X-ray properties of 4U 1543-624
  4U 1543-624 is a relatively bright persistent low-mass X-ray binary. Analysis of archival data from ASCA, SAX and RXTE is presented. The X-ray continuum be can modeled with the standard low-mass X-ray binary spectrum, an isothermal blackbody and a Comptonized component. Variations in the luminosity and flux ratio of the continuum components are seen. An increase in luminosity is accompanied by a decrease in the blackbody luminosity and a hardening of the spectrum. Most low-mass X-ray binaries have softer spectra and higher blackbody luminosities in high luminosity states. The Fe K-alpha line is seen only in the high luminosity spectra. A narrow feature near 0.7 keV, previously detected in the ASCA data, is also seen in the SAX data. A qualitative model of the system is presented. The X-ray observations can be explained by a low inclination system (face-on disk) containing a slowly (P >> ms) rotating neutron star. A slowly rotating neutron star would imply either that the system is a young low-mass X-ray binary, or that the accretion rate is unusually low. The empirical relation between optical and X-ray luminosity and orbital period suggests a relatively short period. 
J. Schultz (Observatory, University of Helsinki)