Joint Astrophysics Colloquium

The Puzzles of Very Faint X-ray Transients

Craig Heinke

University of Alberta

The evolution and final fate of low-mass X-ray binaries (LMXBs) are difficult to model, due to the wide range of physics that must be included (from common envelopes, to radiative efficiency, to the transition from neutron star LMXBs to millisecond pulsars). Simple calculations indicate that the majority of LMXBs should have short orbital periods (<~3 hours) and low mass-transfer rates, which suggest they should have infrequent, short, low-luminosity outbursts.

Such outbursts are difficult to observe with current X-ray monitoring missions (which are generally not sensitive enough at typical Galactic distances), or with pointed X-ray telescopes (due to their small field of view). However, long-term monitoring of the Galactic Center (by Degenaar, Wijnands, Muno, etc.) has revealed a population of very faint X-ray transients with peak luminosities of 10³⁵-10³⁶ ergs/s. This suggests that the expected population of old LMXBs may have been found, though their location in the crowded, heavily extincted Galactic Center prevents follow-up to identify the donor stars and characterize the systems.

We have just been awarded a large Swift proposal, the Swift Galactic Bulge Survey, to survey 16 square degrees of the Galactic Bulge every two weeks for ~8 months, which should find ~15 new very faint X-ray transients. Through multiwavelength follow-up (possible at the lower typical extinctions of our survey), we should be able to identify their natures. This will help resolve key questions about how the LMXB population ages, including the transition of NS LMXBs to millisecond pulsars, and whether black hole LMXBs become more radiatively inefficient at short orbital periods, enabling them to “disappear”.