Orbital and Superorbital Periods of 1E 1740.7−2942 and GRS 1758−258

Abstract

GRS 1758-258, 1E 1740.7-2942, and Cyg X-1 are the three persistently bright black-hole candidates in the Galaxy. Weekly monitoring of the first two by the \it Rossi X-ray Timing Explorer (RXTE) has produced the first detection of their orbital periods: 18.45 +/- 0.03 dy for GRS 1758-258 and 12.70 +/- 0.03 dy for 1E 1740.7-2942. Such long periods require the companion stars to be giants in order to fill their Roche lobes. I- and K-band observations of the field around GRS 1758-258 (Martí et al. 1998, A&A 338, L95) produced two candidates, a K giant and a main-sequence F star. The orbital period points to the former as the correct companion. In addition, both objects show long periods, presumably from disk precession: ~ 625 dy in 1E 1740.7-2942 and ~ 500 dy in GRS 1758-258, the longest such periods on record. Additional evidence for very large accretion disks comes from long-term observations of x-ray spectral evolution, which differ fundamentally from the state changes in Cyg X-1. This work was supported by NASA grant NAG5-7265 and uses public data provided by the \it RXTE ASM team.