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

Discovery of narrow X-ray absorption features from the low-mass X-ray binary X 1254-690 with XMM-Newton

We report on two XMM-Newton observations of the low-mass X-ray binary X 1254-690. During an XMM-Newton observation of the low-mass X-ray binary in 2001 January a deep X-ray dip was seen while in a second observation one year later no dips were evident. The 0.5-10 keV EPIC spectra from both non-dipping intervals are very similar being modeled by a disk-blackbody and a power-law continuum with additional structure around 1 keV and narrow absorption features at 7.0 keV and 8.2 keV which are identified with the K alpha and K beta absorption lines of Fe XXVI. The low-energy structure may be modeled as a 175 eV (sigma) wide emission line at ~0.95 keV. This feature is probably the same structure that was modeled as an absorption edge in an earlier BeppoSAX observation. The absorption line properties show no obvious dependence on orbital phase and are similar in both observations suggesting that the occurrence of such features is not directly related to the presence of dipping activity. Narrow Fe absorption features have been observed from the two superluminal jet sources GRO J1655-40 and GRS 1915+105, and the four low-mass X-ray binaries GX 13+1, MXB 1658-298, X 1624-490 and X 1254-690. Since the latter 3 sources are dipping sources, which are systems viewed close to the accretion disk plane, and the two microquasars are thought to be viewed at an inclination of ~70 degrees, this suggests that these features are more prominent when viewed at high-inclination angles. This, together with the lack of any orbital dependence, implies a cylindrical geometry for the absorbing material.Comment: 7 pages, 9 postscript figures, accepted for publication in Astronomy and Astrophysic