The Spectral Evolution of Transient Anomalous X-Ray Pulsar XTE J1810-197

We present a multiepoch spectral study of the transient anomalous X-ray pulsar XTE J1810-197 obtained with the X-Ray Multi-Mirror Mission (XMM-Newton). Four observations taken over the course of a year reveal strong spectral evolution as the source fades from outburst. The origin of this is traced to the individual decay rates of the pulsar's spectral components. A two-temperature fit at each epoch requires that the temperatures remain nearly constant at kT₁=0.25 keV and kT₂=0.67 keV, while the luminosities of these components decrease exponentially with τ₁=900 and τ₂=300 days, respectively. The integrated outburst energy is E₁=1.3×10⁴²d²_2.5kpc ergs and E₂=3.9×10⁴²d²_2.5kpc ergs for the two spectral components, respectively. One possible interpretation of the XMM-Newton observations is that the slowly decaying cooler component is the radiation from a deep heating event that affected a large fraction of the crust, while the hotter component is powered by external surface heating at the footpoints of twisted magnetic field lines by magnetospheric currents that are decaying more rapidly. The energy-dependent pulse profile of XTE J1810-197 is well modeled at all epochs by the sum of a broad pulse that dominates in the soft X-rays and a narrower one at higher energies. These profiles peak at the same phase, suggesting a concentric emission geometry on the neutron star surface. The spectral and pulse evolution together argue against the presence of a significant ``power-law'' contribution to the X-ray spectrum below 8 keV. The extrapolated flux is projected to return to the historic quiescent level, characterized by an even cooler blackbody spectrum, by the year 2007.