A cold neutron star in the transient low-mass X-ray binary HETE J1900.1-2455 after 10 years of active accretion

TL;DR

This study observes a neutron star in a low-mass X-ray binary returning to quiescence after a decade of active accretion, providing insights into its thermal state and core heat capacity through X-ray observations and thermal modeling.

Contribution

It presents the first detailed thermal evolution analysis of HETE J1900.1-2455 post-outburst, constraining neutron star crust and core properties after prolonged accretion.

Findings

The neutron star's surface temperature is approximately 54 eV in quiescence.

No evidence of altered crust thermal properties compared to non-pulsating neutron stars.

The star's core heat capacity is strongly constrained by its observed low temperature.

Abstract

The neutron star low-mass X-ray binary and intermittent millisecond X-ray pulsar HETE J1900.1-2455 returned to quiescence in late 2015, after a prolonged accretion outburst of ~10 yr. Using a Chandra observation taken ~180 d into quiescence we detect the source at a luminosity of ~4.5E31 (D/4.7 kpc)^2 erg/s (0.5-10 keV). The X-ray spectrum can be described by a neutron star atmosphere model with a temperature of ~54 eV for an observer at infinity. We perform thermal evolution calculations based on the 2016 quiescent data and a <98 eV temperature upper limit inferred from a Swift observation taken during an unusually brief (<2 weeks) quiescent episode in 2007. We find no evidence in the present data that the thermal properties of the crust, such as the heating rate and thermal conductivity, are different than those of non-pulsating neutron stars. Finding this neutron star so cold after its long outburst imposes interesting constraints on the heat capacity of the stellar core; these become even stronger if further cooling were to occur.