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 carried out 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 \approx 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 \lesssim 0.8 M _ { \odot } .
A neutron star can not 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 .