We have studied the long-term (~ years) temporal variability of the prototype supersoft X-ray source (SSS) CAL 83 in the LMC, using data from the MACHO and OGLE projects. The CAL 83 light curve exhibits dramatic brightness changes of ~1 mag on timesc
ales of ~450 days, and spends typically ~200 days in the optical low state. Combined with archival XMM-Newton X-ray observations these represent the most extensive X-ray/optical study to date of this system, and reveal in much greater detail that the X-ray light curve is anti-correlated with the optical behaviour. This is remarkably similar to the behaviour of the transient SSS, RX J0513.9-6951, where the SSS outbursts recur on a timescale of ~168 days, and also anti-correlate with the optical flux. We performed simple blackbody fits to both high and low state X-ray spectra, and find that the blackbody temperature and luminosity decrease when the optical counterpart brightens. We interpret these long-term variations in terms of the limit-cycle model of Hachisu & Kato (2003a), which provides further support for these systems containing massive (~1.3 Msun) white dwarfs. In addition, we have refined their orbital periods in the MACHO and OGLE-III light curves to values of 1.047529(1) days and 0.762956(5) days for CAL 83 and RX J0513.9-6951, respectively.
We report on our search for very-long-term variability (weeks to years) in X-ray binaries (XRBs) in the giant elliptical galaxy M87. We have used archival Chandra imaging observations to characterise the long-term variability of 8 of the brightest me
mbers of the XRB population in M87. The peak brightness of some of the sources exceeded the ultra luminous X-ray source (ULX) threshold luminosity of ~ 10^{39} erg/s, and one source could exhibit dips or eclipses. We show that for one source, if it has similar modulation amplitude as in SS433, then period recoverability analysis on the current data would detect periodic modulations, but only for a narrow range of periods less than 120 days. We conclude that a dedicated monitoring campaign, with appropriately defined sampling, is essential if we are to investigate properly the nature of the long-term modulations such as those seen in Galactic sources.
We have discovered a sim420d modulation, with associated X-ray dips, in RXTE-ASM/MAXI/Swift-BAT archival light-curves of the short-period (3.2h) black-hole X-ray transient, Swift J1753.5-0127. This modulation only appeared at the end of a gradual reb
rightening, approximately 3 years after the initial X-ray outburst in mid-2005. The same periodicity is present in both the 2-20 keV and 15-50 keV bands, but with a sim0.1 phase offset (sim40d). Contemporaneous photometry in the optical and near-IR reveals a weaker modulation, but consistent with the X-ray period. There are two substantial X-ray dips (very strong in the 15-50 keV band, weaker at lower energies) that are separated by an interval equal to the X-ray period. This likely indicates two physically separated emitting regions for the hard X-ray and lower energy emission. We interpret this periodicity as a property of the accretion disc, most likely a long-term precession, where the disc edge structure and X-ray irradiation is responsible for the hard X-ray dips and modulation, although we discuss other possible explanations, including Lense-Thirring precession in the inner disc region and spectral state variations. Such precession indicates a very high mass ratio LMXB, which even for a sim10M_sun BH requires a brown dwarf donor (sim0.02M_sun), making Swift J1753.5-0127 a possible analogue of millisecond X-ray pulsars.We compare the properties of Swift J1753.5-0127 with other recently discovered short-period transients, which are now forming a separate population of high latitude BH transients located in the galactic halo.
A Chandra observation of the Large Magellanic Cloud supernova remnant DEM L241 reveals an interior unresolved source which is probably an accretion-powered binary. The optical counterpart is an O5III(f) star making this a High-Mass X-ray Binary (HMXB
) with orbital period likely to be of order tens of days. Emission from the remnant interior is thermal and spectral information is used to derive density and mass of the hot material. Elongation of the remnant is unusual and possible causes of this are discussed. The precursor star probably had mass > 25 solar masses
