An optical study of BG Geminorum: An ellipsoidal binary with an unseen primary star

摘要

We describe optical photometric and spectroscopic observations of the bright variable BG Geminorum. Optical photometry shows a pronounced ellipsoidal variation of the K0 I secondary, with amplitudes of similar to 0.5 mag at VRCIC and a period of 91.645 days. A deep primary eclipse is visible for lambda less than or similar to 4400 Angstrom; a shallower secondary eclipse is present at longer wavelengths. Eclipse timings and the radial velocity curve of the K0 secondary star indicate an interacting binary where a lobe-filling secondary, M-2 similar to 0.5 M., transfers material into a extended disk around a massive primary, M-1 similar to 4.5 M.. The primary star is either an early B-type star or a black hole. If it did contain a black hole, BG Gem would be the longest period black hole binary known by a factor of 10, as well as the only eclipsing black hole binary system. References: 51