The X‐Ray Source at the Center of the Cassiopeia A Supernova Remnant

We present the first results of an XMM-Newton observation of the central X-ray source in the Cas A supernova remnant. The spectrum can be fitted equally well with an absorbed steep power law (α ph ~ 3, N H ~ 1.5 × 10 22 cm -2 ) or with a bremsstrahlung with temperature kT ~ 2.4 keV and N H ~ 10 22 cm -2 . A blackbody model ( kT BB ~ 0.7 keV) gives a slightly worse fit and requires a column density N H ~ 5 × 10 21 cm -2 and an emitting area with radius ~0.3 km (for d = 3.4 kpc). A search for pulsations for periods longer than 150 ms gave negative results. The 3 σ upper limits on the pulsed fraction are ~13% for P > 0.3 s and ~7% for P > 3 s. The overall properties of the central X-ray source in Cas A are difficult to explain in terms of a rapidly spinning neutron star with a canonical magnetic field of ~10 12 G, and are more similar to those of slowly rotating neutron stars such as the anomalous X-ray pulsars.