Keck spectroscopy of millisecond pulsar J2215+5135: A moderate- M_NS, high-inclination binary

We present Keck spectroscopic measurements of the millisecond pulsar binary J2215+5135. These data indicate a neutron-star (NS) mass $M_{NS}=1.6M_{\odot }$, much less than previously estimated. The pulsar heats the companion face to $T_D\approx 9000$ K; the large heating efficiency may be mediated by the intrabinary shock dominating the X-ray light curve. At the best-fit inclination i = 88.°8, the pulsar should be eclipsed. We find weak evidence for such eclipses in the pulsed gamma-rays; an improved radio ephemeris allows use of up to five times more Fermi-Large Area Telescope gamma-ray photons for a definitive test of this picture. If confirmed, the gamma-ray eclipse provides a novel probe of the dense companion wind and the pulsar magnetosphere.