Soft X-ray transients in quiescence : observations of AQL X-1 and CenX-4.

The authors observed the soft X-ray transients Aql X-1 and Cen X-4 during quiescence with EXOSAT. For Aql X-1 they obtain a 3σ upper limit to the X-ray luminosity L_x (in the energy range between 0.5 and 4.5 keV) of <3.0×10³³erg s^-1, Cen X-4 was detected at L_x (0.5 - 4.5 keV) ≅ 1.5 to 4.2×10³³erg s^-1. An optical spectrum of Cen X-4 obtained two years after its 1979 outburst shows a K3 V continuum with Balmer emission lines. Combining their EXOSAT observations with a previous EINSTEIN detection the authors argue that the X-ray luminosity of Cen X-4 is due to continued accretion, and not to thermal radiation of a cooling neutron star.