A Millisecond Pulsar in a 6 Hour Orbit: PSR J0751+1807

A 3.48 ms pulsar in a 6.3 hr binary orbit has been discovered in a systematic survey of error boxes around unidentified gamma-ray sources. The spin-down energy of the pulsar is not sufficient to power the estimated γ-ray emission from the putative γ-ray source. The mass function, small orbit, low eccentricity, and large timing age imply that the companion is most likely a 0.12 M_sun <m_c < 0.6 M_sun white dwarf. Lack of eclipses or propagation delays varying with orbital phase imply that the companion is well contained inside its Roche lobe. Pulsar J0751 + 1807 fits into a class of small circular orbit (<30 days) objects which have undergone spiral-in. Examining the known population of short orbital period objects, we find a positive correlation between orbital period and magnetic field strength.