4,872 research outputs found
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
X-ray Polarization Signatures of Compton Scattering in Magnetic Cataclysmic Variables
Compton scattering within the accretion column of magnetic cataclysmic
variables (mCVs) can induce a net polarization in the X-ray emission. We
investigate this process using Monte Carlo simulations and find that
significant polarization can arise as a result of the stratified flow structure
in the shock-ionized column. We find that the degree of linear polarization can
reach levels up to ~8% for systems with high accretion rates and low
white-dwarf masses, when viewed at large inclination angles with respect to the
accretion column axis. These levels are substantially higher than previously
predicted estimates using an accretion column model with uniform density and
temperature. We also find that for systems with a relatively low-mass white
dwarf accreting at a high accretion rate, the polarization properties may be
insensitive to the magnetic field, since most of the scattering occurs at the
base of the accretion column where the density structure is determined mainly
by bremsstrahlung cooling instead of cyclotron cooling.Comment: 7 pages, 8 figures, accepted by MNRA
Compton Scattering of Fe K alpha Lines in Magnetic Cataclysmic Variables
Compton scattering of X-rays in the bulk flow of the accretion column in
magnetic cataclysmic variables (mCVs) can significantly shift photon energies.
We present Monte Carlo simulations based on a nonlinear algorithm demonstrating
the effects of Compton scattering on the H-like, He-like and neutral Fe K alpha
lines produced in the post-shock region of the accretion column. The peak line
emissivities of the photons in the post-shock flow are taken into consideration
and frequency shifts due to Doppler effects are also included. We find that
line profiles are most distorted by Compton scattering effects in strongly
magnetized mCVs with a low white dwarf mass and high mass accretion rate and
which are viewed at an oblique angle with respect to the accretion column. The
resulting line profiles are most sensitive to the inclination angle. We have
also explored the effects of modifying the accretion column width and using a
realistic emissivity profile. We find that these do not have a significant
overall effect on the resulting line profiles. A comparison of our simulated
line spectra with high resolution Chandra/HETGS observations of the mCV GK Per
indicates that a wing feature redward of the 6.4 keV line may result from
Compton recoil near the base of the accretion column.Comment: Accepted for publication in MNRAS, 10 pages with 8 figure
Compton Scattering by Static and Moving Media I. The Transfer Equation and Its Moments
Compton scattering of photons by nonrelativistic particles is thought to play
an important role in forming the radiation spectrum of many astrophysical
systems. Here we derive the time-dependent photon kinetic equation that
describes spontaneous and induced Compton scattering as well as absorption and
emission by static and moving media, the corresponding radiative transfer
equation, and their zeroth and first moments, in both the system frame and in
the frame comoving with the medium. We show that it is necessary to use the
correct relativistic differential scattering cross section in order to obtain a
photon kinetic equation that is correct to first order in epsilon/m_e, T_e/m_e,
and V, where epsilon is the photon energy, T_e and m_e are the electron
temperature and rest mass, and V is the electron bulk velocity in units of the
speed of light. We also demonstrate that the terms in the radiative transfer
equation that are second-order in V usually should be retained, because if the
radiation energy density is sufficiently large compared to the radiation flux,
the effects of bulk Comptonization described by the terms that are second-order
in V are at least as important as the effects described by the terms that are
first-order in V, even when V is small. Our equations are valid for systems of
arbitrary optical depth and can therefore be used in both the free-streaming
and the diffusion regimes. We demonstrate that Comptonization by the electron
bulk motion occurs whether or not the radiation field is isotropic or the bulk
flow converges and that it is more important than thermal Comptonization if V^2
> 3 T_e/m_e.Comment: 31 pages, accepted for publication in The Astrophysical Journa
Accretion column disruption in GX 1+4
Daily observations of the binary X-ray pulsar GX 1+4 were made with the Rossi
X-ray Timing Explorer (RXTE) satellite between 1997 May 16-20 as part of a
four-month monitoring program. On May 17 the sharp dips normally observed in
the lightcurve were all but absent, resulting in a pulse fraction f_p approx.
0.5 instead of the more typical value of approx. 0.8 measured before and after.
Also observed was a dramatic hardening of the 2-40 keV phase-averaged spectrum.
The power-law photon index was 1.16 +/- 0.02, whereas values of 1.6-2.0 are
more typical. In terms of a Comptonization continuum component, the optical
depth for scattering was tau approx 19, with 4-6 the usual range for RXTE
spectra (Galloway 2000). Pulse-phase spectrosopy indicates that tau is
decreased relative to the phase-averaged value around the primary minimum,
where an increase is normally observed. The reduced depth of the dip is
interpreted as disruption of the accretion column, and the accompanying
spectral variation suggests a substantially different accretion regime than is
usual for this source.Comment: 10 pages, 2 figures, minor abstract typo and wording of final
paragraph correcte
X-ray line formation in the spectrum of SS 433
The mechanisms for the formation of X-ray lines in the spectrum of SS 433 are
investigated by taking into account the radiative transfer inside the jets. The
results of Monte Carlo numerical simulations are presented. The effect of a
decrease in line intensity due to scattering inside the jet turns out to be
pronounced, but it does not exceed 60% in magnitude on the entire grid of
parameters. The line broadening due to scattering, nutational motion, and the
contribution of satellites can lead to overestimates of the jet opening angle
from the line widths in Chandra X-ray observations. The fine structure
of the lines turns out to be very sensitive to the scattering effects. This
makes its investigation by planned X-ray observatories equipped with
high-resolution spectrometers (primarily Astro-H) a powerful tool for
diagnosing the parameters of the jets in SS 433.Comment: 23 pages, 14 figures, to be published in Astronomy Letters, v. 38, n.
7, p. 443 (2012
Compton Scattering in Static and Moving Media. II. System-Frame Solutions for Spherically Symmetric Flows
I study the formation of Comptonization spectra in spherically symmetric,
fast moving media in a flat spacetime. I analyze the mathematical character of
the moments of the transfer equation in the system-frame and describe a
numerical method that provides fast solutions of the time-independent radiative
transfer problem that are accurate in both the diffusion and free-streaming
regimes. I show that even if the flows are mildly relativistic (V~0.1, where V
is the electron bulk velocity in units of the speed of light), terms that are
second-order in V alter the emerging spectrum both quantitatively and
qualitatively. In particular, terms that are second-order in V produce
power-law spectral tails, which are the dominant feature at high energies, and
therefore cannot be neglected. I further show that photons from a static source
are upscattered by the bulk motion of the medium even if the velocity field
does not converge. Finally, I discuss these results in the context of radial
accretion onto and outflows from compact objects.Comment: 28 pages, 9 figures; minor changes, to appear in the Astrophysical
Journa
Chandra Observations of Type Ia Supernovae: Upper Limits to the X-ray Flux of SN 2002bo, SN 2002ic, SN 2005gj, and SN 2005ke
We set sensitive upper limits to the X-ray emission of four Type Ia
supernovae (SNe Ia) using the Chandra X-ray Observatory. SN 2002bo, a normal,
although reddened, nearby SN Ia, was observed 9.3 days after explosion. For an
absorbed, high temperature bremsstrahlung model the flux limits are 3.2E-16
ergs/cm^2/s (0.5-2 keV band) and 4.1E-15 ergs/cm^2/s (2-10 keV band). Using
conservative model assumptions and a 10 km/s wind speed, we derive a mass loss
rate of \dot{M} ~ 2E-5 M_\odot/yr, which is comparable to limits set by the
non-detection of Halpha lines from other SNe Ia. Two other objects, SN 2002ic
and SN 2005gj, observed 260 and 80 days after explosion, respectively, are the
only SNe Ia showing evidence for circumstellar interaction. The SN 2002ic X-ray
flux upper limits are ~4 times below predictions of the interaction model
currently favored to explain the bright optical emission. To resolve this
discrepancy we invoke the mixing of cool dense ejecta fragments into the
forward shock region, which produces increased X-ray absorption. A modest
amount of mixing allows us to accommodate the Chandra upper limit. SN 2005gj is
less well studied at this time. Assuming the same circumstellar environment as
for SN 2002i, the X-ray flux upper limits for SN 2005gj are ~4 times below the
predictions, suggesting that mixing of cool ejecta into the forward shock has
also occurred here. Our reanalysis of Swift and Chandra data on SN 2005ke does
not confirm a previously reported X-ray detection. The host galaxies NGC 3190
(SN 2002bo) and NGC 1371 (SN 2005ke) each harbor a low luminosity (L_X ~ 3-4E40
ergs/s) active nucleus in addition to wide-spread diffuse soft X-ray emission.Comment: 16 pages, to appear in ApJ (20 Nov 2007
Vertical Structure of the Outer Accretion Disk in Persistent Low-Mass X-Ray Binaries
We have investigated the influence of X-ray irradiation on the vertical
structure of the outer accretion disk in low-mass X-ray binaries by performing
a self-consistent calculation of the vertical structure and X-ray radiation
transfer in the disk. Penetrating deep into the disk, the field of scattered
X-ray photons with energy \,keV exerts a significant influence on
the vertical structure of the accretion disk at a distance
\,cm from the neutron star. At a distance \,cm,
where the total surface density in the disk reaches
\,g\,cm, X-ray heating affects all layers of an
optically thick disk. The X-ray heating effect is enhanced significantly in the
presence of an extended atmospheric layer with a temperature
\,K above the accretion disk. We have derived
simple analytic formulas for the disk heating by scattered X-ray photons using
an approximate solution of the transfer equation by the Sobolev method. This
approximation has a \,% accuracy in the range of X-ray photon
energies \,keV.Comment: 19 pages, 8 figures, published in Astronomy Letter
Turbulent Comptonization in Black Hole Accretion Disks
In the inner-most regions of radiation pressure supported accretion disks,
the turbulent magnetic pressure may greatly exceed that of the gas. If this is
the case, it is possible for bulk Alfvenic motions driven by the
magnetorotational instability (MRI) to surpass the electron thermal velocity.
Bulk rather than thermal Comptonization may then be the dominant radiative
process which mediates gravitational energy release. For sufficiently large
turbulent stresses, we show that turbulent Comptonization produces a
significant contribution to the far-UV and X-ray emission of black hole
accretion disks. The existence of this spectral component provides a means of
obtaining direct observational constraints on the nature of the turbulence
itself. We describe how this component may affect the spectral energy
distributions and variability properties of X-ray binaries and active galactic
nuclei.Comment: Submitted to Ap
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