The Variable Quiescent X-ray Emission of the Neutron Star Transient XTE J1701-462

TL;DR

This study monitors the cooling of a neutron star in XTE J1701-462 over several years, revealing slow crust cooling, low-level flares, and insights into thermal equilibrium and crust-core cooling processes.

Contribution

It provides extended observational data on neutron star cooling and low-level activity, improving understanding of crust cooling behavior over more than a year after outburst.

Findings

Neutron star crust may still be cooling towards thermal equilibrium.

Low-level flares up to 1e35 erg/s observed, likely not affecting the overall cooling.

Cooling curve consistent with a broken power law, exponential decay cannot be excluded.

Abstract

We present the results of continued monitoring of the quiescent neutron star low-mass X-ray binary XTE J1701-462 with Chandra and Swift. A new Chandra observation from 2010 October extends our tracking of the neutron star surface temperature from ~800 days to ~1160 days since the end of an exceptionally luminous 19 month outburst. This observation indicates that the neutron star crust may still be slowly cooling toward thermal equilibrium with the core; another observation further into quiescence is needed to verify this. The shape of the overall cooling curve is consistent with that of a broken power law, although an exponential decay to a constant level cannot be excluded with the present data. To investigate possible low-level activity, we conducted a monitoring campaign of XTE J1701-462 with Swift during 2010 April-October. Short-term flares - presumably arising from episodic low-level accretion - were observed up to a luminosity of ~1e35 erg/s, ~20 times higher than the normal quiescent level. We conclude that flares of this magnitude are not likely to have significantly affected the equilibrium temperature of the neutron star and are probably not able to have a measurable impact on the cooling curve. However, it is possible that brighter and longer periods of low-level activity have had an appreciable effect on the equilibrium temperature.