Discovery of the accretion-powered millisecond pulsar SWIFT J1756.9-2508 with a low-mass companion

We report on the discovery by the Swift Gamma- Ray Burst Explorer of the eighth known transient accretion- powered millisecond pulsar, SWIFT J1756.9 - 2508, as part of routine observations with the Swift Burst Alert Telescope hard X- ray transient monitor. The pulsar was subsequently observed by both the X- Ray Telescope on Swift and the Rossi X- Ray Timing Explorer Proportional Counter Array. It has a spin frequency of 182 Hz ( 5.5 ms) and an orbital period of 54.7 minutes. The minimum companion mass is between 0.0067 and, depending on the mass of the 0.0086 M-circle dot, neutron star, and the upper limit on the mass is 0.030 M-circle dot ( 95% confidence level). Such a low mass is inconsistent 0.030 M, with brown dwarf models, and comparison with white dwarf models suggests that the companion is a He- dominated donor whose thermal cooling has been at least modestly slowed by irradiation from the accretion flux. No X- ray bursts, dips, eclipses, or quasi- periodic oscillations were detected. The current outburst lasted approximate to 13 days, and no earlier outbursts were found in archival data.