Continued Neutron Star Crust Cooling of the 11 Hz X-Ray Pulsar in Terzan 5: A Challenge to Heating and Cooling Models?

TL;DR

This study monitors the cooling of a neutron star's crust after an accretion outburst, revealing ongoing cooling behavior that challenges existing models of neutron star thermal evolution.

Contribution

It provides extended observational data showing that the neutron star crust has not fully cooled, questioning current heating and cooling models for transient neutron stars.

Findings

Crust temperature remains above pre-outburst levels after 2.2 years.

Crust cooling is ongoing and not yet thermally relaxed.

Current models struggle to explain the observed prolonged cooling.

Abstract

The transient neutron star low-mass X-ray binary and 11 Hz X-ray pulsar IGR J17480-2446 in the globular cluster Terzan 5 exhibited an 11-week accretion outburst in 2010. Chandra observations performed within five months after the end of the outburst revealed evidence that the crust of the neutron star became substantially heated during the accretion episode and was subsequently cooling in quiescence. This provides the rare opportunity to probe the structure and composition of the crust. Here, we report on new Chandra observations of Terzan 5 that extend the monitoring to ~2.2 yr into quiescence. We find that the thermal flux and neutron star temperature have continued to decrease, but remain significantly above the values that were measured before the 2010 accretion phase. This suggests that the crust has not thermally relaxed yet, and may continue to cool. Such behavior is difficult to explain within our current understanding of heating and cooling of transiently accreting neutron stars. Alternatively, the quiescent emission may have settled at a higher observed equilibrium level (for the same interior temperature), in which case the neutron star crust may have fully cooled.