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Astronomers at Stanford University in California performed joint X-ray and optical observations of a massive “spider” pulsar designated PSR J2215+5135. Results of the monitoring campaign presented in a report posted May 22 on the preprint server arXivprovide more hints about the nature of this pulsar.

The fastest rotating pulsars, those with rotation periods of less than 30 milliseconds, are known as millisecond pulsars (MSPs). The researchers hypothesize that they form in binaries when the initially more massive component becomes a neutron star, which then spins due to accretion of matter from the secondary star.

A class of extreme binary pulsars with semi-degenerate companion stars are called “spider pulsars”. These objects are further categorized as “black widows” if the companion has an extremely low mass (less than 0.1 solar masses), while if the secondary star is more massive, they are called “redbacks”.

In spider pulsars, gamma-ray emission and relativistic particles from the pulsar wind irradiate the companion, subsequently driving away the massive stellar wind. Observations show that when the pulsar wind and the accompanying wind collide, they form a so-called intrabinary shock (IBS).

Located about 9,800 light-years away, PSR J2215+5135 (or J2215 for short) is a red-backed spider MSP with a rotation period of 2.61 milliseconds and a spin-down power of about 50 deciillion erg/s. The neutron star in the system has a mass of approximately 2.24 solar masses, while the mass of the companion is estimated to be about 0.3 solar masses. J2215’s orbital period is 4.14 hours and its measure of dispersion is 225.6 pc/cm³.

Recently, Andrew Sullivan and Roger Romani of Stanford University used ESA’s XMM-Newton spacecraft to take a closer look at J2215. Based on the XMM-Newton data, they created orbital light curves of J2215 and used them to model the system’s properties.

The new observations found that the neutron star in J2215 has a mass of approximately 2.15 solar masses and that the companion star is losing mass at a rate of 0.0003 Earth masses per year. Therefore, the researchers estimate that J2215 may become an isolated MSP.

Based on the X-ray analysis of J2215, the researchers determined that the IBS is still wrapping around the pulsar. This is typical of redbacks because in such systems the companion wind dominates the pulsar wind, so the IBS wraps around the pulsar, while in black widows the IBS wraps around the companion object.

The study also found that J2215 is located about 10,800 light-years away and its spin-down power is on the order of 52 deciillion erg/s. The paper’s authors suggest that J2215’s IBS may recycle much of its spin-down power.