1,080 research outputs found
On the Origin of Polarization near the Lyman Edge in Quasars
Optical/UV radiation from accretion disks in quasars is likely to be partly
scattered by a hot plasma enveloping the disk. We investigate whether the
scattering may produce the steep rises in polarization observed blueward of the
Lyman limit in some quasars. We suggest and assess two models. In the first
model, primary disk radiation with a Lyman edge in absorption passes through a
static ionized "skin" covering the disk, which has a temperature about 3 keV
and a Thomson optical depth about unity. Electron scattering in the skin smears
out the edge and produces a steep rise in polarization at lambda < 912 A. In
the second model, the scattering occurs in a hot coronal plasma outflowing from
the disk with a mildly relativistic velocity. We find that the second model
better explains the data. The ability of the models to fit the observed rises
in polarization is illustrated with the quasar PG 1630+377.Comment: submitted to ApJ Letter
Ultraluminous X-ray Sources Powered by Radiatively Efficient Two-Phased Super-Eddington Accretion onto Stellar Mass Black holes
The radiation spectra of many of the brightest ultraluminous X-ray sources
(ULXs) are dominated by a hard power law component, likely powered by a hot,
optically thin corona that Comptonizes soft seed photons emitted from a cool,
optically thick black hole accretion disk. Before its dissipation and
subsequent conversion into coronal photon power, the randomized gravitational
binding energy responsible for powering ULX phenomena must separate from the
mass of its origin by a means other than, and quicker than, electron
scattering-mediated radiative diffusion. Therefore, the release of accretion
power in ULXs is not necessarily subject to Eddington-limited photon trapping,
as long as it occurs in a corona. Motivated by these basic considerations, we
present a model of ULXs powered by geometrically thin accretion onto stellar
mass black holes. We argue that the radiative efficiency of the flow remains
high if the corona is magnetized or optically thin and the majority of the
accretion power escapes in the form of radiation rather than an outflow. Within
the context of the current black hole X-ray binary paradigm, our ULX model may
be viewed as an extension of the very high state observed in Galactic sources.
(abridged)Comment: 11 page
A Complexity-Brightness Correlation in Gamma Ray Bursts
We observe strong correlations between the temporal properties of gamma ray
bursts (GRBs) and their apparent peak brightness. The strongest effect (with a
significance level of 10^{-6}) is the difference between the brightness
distributions of simple bursts (dominated by a single smooth pulse) and complex
bursts (consisting of overlapping pulses). The latter has a break at a peak
flux of 1.5 ph/cm^2/s, while the distribution of simple bursts is smooth down
to the BATSE threshold. We also observe brightness dependent variations in the
shape of the average peak aligned time profile (ATP) of GRBs. The decaying
slope of the ATP shows time dilation when comparing bright and dim bursts while
the rising slope hardly changes. Both slopes of the ATP are deformed for weak
bursts as compared to strong bursts. The interpretation of these effects is
simple: a complex burst where a number of independent pulses overlap in time
appears intrinsically stronger than a simple burst. Then the BATSE sample of
complex bursts covers larger redshifts where some cosmological factor causes
the break in the peak brightness distribution. This break could correspond to
the peak in the star formation rate that was recently shown to occur at a
redshift of z~1.5.Comment: 13 pages; 11 figures; replaced with the published versio
ON THE GEOMETRY OF THE X-RAY EMITTING REGION IN SEYFERT GALAXIES
For the first time, detailed radiative transfer calculations of Comptonized
X-ray and gamma-ray radiation in a hot pair plasma above a cold accretion disk
are performed using two independent codes and methods. The simulations include
both energy and pair balance as well as reprocessing of the X- and gamma-rays
by the cold disk. We study both plane-parallel coronae as well as active
dissipation regions having shapes of hemispheres and pill boxes located on the
disk surface. It is shown, contrary to earlier claims, that plane-parallel
coronae in pair balance have difficulties in selfconsistently reproducing the
ranges of 2-20 keV spectral slopes, high energy cutoffs, and compactnesses
inferred from observations of type 1 Seyfert galaxies. Instead, the
observations are consistent with the X-rays coming from a number of individual
active regions located on the surface of the disk.
A number of effects such as anisotropic Compton scattering, the reflection
hump, feedback to the soft photon source by reprocessing, and an active region
in pair equilibrium all conspire to produce the observed ranges of X-ray
slopes, high energy cutoffs, and compactnesses. The spread in spectral X-ray
slopes can be due to a spread in the properties of the active regions such as
their compactnesses and their elevations above the disk surface. Simplified
models invoking isotropic Comptonization in spherical clouds are no longer
sufficient when interpreting the data.Comment: 9 pages, 3 postscript figures, figures can be obtained from the
authors via e-mail: [email protected]
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
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
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