The Compact Central Object in Cassiopeia A: A Neutron Star with Hot Polar Caps or a Black Hole?

The central pointlike X-ray source of the Cassiopeia A supernova remnant was discovered in the Chandra first light observation and found later in the archival ROSAT and Einstein images. The analysis of these data does not show statistically significant variability of the source. Because of the small number of photons detected, different spectral models can fit the observed spectrum. The power-law fit yields the photon index γ = 2.6-4.1, and luminosity L(0.1-5.0 keV) = (2-60) × 10³⁴ ergs s^-1 for d = 3.4 kpc. The power-law index is higher, and the luminosity 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 = 0.2-0.5 km, and L_bol = (1.4-1.9) × 10³³ ergs s^-1. The inferred radii are too small, and the temperatures too high, for the radiation to be interpreted as emitted from the whole surface of a uniformly heated neutron star. Fits with the 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. One cannot exclude, however, the possibility that the observed emission originates from hot spots on a cooler neutron star surface. An upper limit on the (gravitationally redshifted) surface temperature is T < 1.9-2.3 MK, depending on the chemical composition of the surface and the star's radius. Among several possible interpretations, we favor a model of a strongly magnetized neutron star with magnetically confined hydrogen or helium polar caps (T ≈ 2.8 MK, R_pc ≈ 1 km) on a cooler iron surface (T ≈ 1.7 MK). Such temperatures are consistent with the standard models of neutron star cooling. Alternatively, the observed radiation may be interpreted as emitted by a compact object (more likely, a black hole) accreting from a residual disk or from a late-type dwarf in a close binary.