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 (also known as soft X-ray transients). These binary systems are similar in all respects except that the former con tains a black hole and the latter contains a neutron star. A UV spectrum (1 700-3100 ) is presented for the quiescent state of each system in the conte xt of previously published UV/optical and X-ray data. The nonstellar, conti nuum spectrum of black hole A0620-00 has a prominent UV/optical peak center ed at similar to 3500 Angstrom. In contrast the spectrum of neutron star Ce n X-4 lacks a peak and rises steadily with frequency over the entire UV/opt ical band. In the optical, the two systems are comparably luminous. However , black hole A0620-00 is similar to 6 times less luminous at 1700 Angstrom, and similar to 40 times less luminous in the X-ray band. The broadband spe ctrum of A0620-00 is discussed in terms of the advection-dominated accretio n flow model.