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E.V. Gotthelf, Jules Halpern (Columbia University)
We present the latest results from a multi-epoch timing and spectral study of the Transient Anomalous X-ray Pulsar XTE J1810--197 obtained with the Newton X-Ray Multi-Mirror Mission (XMM). We have acquired six observations over two years to follow the spectral evolution as the source fades from outburst. The spectrum is arguably best characterized as a two-temperature blackbody whose respective luminosities are decrease exponentially with t1 = 870 d and t2 = 280 d. The temperatures of these components are starting to cool from the nearly constant values at earlier epochs of kT1 = 0.25 keV and kT2 = 0.67 keV, respectively. The pulsed fraction increased with increased energy, as expected for the smaller area of the hotter component. The extrapolated flux is projected to return to the historic quiescent level, characterized by an even cooler blackbody spectrum and smaller pulsed fraction, by the year 2007.
We also present the detection of radio emission from XTE J1810--197, the first reported for any AXP. Archival data from the Very Large Array (VLA) with angular resolution 6" reveals a point-source of flux density 4.5±0.5 mJy at 1.4 GHz at the precise location of the transient AXP. This is brighter than published upper limits from all other AXPs and quiescent states of soft gamma-ray repeaters (SGRs). The detection was made in 2004 January, one year after the discovery of XTE J1810--197 during its only known outburst. However, earlier VLA observations both before and after the outburst yield only upper limits that are comparable to or larger than the sole detection, which does not suggest a decaying radio counterpart to the transient X-ray event. We consider possible scenarios for the origin of radio emission from this transient anomalous X-ray pulsar.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.