A dedicated Chandra ACIS observation of the central compact object in the Cassiopeia A supernova remnant

We present results of a recent Chandra X-ray Observatory observation of the central compact object (CCO) in the supernova remnant Cassiopeia A. This observation was obtained in an instrumental configuration that combines a high spatial resolution with a minimum spectral distortion, and it allowed us to search for pulsations with periods longer than 0.68 s. We found no evidence of extended emission associated with the CCO, nor statistically significant pulsations (the 3-sigma upper limit on pulsed fraction is about 16%). The fits of the CCO spectrum with the power-law model yield a large photon index, Gamma\approx 5, and a hydrogen column density larger than that obtained from the SNR spectra. The fits with the blackbody model are statistically unacceptable. Better fits are provided by hydrogen or helium neutron star atmosphere models, with the best-fit effective temperature kT_{eff}^\infty \approx 0.2 keV, but they require a small star's radius, R = 4 - 5.5 km, and a low mass, M < 0.8 M_sol. A neutron star cannot have so small radius and mass, but the observed emission might emerge from an atmosphere of a strange quark star. More likely, the CCO could be a neutron star with a nonuniform surface temperature and a low surface magnetic field (the so-called anti-magnetar), similar to three other CCOs for which upper limits on period derivative have been established. The bolometric luminosity, L_{bol}^\infty \sim 6\times 10^{33} erg s^{-1}, estimated from the fits with the hydrogen atmosphere models, is consistent with the standard neutron star cooling for the CCO age of 330 yr. The origin of the surface temperature nonuniformity remains to be understood; it might be caused by anisotropic heat conduction in the neutron star crust with very strong toroidal magnetic fields.
Comment: 12 pages, including 7 figures; the end of Section 3.3 revised; accepted by ApJ