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G. G. Pavlov, D. Sanwal (Penn State), V. E. Zavlin, B. Aschenbach (MPE, Garching, Germany)
The central pointlike X-ray source of the Cas A supernova remnant was discovered in the Chandra First Light Observation and found later in the archival ROSAT and Einstein data. We present the analysis of the Chandra ACIS spectrum of this source, supplemented by the analysis of the ROSAT HRI and Einstein HRI images. Because of the small number of photons detected, different spectral models can fit the observed spectrum. The photon power-law index, \gamma=2.6--4.1, is higher, and the (unabsorbed) luminosity, L(0.1--5.0~{\rm keV})= (2--60)\times 1034~erg~s-1 for d=3.4~kpc, is lower than those observed from very young pulsars. One can fit the spectrum equally well with a blackbody model with T=6--8~MK, R=(0.2--0.4)~km, L\rm bol=(1.3--1.9)\times 1033~erg~s-1. The inferred radii are too small, and the temperatures too high, for the radiation could be interpreted as emitted from the whole surface of a uniformly heated neutron star. Fits with the light-element neutron star atmosphere models increase the radius and reduce the temperature, but these parameters are still substantially different from those expected for a young neutron star. If the observed emission originates from hot spots on a cooler neutron star surface, the surface gives a small contribution to the observed flux because of the strong interstellar absorption; an upper limit on the (gravitationally redshifted) surface temperature is Ts\infty < 2--3~MK, depending on chemical composition of the surface and star's radius. We will discuss different possible interpretations of these results and critical observational tests to determine the nature of this source.
This work was partially supported through NASA grants NAG5-6907 and NAG5-7017.