Hst/stis Uv Spectroscopy of Two Quiescent X-ray Novae: A0620-00 and Centaurus X-4

In 1998 we made UV spectroscopic observations with HST/STIS of A0620-00 and Cen X-4, which are two X-ray novae (aka 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 (1700-3100Å) is presented for the quiescent state of each system in the context of previously published UV/optical and X-ray data. The non-stellar, continuum spectrum of black hole A0620-00 has a prominent UV/optical peak centered at ∼ 3500Å. 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 1700Å, 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. Subject headings: black hole physics — accretion, accretion disks — stars: individual (A0620-00; Cen X-4) — X-rays: stars