HST/STIS UV Spectroscopy of Two Quiescent X-Ray Novae: A0620-00 and Centaurus X-4

Abstract

In 1998 we made UV spectroscopic observations with HST/STIS of A0620-00 and Cen X-4, which are two X-ray novae (also known as soft X-ray transients). These binary systems are similar in all respects except that the former contains a black hole and the latter contains a neutron star. A UV spectrum (1700-3100 Å) is presented for the quiescent state of each system in the context of previously published UV/optical and X-ray data. The nonstellar, continuum spectrum of black hole A0620-00 has a prominent UV/optical peak centered at ~3500 Å. In contrast the spectrum of neutron star Cen X-4 lacks a peak and rises steadily with frequency over the entire UV/optical band. In the optical, the two systems are comparably luminous. However, black hole A0620-00 is ~6 times less luminous at 1700 Å, and ~40 times less luminous in the X-ray band. The broadband spectrum of A0620-00 is discussed in terms of the advection-dominated accretion flow model. Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.