422 research outputs found
Studies of highly variable galactic X-ray sources with HEAO-1
Analysis HEAO-Al data on MXB1659-29 revealed a 7.1 hour period from this X-ray burst source. The erratic dips seen in both SAS-3 and HEAO-Al data were concentrated within 1.5 hours (about 25%) of the orbital cycle. There appeared to be a stable dip at the end of the 1.5 h of erratic varibility. It was established that the stable dip was a true eclipse of the central X-ray emitting object. MXB1659-29 is the first X-ray burster to show eclipses and a precise orbital clock
Complex outburst behaviour from the black-hole candidate 4U 1630-47
We present data from different epochs in 1978, 1987-1991 and 1996 from the
black-hole candidate 4U1630-47. For the first time we present almost complete
coverage of the outbursts which started in 1987, 1988 and 1996. We find that
the outburst behaviour of 4U1630-47 is more complex than previously realized.
The source shows outbursts with durations on the order of ~100-200 days and
sometimes intervals of long-term X-ray activity.
The relatively short outbursts which occurred in 1987 and 1996 exhibited
different colour behaviour: the outburst in 1987 showed softening of the X-ray
spectrum, whereas the outburst in 1996 showed hardening of the X-ray spectrum,
as the outbursts proceeded.
The outburst which started in 1977 may have lasted for up to ~10 months,
whereas the outburst which started in 1988 showed activity for up to ~2.4
years. Such long-term activity is reminiscent of that seen in GRS1716-249 and
in the galactic superluminal sources GRS1915+105 and GROJ1655-40.
We refine the outburst ephemeris of 4U\1630-47 and find that the outburst
recurrence time scale may have changed from ~600 days to ~690 days between the
1984 and 1987 outbursts. If the recurrence interval of ~690 days continues, the
next outburst of 4U1630-47 is predicted to occur in 1998 January.Comment: 30 pages including 9 figures accepted for publication in MNRA
BeppoSAX observation of the eclipsing dipping X-ray binary X1658-298
Results of a 2000 August 12-13 BeppoSAX observation of the 7.1 hr eclipsing,
dipping, bursting, transient, low-mass X-ray binary (LMXRB) X1658-298 are
presented. The spectrum outside of eclipses, dips and bursts can be modeled by
the combination of a soft disk-blackbody and a harder Comptonized component
with a small amount (1.3 10E21 atom/cm2) of low-energy absorption. In contrast,
an RXTE observation 18 months earlier during the same outburst, measured an
absorption of 5.0 10E22 atom/cm2. Such a change is consistent with a thinning
of the accretion disk as the outburst progresses. Structured residuals from the
best-fit spectral model are present which are tentatively identified with
Ne-K/Fe-L and Fe-K shell emission. The spectral changes during dips are complex
and may be modeled by a strong (~3 10E23 atom/cm2) increase in absorption of
the Comptonized component only, together with reductions in normalizations of
both spectral components. This behavior is in contrast to the ``complex
continuum'' model for X-ray dip sources, where the softer blackbody component
rapidly suffers strong absorption. It is however, similar to that found during
recent XMM-Newton observations of the eclipsing, dipping, LMXRB EXO0748-676.Comment: 11 pages. Accepted for publication in A&A
Anisotropic inverse Compton scattering of photons from the circumstellar disc in PSR B1259-63
The gamma-ray binary system PSR B1259-63 consists of a 48 ms pulsar orbiting
a Be star. The system is particularly interesting because it is the only
gamma-ray binary system where the nature of the compact object is known. The
non-thermal radiation from the system is powered by the spin-down luminosity of
the pulsar and the unpulsed radiation originates from the stand-off shock front
which forms between the pulsar and stellar wind. The Be star/optical companion
in the system produces an excess infrared flux from the associated
circumstellar disc. This infrared excess provides an additional photon source
for inverse Compton scattering. We discuss the effects of the IR excess near
periastron, for anisotropic inverse Compton scattering and associated gamma-ray
production. We determine the infrared excess from the circumstellar disc using
a modified version of a curve of growth method, which takes into account the
changing optical depth through the circumstellar disc during the orbit. The
model is constrained using archive data and additional mid-IR observations
obtained with the VLT during January 2011. The inverse Compton scattering rate
was calculated for three orientations of the circumstellar disc. The predicted
gamma-ray light curves show that the disc contribution is a maximum around
periastron and not around the disc crossing epoch. This is a result of the disc
being brightest near the stellar surface. Additional spectroscopic and
near-infrared observations were obtained of the system and these are discussed
in relation to the possibility of shock heating during the disc crossing epoch.Comment: 15 pages, 14 figures and 4 tables. MNRAS in press. Updated
references, correction of typos in a few of the equations in sec. 5.2 and
appendix, and other minor typo correction
A Search for Aperiodic Millisecond Variability in Cygnus X-1
We have conducted a search for aperiodic millisecond variability in the
integrated 1 to 25 keV X-ray region of Cyg X-1. We have examined HEAO A-1
archival data and Rossi X-ray Timing Explorer (RXTE) guest observer data for
evidence of excess power above the Poisson noise floor using the relative
integral power analysis and the Fourier transform method. Our results are in
disagreement with the results of Meekins et al. (1984). We attribute the
discrepancy to an instrumental effect for which Meekins et al.(1984) did not
apply a correction. With the correction we see no evidence for excess power
above 25 Hz in the HEAO A-1 data. Our analysis of RXTE data is in agreement
with previously published results of different data sets and shows no sign of
excess power above 30 Hz.Comment: 29 pages, 12 figures, to be submitted to Ap
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