X-ray Transients Observed with the RXTE All Sky Monitor 1996-1997

Abstract

Highlights from the RXTE All Sky Monitor (ASM) during 1996 and 1997 are reviewed with particular attention to X-ray transients. The ASM has detected 117 sources. These include 12 recurrent transients and 10 new X-ray sources, some of which began their outbursts before the launch of RXTE. The majority of the outburst profiles are strikingly different from the classical form, with its fast rise and slow decay. Some of the light curves appear quasi-persistent, which suggests that they are associated with secular changes in the accretion rate from the donor star rather than with a cyclic instability in the accretion disk. The nature of the compact object is uncertain in many cases, but there is a likelihood that the majority of new sources are black hole systems, while the majority of recurrent transients contain neutron stars. There is a correlation between the gross shape of the energy spectrum, as seen in the ASM hardness ratio, HR2 (5-12 keV / 3-5 keV), and the accretion subclass. These data, with important contributions from SAX, CGRO, and Granat, provide a deeper probe of the X-ray sky than has been available previously. However, there are important systematic effects to be investigated before a meaningful evaluation of transient occurrence rates can be derived.

Comment: 8 pages, including 2 figs.; latex; memsait.sty epsf.sty in Procs. of Frascati Workshop 1997, "Multifrequency Behavior of High Energy Cosmic Sources", (Vulcano, May 1997), eds. F. Giovannelli & L. Sabau-Graziati, in press