MONDAY, June 26, 2017

Astronomy Tea Talk, 4 pm, 312 Cahill

"A discovery of a dormant stellar black hole orbiting a giant star in the LMC," Professor Tsevi Mazeh, Tel Aviv University.

Abstract: All stellar black holes (BHs) found in binary systems were discovered by detecting their X-ray luminosity, which is generated by matter transferred from the optical stellar companion and falls onto the horizon of the BH. The bright X-ray phase of these systems lasts as long as the optical companion is large enough to spill mass onto its compact companion, and therefore the active phase can be short on stellar evolution timescale, leaving most BHs in binaries dormant. The talk will report on the discovery of a dormant BH in the Large Magellanic Cloud (LMC) which is being orbited by a giant evolved star. The giant, DBH1, was identified by the OGLE project as displaying large ellipsoidal periodic brightness modulation induced by an unseen companion. Radial velocities showed that DBH1 has a large modulation of about 70 km/s with the same period as the photometric variation of 101 d, indicating a massive companion. Following this analysis, and performing further spectroscopy, we have found that the mass of the unseen companion of DBH1 is 8 ± 1.5 M_solar, suggesting a BH. The spectra reveal double-peaked H_alpha (and faint H_beta) emission, probably emitted by an optically thin disc around the BH, typically seen in BH binaries during their quiescence period. The previously known BHs in low-mass binary systems have short orbital periods, with a median of about 0.5 d. These systems were probably formed through a common envelope phase through which the optical star spiraled-in within the envelope of the BH progenitor to form a short-period binary. DBH1, with separation of about 1 AU, probably went through a different evolutionary track, the results of which have not been observed before.