23 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: 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
Spectral and Timing Nature of the Symbiotic X-ray Binary 4U 1954+319: The Slowest Rotating Neutron Star in an X-ray Binary System
The symbiotic X-ray binary 4U 1954+319 is a rare system hosting a peculiar
neutron star (NS) and an M-type optical companion. Its ~5.4h NS spin period is
the longest among all known accretion-powered pulsars and exhibited large (~7%)
fluctuations over 8 years. A spin trend transition was detected with Swift/BAT
around an X-ray brightening in 2012. The source was in quiescent and bright
states before and after this outburst based on 60 ks Suzaku observations in
2011 and 2012. The observed continuum is well described by a Comptonized model
with the addition of a narrow 6.4 keV Fe Kalpha line during the outburst.
Spectral similarities to slowly rotating pulsars in high-mass X-ray binaries,
its high pulsed fraction (~60-80%), and the location in the Corbet diagram
favor high B-field (>~1e+12 G) over a weak field as in low-mass X-ray binaries.
The observed low X-ray luminosity (1e+33-1e+35 erg/s), probable wide orbit, and
a slow stellar wind of this SyXB make quasi-spherical accretion in the subsonic
settling regime a plausible model. Assuming a ~1e+13 G NS, this scheme can
explain the ~5.4 h equilibrium rotation without employing the magnetar-like
field (~1e+16 G) required in the disk accretion case. The time-scales of
multiple irregular flares (~50 s) can also be attributed to the free-fall time
from the Alfven shell for a ~1e+13 G field. A physical interpretation of SyXBs
beyond the canonical binary classifications is discussed.Comment: 20 pages, 18 figures, 3 tables, accepted for publication in the
Astrophysical Journa
Tracking the Orbital and Super-orbital Periods of SMC X-1
The High Mass X-ray Binary (HMXB) SMC X-1 demonstrates an orbital variation
of 3.89 days and a super-orbital variation with an average length of 55 days.
As we show here, however, the length of the super-orbital cycle varies by
almost a factor of two, even across adjacent cycles. To study both the orbital
and super-orbital variation we utilize lightcurves from the Rossi X-ray Timing
Explorer All Sky Monitor (RXTE-ASM). We employ the orbital ephemeris from
Wojdowski et al. (1998) to obtain the average orbital profile, and we show that
this profile exhibits complex modulation during non-eclipse phases.
Additionally, a very interesting ``bounceback'' in X-ray count rate is seen
during mid-orbital eclipse phases, with a softening of the emission during
these periods. This bounceback has not been previously identified in pointed
observations. We then define a super-orbital ephemeris (the phase of the
super-orbital cycle as a function of date) based on the ASM lightcurve and
analyze the trend and distribution of super-orbital cycle lengths. SMC X-1
exhibits a bimodal distribution of these lengths, similar to what has been
observed in other systems (e.g., Her X-1), but with more dramatic changes in
cycle length. There is some hint, but not conclusive evidence, for a dependence
of the super-orbital cycle length upon the underlying orbital period, as has
been observed previously for Her X-1 and Cyg X-2. Using our super-orbital
ephemeris we are also able to create an average super-orbital profile over the
71 observed cycles, for which we witness overall hardening of the spectrum
during low count rate times. We combine the orbital and super-orbital
ephemerides to study the correlation between the orbital and super-orbital
variations in the system.Comment: 10 pages, using emulateapj style. To be published in the
Astrophysical Journa
Is the `IR Coincidence' Just That?
(Abridged) Motch (1985) suggested that in the hard state of GX 339-4 the soft
X-ray power-law extrapolated backward in energy agrees with the IR flux. Corbel
& Fender (2002) showed that the hard state radio power-law extrapolated forward
in energy meets the extrapolated X-ray power-law at an IR break, which was
explicitly observed twice in GX 339-4. This `IR coincidence' has been cited as
further evidence that a jet might make a significant contribution to the X-rays
in hard state systems. We explore this hypothesis with a series of simultaneous
radio/X-ray observations of GX 339-4, taken during its 1997, 1999, and 2002
hard states. We fit these spectra, in detector space, with a simple, but
remarkably successful, doubly broken power-law that requires an IR spectral
break. For these observations, the break position and the integrated radio/IR
flux have stronger dependences upon the X-rays than the simplest jet
predictions. If one allows for a softening of the X-ray power law with
increasing flux, then the jet model agrees with the correlation. We also find
evidence that the radio/X-ray fcorrelation previously observed in GX 339-4
shows a `parallel track' for the 2002 hard state. The slope of the 2002
correlation is consistent with prior observations; however, the radio amplitude
is reduced. We then examine the correlation in Cyg X-1 through the use of radio
data, obtained with the Ryle radio telescope, and RXTE data, from the ASM and
pointed observations. We again find evidence of `parallel tracks', and here
they are associated with `failed transitions' to the soft state. We also find
that for Cyg X-1 the radio flux is more fundamentally correlated with the hard
X-ray flux.Comment: To Appear in the July 2005 Astrophysical Journal; 9 Pages, uses
emulateapj.st
Low Luminosity States of the Black Hole Candidate GX~339--4. II. Timing Analysis
Here we present timing analysis of a set of eight Rossi X-ray Timing Explorer
(RXTE) observations of the black hole candidate GX 339-4 that were taken during
its hard/low state. On long time scales, the RXTE All Sky Monitor data reveal
evidence of a 240 day periodicity, comparable to timescales expected from
warped, precessing accretion disks. On short timescales all observations save
one show evidence of a persistent f approximately equal to 0.3 Hz QPO. The
broad band (10^{-3}-10^2 Hz) power appears to be dominated by two independent
processes that can be modeled as very broad Lorentzians with Q approximately
less than 1. The coherence function between soft and hard photon variability
shows that if these are truly independent processes, then they are individually
coherent, but they are incoherent with one another. This is evidenced by the
fact that the coherence function between the hard and soft variability is near
unity between 0.005-10 Hz but shows evidence of a dip at f approximately equal
to 1 Hz. This is the region of overlap between the broad Lorentzian fits to the
PSD. Similar to Cyg X-1, the coherence also drops dramatically at frequencies
approximately greater than 10 Hz. Also similar to Cyg X-1, the hard photon
variability is seen to lag the soft photon variability with the lag time
increasing with decreasing Fourier frequency. The magnitude of this time lag
appears to be positively correlated with the flux of GX 339-4. We discuss all
of these observations in light of current theoretical models of both black hole
spectra and temporal variability.Comment: To Appear in the AStrophysical Journa
The Next Generation X-ray Galaxy Survey with eROSITA
eROSITA, launched on 13 July 2019, will be completing the first all-sky
survey in the soft and medium X-ray band in nearly three decades. This 4-year
survey, finishing in late 2023, will present a rich legacy for the entire
astrophysics community and complement upcoming multi-wavelength surveys (with,
e.g. the Large Synoptic Survey Telescope and the Dark Energy Survey). Besides
the major scientific aim to study active galactic nuclei (AGN) and galaxy
clusters, eROSITA will contribute significantly to X-ray studies of normal
(i.e., not AGN) galaxies. Starting from multi-wavelength catalogues, we measure
star formation rates and stellar masses for 60 212 galaxies constrained to
distances of 50-200 Mpc. We chose this distance range to focus on the
relatively unexplored volume outside the local Universe, where galaxies will be
largely spatially unresolved and probe a range of X-ray luminosities that
overlap with the low luminosity and/or highly obscured AGN population. We use
the most recent X-ray scaling relations as well as the on-orbit eROSITA
instrument performance to predict the X-ray emission from XRBs and diffuse hot
gas and to perform both an analytic prediction and an end-to-end simulation
using the mission simulation software, SIXTE. We consider potential
contributions from hidden AGN and comment on the impact of normal galaxies on
the measurement of the faint end of the AGN luminosity function. We predict
that the eROSITA 4-year survey, will detect 15 000 galaxies (3
significance) at 50-200 Mpc, which is ~100X more normal galaxies than
detected in any X-ray survey to date.Comment: 18 pages, 15 figures. Accepted for publication in MNRA
Going with the flow: can the base of jets subsume the role of compact accretion disk coronae?
The hard state of X-ray binaries (XRBs) is characterized by a power law
spectrum in the X-ray band, and a flat/inverted radio/IR spectrum associated
with occasionally imaged compact jets. It has generally been thought that the
hard X-rays result from Compton upscattering of thermal accretion disk photons
by a hot, coronal plasma whose properties are inferred via spectral fitting.
Interestingly, these properties-especially those from certain magnetized corona
models-are very similar to the derived plasma conditions at the jet footpoints.
Here we explore the question of whether the `corona' and `jet base' are in fact
related, starting by testing the strongest premise that they are synonymous. In
such models, the radio through the soft X-rays are dominated by synchrotron
emission, while the hard X-rays are dominated by inverse Compton at the jet
base - with both disk and synchrotron photons acting as seed photons. The
conditions at the jet base fix the conditions along the rest of the jet, thus
creating a direct link between the X-ray and radio emission. We also add to
this model a simple iron line and convolve the spectrum with neutral
reflection. After forward-folding the predicted spectra through the detector
response functions, we compare the results to simultaneous radio/X-ray data
obtained from the hard states of the Galactic XRBs GX339-4 and Cygnus X-1.
Results from simple Compton corona model fits are also presented for
comparison. We demonstrate that the jet model fits are statistically as good as
the single-component corona model X-ray fits, yet are also able to address the
simultaneous radio data.Comment: Accepted to the Astrophysical Journal. 14 pages, emulateapj.st
Timing variability of Vela X-1 during a bright flare
The X-ray Universe 2014, edited by Jan-Uwe Ness. Online at http://www.cosmos.esa.int/web/xmm-newton/2014-symposium/, id.130We present an in-depth analysis of the temporal behaviour of the HMXB Vela X-1 which exhibited strongly varying flux levels during an observation by XMM-Newton. During the 100 ks observation the source went from being in a highly absorbed initial state, to one of increased activity which was followed by a giant X-ray flare, before finally reaching a settled state with low absorption. The lower absorption state allows the normally absorbed pulse profile below 1 keV to be studied. Vela X-1 also showed multiple changes in the structure of the pulse profile including a phase lag with the harder photons leading the softer ones. A change in the pulse period, significant at approximately the 2Ï level, was observed at the beginning of the powerful flare and is likely related to the large accreted mass. For comparison purposes, an earlier XMM-Newton observation in a lower active state is used to put the variations in pulse profile and phase lag in context
On the enigmatic X-ray Source V1408 Aql (=4U 1957+11)
Models for the characteristically soft X-ray spectrum of the compact X-ray
source V1498 Aql (=4U 1957+11) have ranged from optically thick Comptonization
to multicolor accretion disk models. We critically examine the X-ray spectrum
of V1408 Aql via archival Advanced Satellite for Cosmology and Astrophysics
(ASCA) data, archival Roentgensatellit (ROSAT) data, and recent Rossi X-Ray
Timing Explorer (RXTE) data. Although we are able to fit a variety of X-ray
spectral models to these data, we favor an interpretation of the X-ray spectrum
as being due to an accretion disk viewed at large inclination angles. Evidence
for this hypothesis includes long term (117 day, 235 day, 352 day)
periodicities seen by the RXTE All Sky Monitor (ASM), which we interpret as
being due to a warped precessing disk, and a 1 keV feature in the ASCA data,
which we interpret as being the blend of L fluorescence features from a disk
atmosphere or wind. We also present timing analysis of the RXTE data and find
upper limits of 4% for the root mean square (rms) variability between
f=0.001-16 Hz. The situation of whether the compact object is a black hole or
neutron star is still ambiguous; however, it now seems more likely that an
X-ray emitting, warped accretion disk is an important component of this system.Comment: High Frequency Power Spectrum corrected for unflagged `data dropouts'
(described in Appendix) and correct upper limits for variability presented.
All energy spectra and long term variability sections unchanged. Additional
references and acknowledgements added. 13 pages in emulateapj.st