Further X-ray observations of EXO 0748-676 in quiescence: evidence for a cooling neutron star crust

TL;DR

This study presents extended X-ray observations of EXO 0748-676 in quiescence, providing evidence for neutron star crust cooling through spectral analysis and luminosity decay over 1.6 years.

Contribution

It offers new long-term X-ray monitoring data that demonstrate neutron star crust cooling behavior after a prolonged accretion period.

Findings

Thermal luminosity decreases from ~1E34 to 6E33 erg/s.

Neutron star temperature drops from ~124 to 109 eV.

Hard X-ray tail varies non-monotonically over time.

Abstract

In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748-676 started a transition from outburst to quiescence, after it had been actively accreting for more than 24 years. In a previous work, we discussed Chandra and Swift observations obtained during the first five months after this transition. Here, we report on further X-ray observations of EXO 0748-676, extending the quiescent monitoring to 1.6 years. Chandra and XMM-Newton data reveal quiescent X-ray spectra composed of a soft, thermal component that is well-fitted by a neutron star atmosphere model. An additional hard powerlaw tail is detected that changes non-monotonically over time, contributing between 4 and 20 percent to the total unabsorbed 0.5-10 keV flux. The combined set of Chandra, XMM-Newton and Swift data reveals that the thermal bolometric luminosity fades from ~1E34 to 6E33 (D/7.4 kpc)^2 erg/s, whereas the inferred neutron star effective temperature decreases from ~124 to 109 eV. We interpret the observed decay as cooling of the neutron star crust and show that the fractional quiescent temperature change of EXO 0748-676 is markedly smaller than observed for three other neutron star X-ray binaries that underwent prolonged accretion outbursts.