The accretion-heated crust of the transiently accreting 11 Hz X-ray pulsar in the globular cluster Terzan 5

TL;DR

This study observes the neutron star in Terzan 5 after an accretion outburst, providing evidence of crustal heating and cooling in a transient system with a short outburst duration, which is a novel finding.

Contribution

First detection of crustal heating in a transient neutron star with a weeks-long outburst, expanding understanding of neutron star thermal evolution.

Findings

Elevated thermal emission post-outburst indicates crust heating.

Neutron star temperature ~100 eV, luminosity ~2E33 erg/s.

Crust cooling observed as emission decreases over time.

Abstract

We report on a Chandra Director's Discretionary Time observation of the globular cluster Terzan 5, carried out ~7 weeks after the cessation of the 2010 outburst of the newly discovered transiently accreting 11 Hz X-ray pulsar. We detect a thermal spectrum that can be fitted with a neutron star atmosphere model with a temperature for an observer at infinity of kT~100 eV, and a quiescent thermal bolometric luminosity of Lq~2E33 erg/s for an assumed distance of 5.5 kpc. The thermal emission is elevated above the quiescent base level measured in 2003 and 2009, i.e., prior to the recent accretion outburst. A likely explanation is that the neutron star crust was significantly heated during the recent accretion episode and needs to cool until it restores thermal equilibrium with the core. Although this has been observed for neutron star low-mass X-ray binaries that undergo accretion episodes of years to decades, it is the first time that evidence for crustal heating is detected for a transient system with a regular outburst duration of weeks. This opens up a new window to study heating and cooling of transiently accreting neutron stars.