111 research outputs found
Coronal--Temporal Correlations in GX339-4: Hysteresis, Possible Reflection Changes, and Implications for ADAFs
We present spectral fits and timing analysis of Rossi X-ray Timing Explorer
observations of GX339-4. These observations were carried out over a span of
more than two years and encompassed both the soft/high and hard/low states.
Hysteresis in the soft state/hard state transition is observed. The hard state
exhibits a possible anti-correlation between coronal compactness (i.e.,
spectral hardness) and the covering fraction of cold, reflecting material. The
correlation between `reflection fraction' and soft X-ray flux, however, appears
to be more universal. Furthermore, low flux, hard state observations - taken
over a decline into quiescence- show that the Fe line, independent of
`reflection fraction', remains broad and at a roughly constant equivalent
width, counter to expectations from ADAF models. All power spectral densities
(PSD) of the hard state X-ray lightcurves are describable as the sum of just a
few broad, quasi-periodic features with frequencies that roughly scale as
coronal compactness to the -3/2 power. Similar to observations of Cyg X-1, time
lags between soft and hard variability anti-correlate with coronal compactness.
A stronger correlation is seen between the time lags and the `reflection
fraction'.Comment: 29 Pages, 17 Figures, 6 Tables. Accepted for Publication in MNRAS.
(Abstract Abridged
The column density towards LMC X-1
We measure the neutral absorption towards the black hole X-ray binary system
LMC X-1 from six archival soft X-ray spectra obtained with the gratings and/or
CCD detectors on Chandra, XMM-Newton, and Swift. Four spectral models for the
soft continuum have been investigated. While the powerlaw model may
overestimate NH considerably, the others give consistent results. Taking the
lower metalicity of the Large Magellanic Cloud into account, we find equivalent
hydrogen column densities of N_H = (1.0-1.3)*10^22 cm^-2, with a systematic
dependence on the orbital phase. This variation of the neutral absorption can
nearly explain the orbital modulation of the soft X-ray flux recently detected
with the All Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE).Comment: 4 pages, accepted for publication as a Letter in Astronomy and
Astrophysic
Low Luminosity States of the Black Hole Candidate GX 339-4. I. ASCA and Simultaneous Radio/RXTE Observations
We discuss a series of observations of the black hole candidate GX 339-4 in
low luminosity, spectrally hard states. We present spectral analysis of three
separate archival Advanced Satellite for Cosmology and Astrophysics (ASCA) data
sets and eight separate Rossi X-ray Timing Explorer (RXTE) data sets. Three of
the RXTE observations were strictly simultaneous with 843 MHz and 8.3-9.1 GHz
radio observations. All of these observations have (3-9 keV) flux approximately
< 10^{-9} ergs s^{-1} cm^{-2}. The ASCA data show evidence for an 6.4 keV Fe
line with equivalent width 40 eV, as well as evidence for a soft excess that is
well-modeled by a power law plus a multicolor blackbody spectrum with peak
temperature 150-200 eV. The RXTE data sets also show evidence of an Fe line
with equivalent widths 20-140 eV. Reflection models show a hardening of the
RXTE spectra with decreasing X-ray flux; however, these models do not exhibit
evidence of a correlation between the photon index of the incident power law
flux and the solid angle subtended by the reflector. `Sphere+disk'
Comptonization models and Advection Dominated Accretion Flow (ADAF) models also
provide reasonable descriptions of the RXTE data. The former models yield
coronal temperatures in the range 20-50 keV and optical depths of \tau ~ 3. The
model fits to the X-ray data, however, do not simultaneously explain the
observed radio properties. The most likely source of the radio flux is
synchrotron emission from an extended outflow of size greater than O(10^7
GM/c^2).Comment: 18 pages in latex emulateapj.sty. Accepted for publication in the
Astrophysical Journa
An X-ray variable absorber within the Broad Line Region in Fairall 51
Fairall 51 is a polar-scattered Seyfert 1 galaxy, a type of active galaxies
believed to represent a bridge between unobscured type-1 and obscured type-2
objects. Fairall 51 has shown complex and variable X-ray absorption but only
little is known about its origin. In our research, we observed Fairall 51 with
the X-ray satellite Suzaku in order to constrain a characteristic time-scale of
its variability. We performed timing and spectral analysis of four observations
separated by 1.5, 2 and 5.5 day intervals. We found that the 0.5-50 keV
broadband X-ray spectra are dominated by a primary power-law emission (with the
photon index ~ 2). This emission is affected by at least three absorbers with
different ionisations (log(xi) ~ 1-4). The spectrum is further shaped by a
reprocessed emission, possibly coming from two regions -- the accretion disc
and a more distant scattering region. The accretion disc emission is smeared by
the relativistic effects, from which we measured the spin of the black hole as
a ~ 0.8 (+-0.2). We found that most of the spectral variability can be
attributed to the least ionised absorber whose column density changed by a
factor of two between the first (highest-flux) and the last (lowest-flux)
observation. A week-long scale of the variability indicates that the absorber
is located at the distance ~ 0.05 pc from the centre, i.e., in the Broad Line
Region.Comment: 12 pages, 9 figures, accepted to A&
A Clumpy Stellar Wind and Luminosity-Dependent Cyclotron Line Revealed by The First Suzaku Observation of the High-Mass X-ray Binary 4U 1538-522
We present results from the first Suzaku observation of the high-mass X-ray
binary 4U 1538-522. The broad-band spectral coverage of Suzaku allows for a
detailed spectral analysis, characterizing the cyclotron resonance scattering
feature at keV and the iron K line at
keV, as well as placing limits on the strengths of the iron K line and
the iron K edge. We track the evolution of the spectral parameters both in time
and in luminosity, notably finding a significant positive correlation between
cyclotron line energy and luminosity. A dip and spike in the lightcurve is
shown to be associated with an order-of-magnitude increase in column density
along the line of sight, as well as significant variation in the underlying
continuum, implying the accretion of a overdense region of a clumpy stellar
wind. We also present a phase-resolved analysis, with most spectral parameters
of interest showing significant variation with phase. Notably, both the
cyclotron line energy and the iron K line intensity vary significantly
with phase, with the iron line intensity significantly out-of-phase with the
pulse profile. We discuss the implications of these findings in the context of
recent work in the areas of accretion column physics and cyclotron resonance
scattering feature formation.Comment: 15 pages, 8 figures. Accepted to ApJ on 2 July 201
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
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