The Intrabinary Shock and Companion Star of Redback Pulsar J2215+5135

PSR J2215+5135 (J2215) is a `redback' spider pulsar, where the intrabinary shock (IBS) wraps around the pulsar rather than the stellar-mass companion. Spider orbital light curves are modulated, dominated by their binary companion thermal emission in the optical bands and by IBS synchrotron emission in the X-rays. We report on new XMM-Newton X-ray and U-band observations of J2215. We produce orbital light curves and use them to model the system properties. Our best-fit optical light model gives a neutron star mass $M_{NS}=1.98\pm0.08$ M$_\odot$, lower than previously reported. However, uncertainty in the stellar atmosphere metallicity, a parameter to which J2215 is unusually sensitive, requires us to consider an acceptable systematic plus statistical range of $M_{NS}\sim1.85-2.3$ M$_\odot$. From the X-ray analysis, we find that the IBS wraps around the pulsar, but with a pulsar wind to companion wind momentum ratio unusually close to unity, implying a flatter IBS geometry than seen in other spiders. Estimating the companion wind momentum and speed from the X-ray light curve, we find a companion mass-loss rate of ${\dot M}_c\gtrsim10^{-10}$ M$_\odot$ yr$^{-1}$, so that J2215 may become an isolated millisecond pulsar in $\sim 1$ Gyr. Our X-ray analyses place constraints on the magnetization and particle density of the pulsar wind and support models of magnetic reconnection and particle acceleration in the highly magnetized relativistic IBS.