Evidence for crust cooling in the transiently accreting 11-Hz X-ray pulsar in the globular cluster Terzan 5

TL;DR

This study observes crust cooling in a transiently accreting neutron star in Terzan 5, providing evidence of observable cooling after short accretion episodes and challenging existing nuclear heating models.

Contribution

First detection of crust cooling in a short outburst neutron star, revealing shallow heat release not explained by current models.

Findings

Crust temperature decreased by ~10% over 125 days

Cooling observed after a short 10-week accretion episode

Indicates need for revised nuclear heating models at shallow depths

Abstract

The temporal heating and subsequent cooling of the crusts of transiently accreting neutron stars carries unique information about their structure and a variety of nuclear reaction processes. We report on a new Chandra Director's Discretionary Time observation of the globular cluster Terzan 5, aimed to monitor the transiently accreting 11-Hz X-ray pulsar IGR J17480-2446 after the cessation of its recent 10-week long accretion outburst. During the observation, which was performed ~125 days into quiescence, the source displays a thermal spectrum that fits to a neutron star atmosphere model with a temperature for an observer at infinity of kT~92 eV. This is ~10% lower than found ~75 days earlier, yet ~20% higher than the quiescent base level measured prior to the recent outburst. This can be interpreted as cooling of the accretion-heated neutron star crust, and implies that crust cooling is observable after short accretion episodes. Comparison with neutron star thermal evolution simulations indicates that substantial heat must be released at shallow depth inside the neutron star, which is not accounted for in current nuclear heating models.