Chandra and Swift observations of the quasi-persistent neutron star transient EXO 0748-676 back to quiescence

TL;DR

This study reports on Chandra and Swift observations of the neutron star transient EXO 0748-676 transitioning to quiescence after over 24 years of outburst, revealing a hot, luminous state consistent with neutron star cooling models.

Contribution

First detailed X-ray spectral analysis of EXO 0748-676 during early quiescence after a prolonged outburst, providing insights into neutron star cooling behavior.

Findings

Neutron star surface temperature ~0.11-0.13 keV.

Bolometric thermal luminosity ~8.1E33-1.6E34 erg/s.

Presence of a hard powerlaw tail contributing ~20% of flux.

Abstract

The quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748-676 recently started the transition to quiescence following an accretion outburst that lasted more than 24 years. We report on two Chandra and twelve Swift observations performed within five months after the end of the outburst. The Chandra spectrum is composed of a soft, thermal component that fits to a neutron star atmosphere model with kT^inf~0.12 keV, joined by a hard powerlaw tail that contributes ~20% of the total 0.5-10 keV unabsorbed flux. The combined Chandra/Swift data set reveals a relatively hot and luminous quiescent system with a temperature of kT^inf~0.11-0.13 keV and a bolometric thermal luminosity of ~8.1E33-1.6E34 (d/7.4 kpc)^2 erg/s. We discuss our results in the context of cooling neutron star models.