A new class of X-ray Pulsars?

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While the distribution of spin periods of High Mass X-ray Binaries (HMXBs) spans more than four orders of magnitude (69ms - 25min), the few known X-ray pulsars accreting from very low mass companions (<1 Msun) have very similar periods between 5.4 and 8.7s. These pulsars display also several other similarities and they are probably members of a subclass of Low Mass X-ray Binaries (LMXBs) with similar magnetic field (a few 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 10³5 erg/s. These pulsars might represent the closest members of a subclass of LMXBs characterized by lower luminosities, higher magnetic fields and smaller ages than non-pulsating LMXBs.