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 0.0086 M _ { \odot } , depending on the mass of the neutron star , and the upper limit on the mass is 0.030 M _ { \odot } ( 95 % confidence level ) .
Such a low mass is inconsistent 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 \approx 13 days and no earlier outbursts were found in archival data .