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 ≃700 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 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 (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 highly uncertain; one possibility is residual accretion from the companion star