1,117 research outputs found
On the light-bending model of X-ray variability of MCG-6-30-15
We apply the light bending model of X-ray variability to Suzaku data of the
Seyfert 1 galaxy MCG-6-30-15. We analyze the energy dependence of the root mean
square (rms) variability, and discuss conditions necessary for the model to
explain the characteristic decrease of the source variability around 5-8 keV. A
model, where the X-ray source moves radially rather than vertically close to
the disk surface, can indeed reproduce the reduced variability near the energy
of the Fe Kalpha line, although the formal fit quality is poor. The model then
predicts the energy spectra, which can be compared to observational data. The
spectra are strongly reflection dominated, and do not provide a good fit to
Suzaku spectral data of the source. The inconsistency of this result with some
previous claims can be traced to our using data in a broader energy band, where
effects of warm absorber in the spectrum cannot be neglected.Comment: 6 pages, PASJ, accepte
Timing and Spectral Properties of X-ray Emission from the Converging Flows onto Black hole: Monte-Carlo Simulations
We demonstrate that a X-ray spectrum of a converging inflow (CI) onto a black
hole is the sum of a thermal (disk) component and the convolution of some
fraction of this component with the Comptonization spread (Green's) function.
The latter component is seen as an extended power law at energies much higher
than the characteristic energy of the soft photons. We show that the high
energy photon production (source function) in the CI atmosphere is distributed
with the characteristic maximum at about the photon bending radius, 1.5r_S,
independently of the seed (soft) photon distribution. We show that high
frequency oscillations of the soft photon source in this region lead to the
oscillations of the high energy part of the spectrum but not of the thermal
component. The high frequency oscillations of the inner region are not
significant in the thermal component of the spectrum. We further demonstrate
that Doppler and recoil effects (which are responsible for the formation of the
CI spectrum) are related to the hard (positive) and soft (negative) time lags
between the soft and hard photon energy channels respectively.Comment: 9 pages and 4 figures, to be published in the Astrophysical Journal
Letter
Chandra deep X-ray observation on the Galactic plane
Using the Chandra ACIS-I instruments, we have carried out the deepest X-ray
observation on a typical Galactic plane region at l 28.5 deg, where no discrete
X-ray sources have been known previously. We have detected, as well as strong
diffuse emission, 275 new point X-ray sources (4 sigma confidence) within two
partially overlapping fields (~250 arcmin^2 in total) down to ~3 x 10^{-15} erg
s^{-1} cm^{-2} (2 -- 10 keV) or ~ 7 x 10^{-16} erg s^{-1} cm^{-2} (0.5 -- 2
keV). We have studied spectral distribution of these point sources, and found
that very soft sources detected only below ~ 3 keV are more numerous than hard
sources detected only above ~ 3 keV. Only small number of sources are detected
both in the soft and hard bands. Surface density of the hard sources is almost
consistent with that at high Galactic regions, thus most of the hard sources
are considered to be Active Galactic Nuclei seen through the milky way. On the
other hand, some of the bright hard X-ray sources which show extremely flat
spectra and iron line or edge features are considered to be Galactic,
presumably quiescent dwarf novae. The soft sources show thermal spectra and
small interstellar hydrogen column densities, and some of them exhibit X-ray
flares. Therefore, most of the soft sources are probably X-ray active nearby
late type stars.Comment: Contribution to the proceedings of the "New Visions of the X-Ray
Universe in the XMM-Newton and Chandra Era" symposium at ESTEC, The
Netherlands. 26-30 Nov. 200
Spectral Properties of Accretion Disks Around Black Holes II -- Sub-Keplerian Flows With and Without Shocks
Close to a black hole, the density of the sub-Keplerian accreting matter
becomes higher compared to a spherical flow due to the presence of a
centrifugal barrier independent of whether or not a standing shock actually
forms. This hot dense flow intercepts soft photons from a cold Keplerian disk
and reprocesses them to form high energy X-rays and gamma rays. We study the
spectral properties of various models of accretion disks where a Keplerian disk
on the equatorial plane may or may not be flanked by a sub-Keplerian disk and
the sub-Keplerian flow may or may not possess standing shocks. From comparison
with the spectra, we believe that the observed properties could be explained
better when both the components (Keplerian and sub-Keplerian) are
simultaneously present close to a black hole, even though the sub-Keplerian
halo component may have been produced out of the Keplerian disk itself at
larger radii. We are able to understand soft and hard states of black hole
candidates, properties of X-ray novae outbursts, and quasi-periodic
oscillations of black hole candidates using these two component models. We fit
spectra of X-ray novae GS1124-68 and GS2000+25 and satisfactorily reproduce the
light curves of these objects.Comment: 15 Latex pages plus 12 figures. Macros included. Astrophysical
Journal (In press
Detection of anti-correlated hard X-ray time lag in Cygnus X-3
The wide-band X-ray spectra of the high mass X-ray binary Cygnus X-3 exhibits
a pivoting behavior in the `low' (as well as `hard') state, correlated to the
radio emission. The time scale of the soft and hard X-rays' anti-correlation,
which gave rise to the pivoting feature, was found to be less than a day from
the monitoring observations by RXTE--ASM and CGRO--BATSE. In this Letter we
report the detection of a lag of 1000s in the anti-correlation of
the hard X-ray emission (20--50 keV) to that of the soft X-ray emission (2--7
keV), which may be attributed to the viscous time scale of flow of matter in
the accretion disk. This suggests the geometrical picture of a truncated
accretion disc with a Compton cloud inside the disc, the relative sizes of
which determine the spectral shape. Any change in the disc structure will take
place in a viscous time scale, with corresponding anti-correlated change in the
Compton cloud. We also report the pivoting in the spectra in one span of a
pointed observation when an episode of the rearranging of the accretion system
is serendipitously observed. This is the first such observation of hard X-ray
delay seen in the persistent Galactic microquasars, within the precincts of the
hard state.Comment: Accepted in The Astrophysical Journal (Letters): in pres
Broad-band X-ray/gamma-ray spectra and binary parameters of GX 339-4 and their astrophysical implications
We present X-ray/gamma-ray spectra of the binary GX 339-4 observed in the
hard state simultaneously by Ginga and CGRO OSSE during an outburst in 1991
September. The Ginga spectra are well represented by a power law with a photon
spectral index of 1.75 and a moderately-strong Compton reflection component
with a fluorescent Fe K alpha line. The OSSE data require a sharp high-energy
cutoff in the power-law spectrum. The broad-band spectra are very well modelled
by repeated Compton scattering in a thermal plasma with tau=1 and kT=50 keV. We
also find the distance to the system to be > 3 kpc, ruling out earlier
determinations of 1.3 kpc. Using this limit, the observed reddening and the
orbital period, we find the allowed range of the mass of the primary is
consistent with it being a black hole. The data are inconsistent with models of
either homogenous or patchy coronae above the surface of an accretion disc.
Rather, they are consistent with the presence of a hot inner hot disc accreting
at a rate close to the maximum set by advection and surrounded by a cold outer
disc. The seed photons for Comptonization are supplied by the outer cold disc
and/or cold clouds within the hot disc. Pair production is negligible if
electrons are thermal. The hot disc model, which scaled parameters are
independent of the black-hole mass, is supported by the similarity of the
spectrum of GX 339-4 to those of other black-hole binaries and Seyfert 1s. On
the other hand, their spectra in the soft gamma-ray regime are significantly
harder than those of weakly-magnetized neutron stars. Based on this difference,
we propose that the presence of broad-band spectra corresponding to thermal
Comptonization with kT of 50 keV or more represents a black-hole signature.Comment: 17 pages, 9 figures, accepted to MNRA
Radiation mechanisms and geometry of Cygnus X-1 in the soft state
We present X-ray/gamma-ray spectra of Cyg X-1 observed during the transition
from the hard to the soft state and in the soft state by ASCA, RXTE and OSSE in
1996 May and June. The spectra consist of a dominant soft component below ~2
keV and a power-law-like continuum extending to at least ~800 keV. We interpret
them as emission from an optically-thick, cold accretion disc and from an
optically-thin, non-thermal corona above the disc. A fraction f ~ 0.6 of total
available power is dissipated in the corona. We model the soft component by
multi-colour blackbody disc emission taking into account the torque-free
inner-boundary condition. If the disc extends down to the minimum stable orbit,
the ASCA/RXTE data yield the most probable black hole mass of about 10 solar
masses and an accretion rate about 0.5 L_E/c^2, locating Cyg X-1 in the soft
state in the upper part of the stable, gas-pressure dominated, accretion-disc
solution branch. The spectrum of the corona is well modelled by repeated
Compton scattering of seed photons from the disc off electrons with a hybrid,
thermal/non-thermal distribution. The electron distribution can be
characterized by a Maxwellian with an equilibrium temperature of kT ~ 30--50
keV and a Thomson optical depth of ~0.3 and a quasi-power-law tail. The
compactness of the corona is between 2 and 7, and a presence of a significant
population of electron-positron pairs is ruled out. We find strong signatures
of Compton reflection from a cold and ionized medium, presumably an accretion
disc, with an apparent reflector solid angle ~0.5--0.7. The reflected continuum
is accompanied by a broad iron K-alpha line.Comment: 18 pages, 12 figures, 2 landscape tables in a separate file. Accepted
to MNRA
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