8th HEAD Meeting, 8-11 September, 2004
Session 8 Pulsars and Magnetars

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[8.13] Fading of the Transient Anomalous X-ray Pulsar XTE J1810-197

J. P. Halpern, E. V. Gotthelf (Columbia U.)

Two observations of the 5.54~s Transient Anomalous X-ray Pulsar XTE J1810-197 obtained 6 months apart with XMM-Newton are used to study its spectrum and pulsed light curve as the source fades from outburst. The decay is consistent with an exponential of time constant \approx 300 days, but not a power law as predicted in some models of sudden deep crustal heating events. Both spectra are well fitted by a model that commonly describes anomalous X-ray pulsars (AXPs), an absorbed blackbody plus a steep power law. However, a two-temperature blackbody fit is also acceptable, and better motivated physically in view of the faint optical/IR fluxes, the X-ray pulse shapes that hardly depend on energy in XTE J1810-197, and the inferred emitting areas that are less than or equal to the surface area of a neutron star. The fitted temperatures remained the same while the flux declined by 46%, which can be interpreted as a decrease in area of the emitting regions. The pulsar continues to spin down, albeit at a reduced rate of (5.1 ±1.6)\times10-12 s~s-1. The inferred characteristic age \tau\rm c \equiv P/2\dot P \approx 17,000~yr, magnetic field strength B\rm s \approx 1.7\times 1014~G, and outburst properties are consistent with both the outburst and quiescent X-ray luminosities being powered by magnetic field decay, i.e., XTE J1810-197 is a magnetar. Continuing study of XTE J1810-197 in various states of luminosity is important for understanding the range of thermal and non-thermal emission mechanisms to which magnetars convert their energy, and for possibly unifying the growing classes of isolated, young neutron stars that are not powered by rotation.

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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.