ASCA observations of soft X-ray transients in quiescence: X1608-52 and Cen X-4

Abstract

We observed soft X-ray transients, X1608-52 and Cen X-4, in quiescence with ASCA at X-ray luminosities on the order of 1032-33 erg s-1 . The energy spectra were rather soft for both sources. Blackbody fits to the data required characteristic temperatures of 0.2-0.3 keV and emission regions of ∼ 10 km2. A conspicuous hard tail remained after fitting Cen X-4 with a blackbody curve. An extremely soft spectrum in quiescence seems to be a common property of soft X-ray transients.