Observations of Soft X-ray Transients & related dwarf novae

The Soft X-ray Transients (SXTs) are a sub-class of low-mass X-ray binary (LMXB) with many similarities to dwarf novae. They are short-period (5hrs - 6days) binaries with mostly K-M type secondaries. Their rare, dramatic X-ray outbursts (typically separated by decades) can reach very high luminosities (> 10(38)erg s(-1)) and are considered to be accretion disc instability events. Remarkably, a very high fraction of SXTs are black-hole candidates. This is largely because they are the only LMXBs for which accurate mass determinations are possible, through the ability to undertake dynamical studies of the secondary star during the long periods of quiescence. The key X-ray, optical and IR properties of the SXTs, largely from observations made over the last 5 years, are summarised. The main similarity with dwarf novae is in the high mass ratios found in SXTs, which are then analogous to the SU UMa (and WZ Sge/TOAD) classes of dwarf novae. Indeed, their outburst light curves are remarkably similar. However, two recent X-ray transients exhibited radio superluminal events (GRO J1655-40 and GRS 1915+105) similar to those seen in AGN. With a much earlier (mid-F star) secondary in J1655-40 (it is the brightest SXT in quiescence), their properties will be compared and contrasted with those of other SXTs.