The Fading of Transient Anomalous X‐Ray Pulsar XTE J1810−197

Abstract

Three observations of the 5.54 s transient anomalous X-ray pulsar XTEJ1810-197 obtained over 6 months with the Newton X-ray Multi-Mirror(XMM-Newton) mission are used to study its spectrum and pulsed light curveas the source fades from outburst. The decay is consistent with anexponential of time constant ~300 days but not a power law as predicted insome models of sudden deep crustal heating events. All spectra arewell fitted by a blackbody plus a steep power law, a problematicmodel that is commonly fitted to anomalous X-ray pulsars (AXPs). Atwo-temperature blackbody fit is also acceptable and better motivatedphysically in view of the faint optical/IR fluxes, the X-ray pulseshapes that weakly depend on energy in XTE J1810-197, and the inferredemitting areas that are less than or equal to the surface area of aneutron star. The fitted temperatures remained the same while the fluxdeclined by 46%, which can be interpreted as a decrease in area of theemitting regions. The pulsar continues to spin down, albeit at a reducedrate of (5.1+/-1.6)�10 -12 s s -1. The inferredcharacteristic age tau c ≡P/2P~17,000 yr, magnetic field strengthB s ~1.7�10 14 G, and outburst properties areconsistent with both the outburst and quiescent X-ray luminosities beingpowered by magnetic field decay, i.e., XTE J1810-197 is a magnetar.