. At these parameters, thermal bremsstrahlung emission of the
cloud becomes important as well. The physical origin of the cloud is likely to
be collision of the very strong stellar wind of the companion Wolf-Rayet star
with a small accretion disc formed by the wind accretion. Our model thus
explains the peculiar X-ray energy and power spectra of Cyg X-3.Comment: MNRAS, the version as printed, the title and abstract change
An absorption origin for the soft excess in Seyfert 1 active galactic nuclei
(abridged) The soft excess seen in the X-ray spectra of many high mass
accretion rate AGN can be well modelled by reflection from a partially ionised
accretion disc. However, the derived parameters are often extreme, both in
terms of the underlying spacetime and the reflection geometry, and these models
require that the disc is far from hydrostatic equilibrium. An alternative model
uses similarly partially ionised, velocity smeared material but seen in
absorption, though again the derived velocities are extreme, requiring magnetic
driving (in the jet?) rather than a simple line driven disc wind. We find that
while both models give comparably good fits to XMM--Newton data, we favour the
absorption model as, unlike reflection, all the derived spectral indices are
soft. This is as expected by analogy with the correspondingly high mass
accretion rate stellar mass black holes. Furthermore, these X-ray spectra are
consistent with a one--to--one mapping between AGN type and spectral state,
with NLS1's having softer spectra corresponding to the very high state, while
the broad line AGN have Gamma~2 as expected for the high/soft state. We also
use the simultaneous OM data to derive the ratio of disc to total accretion
power which is another tracer of spectral state in X-ray binaries. This does
not always show that the disc in NLS1's contributes less than 80 per cent of
the total power, as expected for a very high state. We suggest that this is an
artifact of the standard disc models used to determine the disc luminosity in
our fits. The disc seen in the very high state of black hole binaries is often
observed to be distorted from the standard shape, and a similar effect in
NLS1's could recover the correspondance between black hole binary spectral
state and AGN type.Comment: 12 pages, 12 figures, submitted to MNRA
High-frequency X-ray variability as a mass estimator of stellar and supermassive black holes
There is increasing evidence that supermassive black holes in active galactic nuclei (AGN) are scaled-up versions of Galactic black holes. We show that the amplitude of high-frequency X-ray variability in the hard spectral state is inversely proportional to the black hole mass over eight orders of magnitude. We have analysed all available hard-state data from RXTE of seven Galactic black holes. Their power density spectra change dramatically from observation to observation, except for the high-frequency (≳10 Hz) tail, which seems to have a universal shape, roughly represented by a power law of index -2. The amplitude of the tail, CM (extrapolated to 1 Hz), remains approximately constant for a given source, regardless of the luminosity, unlike the break or quasi-periodic oscillation frequencies, which are usually strongly correlated with luminosity. Comparison with a moderate-luminosity sample of AGN shows that the amplitude of the tail is a simple function of black hole mass, CM = C/M, where C ≈ 1.25 M⊙ Hz -1. This makes CM a robust estimator of the black hole mass which is easy to apply to low- to moderate-luminosity supermassive black holes. The high-frequency tail with its universal shape is an invariant feature of a black hole and, possibly, an imprint of the last stable orbit.</p
Universal spectral shape of AGN with high accretion rate
The spectra of radio quiet and NLS1 galaxies show suprising similarity in
their shape. They seem to scale only with the accretion rate but not with
central black hole mass. We consider two mechanisms modifying the disk
spectrum. First, the outer parts of the disk are irradiated by the flux
emerging from the inner parts. This is due to the scattering of the flux by the
extended hot medium (warm absorber). Second process is connected with the
development of the disk warm Comptonizing skin above the disk and/or coronae.
Our scenario applies only to object with relatively high luminosity to the
Eddington luminosity ratio for which disk evaporation is inefficient.Comment: 4 pages, 5 figures, 1 table, Proc. of the meeting: "The Restless
High-Energy Universe" (Amsterdam, The Netherlands
Masses, Beaming and Eddington Ratios in Ultraluminous X-ray Sources
I suggest that the beaming factor in bright ULXs varies as , where is the Eddington ratio for accretion. This is required
by the observed universal relation between
soft--excess luminosity and temperature, and is reasonable on general physical
grounds. The beam scaling means that all observable properties of bright ULXs
depend essentially only on the Eddington ratio , and that these systems
vary mainly because the beaming is sensitive to the Eddington ratio. This
suggests that bright ULXs are stellar--mass systems accreting at Eddington
ratios of order 10 -- 30, with beaming factors b \ga 0.1. Lower--luminosity
ULXs follow bolometric (not soft--excess) correlations and
probably represent {\it sub}--Eddington accretion on to black holes with masses
\sim 10\msun. High--mass X-ray binaries containing black holes or neutron
stars and undergoing rapid thermal-- or nuclear--timescale mass transfer are
excellent candidates for explaining both types. If the
scaling for bright ULXs can be extrapolated to the Eddington ratios found in
SS433, some objects currently identified as AGN at modest redshifts might
actually be ULXs (`pseudoblazars'). This may explain cases where the active
source does not coincide with the centre of the host galaxy.Comment: MNRAS Letters, in pres
Comptonization in the vicinity of black hole horizon
Using a Monte Carlo method, we derive spectra arising from Comptonization
taking place close to a Kerr black hole. We consider a model consisting of a
hot thermal corona Comptonizing seed photons emitted by a cold accretion disc.
We find that general relativistic effects are crucial for the emerging spectra
in models which involve significant contribution of radiation produced in the
black hole ergosphere. Due to this contribution, spectra of hard X-ray emission
produced in the vicinity of a rapidly rotating black hole strongly depend on
the inclination of the line of sight, with larger inclinations corresponding to
harder spectra. Remarkably, such anisotropy could be responsible for properties
of the X-ray spectra of Seyfert galaxies, which appear to be intrinsically
harder in type 2 objects than in type 1, as reported recently.Comment: 13 pages, 15 figures. Accepted for publication in MNRA
Is the soft excess in active galactic nuclei real?
We systematically analyse all publicly available XMM–Newton spectra of radio-quiet PG quasars. The soft X-ray excess in these objects is well modelled by an additional, cool, Compton scattering region. However, the remarkably constant temperature derived for this component over the whole sample requires a puzzling fine tuning of the parameters. Instead, we propose that the soft excess is an artifact of strong, relativistically smeared, partially ionized absorption. The strong jump in opacity at 0.7 keV from O vii, O viii and iron can lead to an apparent soft excess below this energy, which is trivially constant since it depends on atomic processes. This can have a dramatic effect on the derived spectrum, which has implications for fitting the relativistic smearing of the reflected iron line emission from the disc
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