Discovery of a 50 minute binary period and a likely 22 magnitude optical counterpart for the X-ray burster 4U 1915-05

Abstract

We have observed absorption dips which recur with a period of 2985 s from the X-ray burst source 4U 1915—05 (= MXB 1916 — 05). We suggest that the dips are caused by obscuration of the X-ray source by material at the point where the gas stream from the companion meets the accretion disk, and that the 2985 s periodicity is the orbital period of the binary system. Assuming this model is appropriate, these observations represent the first direct evidence of the binary nature of X-ray burst sources. We have also observed secondary dips that occur approximately half a cycle away from the primary dips; these secondary dips probably reflect the complex interaction of the gas stream with the star. We suggest a new optical identification of 4U 1915 — 05: a 22d magnitude candidate observed in a CCD image of the optical field at the arcsecond-accurate HRI X-ray position. The X-ray emitting region must have an extent of less than ~ 10 8 cm for reasonable system parameters.