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

TL;DR

This study models the intrabinary shock and companion star of the redback pulsar J2215+5135 using new X-ray and optical data, revealing system properties, neutron star mass estimates, and insights into the pulsar wind and shock geometry.

Contribution

It provides the first detailed orbital light curve modeling of J2215+5135, constrains neutron star mass considering metallicity uncertainties, and analyzes the intrabinary shock structure and pulsar wind properties.

Findings

Neutron star mass estimated at 1.85-2.3 M$_\\odot$ considering metallicity uncertainties.

IBS wraps around the pulsar with a near-unity wind momentum ratio, indicating a flatter shock geometry.

Companion mass-loss rate estimated at >10^{-10} M$_\odot$ yr$^{-1}$, suggesting potential evolution into an isolated millisecond pulsar.

Abstract

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.