2,588 research outputs found
Self-Consistent Thermal Accretion Disk Corona Models for Compact Objects: I. Properties of the Corona and the Spectrum of Escaping Radiation
We present the properties of accretion disk corona (ADC) models, where the
radiation field, the temperature, and the total opacity of the corona are
determined self-consistently. We use a non-linear Monte Carlo code to perform
the calculations. As an example, we discuss models where the corona is situated
above and below a cold accretion disk with a plane-parallel (slab) geometry,
similar to the model of Haardt and Maraschi. By Comptonizing the soft radiation
emitted by the accretion disk, the corona is responsible for producing the
high-energy component of the escaping radiation. Our models include the
reprocessing of radiation in the accretion disk. Here, the photons either are
Compton reflected or photo-absorbed, giving rise to fluorescent line emission
and thermal emission. The self-consistent coronal temperature is determined by
balancing heating (due to viscous energy dissipation) with Compton cooling,
determined using the fully relativistic, angle-dependent cross-sections. The
total opacity is found by balancing pair productions with annihilations. We
find that, for a disk temperature kT_bb \lta 200 eV, these coronae are unable
to have a self-consistent temperature higher than \sim 120 keV if the total
optical depth is \gta 0.2, regardless of the compactness parameter of the
corona and the seed opacity. This limitation corresponds to the angle-averaged
spectrum of escaping radiation having a photon index \gta 1.8 within the 5 keV
- 30 keV band. Finally, all models that have reprocessing features also predict
a large thermal excess at lower energies. These constraints make explaining the
X-ray spectra of persistent black hole candidates with ADC models very
problematic.Comment: 15 pages, Latex, 9 .eps figures, uses emulateapj.sty (included). To
be published in ApJ, October 1, 1997, Vol. 48
RXTE Observation of Cygnus X-1: II. Timing Analysis
We present timing analysis for a Rossi X-ray Timing Explorer observation of
Cygnus X-1 in its hard/low state. This was the first RXTE observation of Cyg
X-1 taken after it transited back to this state from its soft/high state.
RXTE's large effective area, superior timing capabilities, and ability to
obtain long, uninterrupted observations have allowed us to obtain measurements
of the power spectral density (PSD), coherence function, and Fourier time lags
to a decade lower in frequency and half a decade higher in frequency than
typically was achieved with previous instruments. Notable aspects of our
observations include a weak 0.005 Hz feature in the PSD coincident with a
coherence recovery; a `hardening' of the high-frequency PSD with increasing
energy; a broad frequency range measurement of the coherence function,
revealing rollovers from unity coherence at both low and high frequency; and an
accurate determination of the Fourier time lags over two and a half decades in
frequency. As has been noted in previous similar observations, the time delay
is approximately proportional to f^(-0.7), and at a fixed Fourier frequency the
time delay of the hard X-rays compared to the softest energy channel tends to
increase logarithmically with energy. Curiously, the 0.01-0.2 Hz coherence
between the highest and lowest energy bands is actually slightly greater than
the coherence between the second highest and lowest energy bands. We carefully
describe all of the analysis techniques used in this paper, and we make
comparisons of the data to general theoretical expectations. In a companion
paper, we make specific comparisons to a Compton corona model that we have
successfully used to describe the energy spectral data from this observation.Comment: To Be Published in the Astrophysical Journal. 18 pages. Uses
emulatepaj.st
RXTE Observation of Cygnus X-1: Spectral Analysis
We present the results of the analysis of the broad-band spectrum of Cygnus
X-1 from 3.0 to 200 keV, using data from a 10 ksec observation by the Rossi
X-ray Timing Explorer. The spectrum can be well described phenomenologically by
an exponentially cut-off power law with a photon index Gamma = 1.45 +/- 0.02 (a
value considerably harder than typically found), e-folding energy E_fold = 162
+/- 9 keV, plus a deviation from a power law that formally can be modeled as a
thermal blackbody with temperature kT_bb = 1.2 +/1 0.2 keV. Although the 3 - 30
keV portion of the spectrum can be fit with a reflected power law with Gamma =
1.81 +/- 0.01 and covering fraction f = 0.35 +/- 0.02, the quality of the fit
is significantly reduced when the HEXTE data in the 30 - 100 keV range is
included, as there is no observed hardening in the power law within this energy
range. As a physical description of this system, we apply the accretion disc
corona models of Dove, Wilms & Begelman (1997) --- where the temperature of the
corona is determined self-consistently. A spherical corona with a total optical
depth tau = 1.6 +/- 0.1 and an average temperature kT_c = 87 +/- 5 keV,
surrounded by an exterior cold disc, does provide a good description of the
data (reduced chi-squared = 1.55). These models deviate from the data by up to
7% in the 5 - 10 keV range, and we discuss possible reasons for these
discrepancies. However, considering how successfully the spherical corona
reproduces the 10 - 200 keV data, such ``photon-starved'' coronal geometries
seem very promising for explaining the accretion processes of Cygnus X-1.Comment: Revised version (added content). 8 pages, 6 figures, 1 table.tex
file, latex, uses mn.sty. Accepted for publication in MNRA
Chandra X-ray spectroscopy of the focused wind in the Cygnus X-1 system. I. The non-dip spectrum in the low/hard state
We present analyses of a 50 ks observation of the supergiant X-ray binary
system Cygnus X-1/HDE 226868 taken with the Chandra High Energy Transmission
Grating Spectrometer (HETGS). Cyg X-1 was in its spectrally hard state and the
observation was performed during superior conjunction of the black hole,
allowing for the spectroscopic analysis of the accreted stellar wind along the
line of sight. A significant part of the observation covers X-ray dips as
commonly observed for Cyg X-1 at this orbital phase, however, here we only
analyze the high count rate non-dip spectrum. The full 0.5-10 keV continuum can
be described by a single model consisting of a disk, a narrow and a
relativistically broadened Fe Kalpha line, and a power law component, which is
consistent with simultaneous RXTE broad band data. We detect absorption edges
from overabundant neutral O, Ne and Fe, and absorption line series from highly
ionized ions and infer column densities and Doppler shifts. With emission lines
of He-like Mg XI, we detect two plasma components with velocities and densities
consistent with the base of the spherical wind and a focused wind. A simple
simulation of the photoionization zone suggests that large parts of the
spherical wind outside of the focused stream are completely ionized, which is
consistent with the low velocities (<200 km/s) observed in the absorption
lines, as the position of absorbers in a spherical wind at low projected
velocity is well constrained. Our observations provide input for models that
couple the wind activity of HDE 226868 to the properties of the accretion flow
onto the black hole.Comment: 16 pages, 15 figures, uses emulateapj, published as ApJ 690:330-346,
2009 January
A possible phase dependent absorption feature in the transient X-ray pulsar SAX J2103.5+4545
We present an X-ray spectral and timing analysis of two observations
of the transient Be X-ray binary SAX J2103.5+4545 during its April 2016
outburst, which was characterized by the highest flux since 's launch.
These observations provide detailed hard X-ray spectra of this source during
its bright precursor flare and subsequent fainter regular outburst for the
first time. In this work, we model the phase-averaged spectra for these
observations with a negative and positive power law with an exponential cut-off
(NPEX) model and compare the pulse profiles at different flux states. We found
that the broad-band pulse profile changes from a three peaked pulse in the
first observation to a two peaked pulse in the second observation, and that
each of the pulse peaks has some energy dependence. We also perform pulse-phase
spectroscopy and fit phase-resolved spectra with NPEX to evaluate how spectral
parameters change with pulse phase. We find that while the continuum parameters
are mostly constant with pulse phase, a weak absorption feature at ~12 keV that
might, with further study, be classified as a cyclotron line, does show strong
pulse phase dependence.Comment: 10 pages, 7 figures, accepted by ApJ, acknowledgements update
Multi-Satellite Observations of Cygnus X-1 to Study the Focused Wind and Absorption Dips
High-mass X-ray binary systems are powered by the stellar wind of their donor
stars. The X-ray state of Cygnus X-1 is correlated with the properties of the
wind which defines the environment of mass accretion. Chandra-HETGS
observations close to orbital phase 0 allow for an analysis of the photoionzed
stellar wind at high resolution, but because of the strong variability due to
soft X-ray absorption dips, simultaneous multi-satellite observations are
required to track and understand the continuum, too. Besides an earlier joint
Chandra and RXTE observation, we present first results from a recent campaign
which represents the best broad-band spectrum of Cyg X-1 ever achieved: On 2008
April 18/19 we observed this source with XMM-Newton, Chandra, Suzaku, RXTE,
INTEGRAL, Swift, and AGILE in X- and gamma-rays, as well as with VLA in the
radio. After superior conjunction of the black hole, we detect soft X-ray
absorption dips likely due to clumps in the focused wind covering >95 % of the
X-ray source, with column densities likely to be of several 10^23 cm^-2, which
also affect photon energies above 20 keV via Compton scattering.Comment: 10 pages, contributed talk at the 7th Microquasar Workshop, Foca,
Turkey, Sept. 1-5, 200
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