The very low mass X-ray binary pulsars: A new class of sources?

While the distribution of spin periods of high-mass X-ray binaries spans more than four orders of magnitude (69 ms-25 minutes) the few known X-ray pulsars accreting from very low mass companions (less than 1 solar mass) have very similar periods between 5.4 and 8.7 s. These pulsars also display several other similarities, and we propose that they are members of a subclass of low-mass X-ray binaries (LMXBs) with similar magnetic field (a few times 1011 G), companion stars and, possibly, evolutionary histories. If they are rotating at, or close to, the equilibrium period, their properties are consistent with luminosities of the order of a few times 1035 ergs/s. These pulsars might represent the closest members of a subclass of LMXBs characterized by lower luminosities, higher magnetic fields, and smaller ages than nonpulsating LMXBs.