Rapid Cooling Of The Neutron Star In The Quiescent Super-Eddington Transient XTE J1701-462

In the past decade the observing of cooling neutron star transients after long-duration (year or longer) outbursts has entered as a new approach to constraining the properties of matter inside neutron stars. We present Chandra and XMM-Newton observations of the super-Eddington neutron star transient XTE J1701-426 in quiescence. The observations cover the first 800 days of quiescence following the end, in August 2007, of a very luminous 19-month-long outburst. This data set represents the best-sampled cooling curve of a neutron star transient to date. We also present Rossi X-Ray Timing Explorer and Swift observations made during the final three weeks of the outburst. The transition from active accretion to a quiescent phase dominated by thermal emission from the neutron star is resolved with unprecedented precision compared to other cooling neutron star transients observed after extended outbursts. The observations of XTE J1701-426 represent new parameter space being covered in neutron star cooling, this source having accreted at a much higher level than other observed sources, and the length of the outburst being intermediate between regular transients (with outbursts typically lasting weeks or months) and quasi-persistent transients (which accrete for many years or decades). The inferred effective surface temperature at the start of the quiescent phase is considerably higher for XTE J1701-426 than other observed sources, and the timescale of the cooling is much faster, strongly indicating a highly conductive neutron star crust. The quiescent spectra also show a prominent non-thermal component, which has exhibited large and irregular variability. The origin of this component is uncertain; one possibility is residual accretion from the companion star