We present a multi-epoch spectral study of the Transient Anomalous X-ray Pulsar XTE J1810 - 197 obtained with the Newton 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 remains nearly constant at kT _ { 1 } = 0.25 keV and kT _ { 2 } = 0.67 keV while the luminosities of these components decrease exponentially with \tau _ { 1 } = 900 days and \tau _ { 2 } = 300 days , respectively .
The integrated outburst energy is E _ { 1 } = 1.3 \times 10 ^ { 42 } d ^ { 2 } _ { 2.5 ~ { } kpc } ergs and E _ { 2 } = 3.9 \times 10 ^ { 42 } d ^ { 2 } _ { 2.5 ~ { } kpc } 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 foot-points 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 .