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 (αph ~ 3, NH ~ 1.5 × 1022 cm-2) or with a bremsstrahlung with temperature kT ~ 2.4 keV and NH ~ 1022 cm-2. A blackbody model (kTBB ~ 0.7 keV) gives a slightly worse fit and requires a column density NH ~ 5 × 1021 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 ~1012 G, and are more similar to those of slowly rotating neutron stars such as the anomalous X-ray pulsars.

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