7,828 research outputs found
X-ray emission from the PSR B1259--63 system near apastron
The PSR B1259--63 system contains a 47 ms radio pulsar in a highly eccentric
binary with a Be-star companion. Strongly time variable X-ray emission was
reported from this system as the pulsar was near apastron in 1992-early 1993.
The variability was primarily deduced from an apparent non-detection of the
\psr system during a first pre-apastron \ros observation in February~1992. We
have re-analyzed the \ros observations of the \psr system. Contrary to the
results of a previous analysis, we find that the \psr system was detected by
\ros during the first off-axis February~1992 observation. The intensity of the
soft X-ray emission of the \psr system before and after the 1992 apastron
appears to vary at most by a factor . Our results sensibly constrain
theoretical models of X-ray emission from the \psr system.Comment: LATEX, Accepted for publ. in ApJ
On the kHz QPO frequency correlations in bright neutron star X-ray binaries
We re-examine the correlation between the frequencies of upper and lower kHz
quasi-periodic oscillations (QPO) in bright neutron-star low-mass X-ray
binaries. By including the kHz QPO frequencies of the X-ray binary Cir X-1 and
two accreting millisecond pulsars in our sample, we show that the full sample
does not support the class of theoretical models based on a single resonance,
while models based on relativistic precession or Alfven waves describe the data
better. Moreover, we show that the fact that all sources follow roughly the
same correlation over a finite frequency range creates a correlation between
the linear parameters of the fits to any sub-sample.Comment: Accepted for publication in MNRAS; 7 pages, 4 figure
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 evolution of the high-energy cut-off in the X-Ray spectrum of the GX 339-4 across a hard-to-soft transition
We report on X-ray observations of the black-hole candidate GX 339-4 during
its 2006/2007 outburst. The hardness-intensity diagram of all RXTE/PCA data
combined shows a q-shaped track similar to that observed in previous outbursts.
The evolution through the HID suggests that in the early phase of the outburst
the source underwent a sequence of state transitions, from the hard to the soft
state, which is supported by our timing analysis. Broadband (4-200 keV)
spectra, fitted with an exponentially cutoff powerlaw, show that the hard
spectral component steepens during the transition from the hard to the soft
state. The high-energy cutoff decreased monotonically from 120 to 60 keV during
the brightening of the hard state, but increased again to 100 keV during the
softening in the hard intermediate state. In the short-lived soft intermediate
state the cutoff energy was ~ 130 keV, but was no longer detected in the soft
state. This is one of the first times that the high-energy cut-off has been
followed in such detail across several state transitions. We find that in
comparison to several other spectral parameters, the cut-off energy changes
more rapidly, just like the timing properties. The observed behaviour of the
high energy cutoff of GX 339-4 is also similar to that observed with
RXTE-INTEGRAL-Swift during the 2005 outburst of GRO J1655-40. These results
constitute a valuable reference to be considered when testing theoretical
models for the production of the hard component in these systems.Comment: 16 pages, 8 figures, 5 tables, accepted for publication in MNRAS Main
Journa
Variability of X-ray binaries from an oscillating hot corona
The spectral and timing properties of an oscillating hot thermal corona are
investigated. This oscillation is assumed to be due to a magneto-acoustic wave
propagating within the corona and triggered by an external, non specified,
excitation. A cylindrical geometry is adopted and, neglecting the rotation, the
wave equation is solved in for different boundary conditions. The resulting
X-ray luminosity, through thermal comptonization of embedded soft photons, is
then computed, first analytically, assuming linear dependence between the local
pressure disturbance and the radiative modulation. These calculations are also
compared to Monte-Carlo simulations. The main results of this study are: (1)
the corona plays the role of a low band-pass medium, its response to a white
noise excitation being a at top noise Power Spectral Density (PSD) at low
frequencies and a red noise at high frequency, (2) resonant peaks are present
in the PSD. Their powers depend on the boundary conditions chosen and, more
specifically, on the impedance adaptation with the external medium at the
corona inner boundary. (3) The flat top noise level and break as well as the
resonant peak frequencies are inversely proportional to the external radius rj.
(4) Computed rms and f-spectra exhibit an overall increase of the variability
with energy. Comparison with observed variability features, especially in the
hard intermediate states of X-ray binaries are discussed.Comment: 12 pages, 7 figures, accepted for publication in MNRA
A Disk--Jet interaction model for the X--Ray Variability in Microquasars
We propose a simple dynamical model that may account for the observed
spectral and temporal properties of GRS 1915+105 and XTE J1550-5634. The model
is based on the assumption that a fraction of the radiation emitted by a hot
spot lying on the accreting disk is dynamically Comptonized by the relativistic
jet that typically accompanies the microquasar phenomenon. We show that
scattering by the jet produces a detectable modulation of the observed flux. In
particular, we found that the phase lag between hard and soft photons depends
on the radial position of the hot spot and, if the angle between the jet and
the line of sight is sufficiently large, the lags of the fundamental and its
harmonics may be either positive or negative.Comment: 14 pages, 4 figures, accepted for publication in ApJ Part
A Unified Description of the Timing Features of Accreting X-ray Binaries
We study an empirical model for a unified description of the power spectra of
accreting neutron stars and black holes. This description is based on a
superposition of multiple Lorentzians and offers the advantage that all QPO and
noise components are dealt with in the same way, without the need of deciding
in advance the nature of each component. This approach also allows us to
compare frequencies of features with high and low coherences in a consistent
manner and greatly facilitates comparison of power spectra across a wide range
of source types and states. We apply the model to six sources, the
low-luminosity X-ray bursters 1E 1724-3045, SLX 1735-269 and GS 1826-24, the
high-latitude transient XTE J1118+480, the bright system Cir X-1, and the Z
source GX 17+2. We find that it provides a good description of the observed
spectra, without the need for a scale-free (1/f) component. We update
previously reported correlations between characteristic frequencies of timing
features in the light of this new approach and discuss similarities between
different types of systems which may point towards similar underlying physics.Comment: 13 pages, to appear in The Astrophysical Journa
The 2009 outburst of H~1743-322 as observed by RXTE
We analyze the RXTE observations of the 2009 outburst of H~1743-322, as well
as the observations of the previous five outbursts for comparison. The
hardness-intensity diagram (HID) shows a complete counter-clockwise q-track for
the 2009 outburst and, interestingly, the track falls in} between a huge one in
2003, with a complete transition to high/soft state, and that of} the failed
outburst in 2008. It leaves the low/hard state but does not reach the leftmost
edge of the overall HID. While the lowest hardness (6--19 keV/3--6 keV) values}
in the HID is about 0.3--0.4 for the 2009 outburst, similar to the ``failed
state transition" seen in the persistent black hole XRB Cyg X-1, the timing
analysis shows that a transition to the high soft state occurred. During the
low/hard state of the 2009 outburst, the inner radius of the accretion disk is
found to be closer to the central black hole and have an anti-correlation with
the disk temperature. These results may be understood as the reprocessing} of
the hot corona on the disk's} soft X-rays, which can lead to an underestimation
of the inner radius of the accretion disk. In the luminosity diagram of the
corona versus the disk, the tracks of the outbursts} in 2003 and 2009 cross the
line which represents a roughly equal contribution to the entire emission from
the thermal and the non-thermal components;} the track of the 2008 outburst has
the turn-over falling} on this line. This may be indicative of an emission
balance between the corona and the disk, which prevents the state transition
from going further than the low/hard state.Comment: accepted by A&
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