Spatially resolved spectroscopy of Cassiopeia A with MECS on board ${\vec Beppo}$SAX

Abstract

We have performed the first detailed spatially resolved spectroscopy of Cas A in the 1.6-10 keV energy range, using data taken with the MECS spectrometer on board the Beppo SAX Observatory. The well calibrated point spread function in the central region of the MECS allowed us to perform a spatial deconvolution of the data at full energy resolution. We eventually generated a set of spectra, covering a region of ∼ 3′ radius around the centre of Cas A. The results obtained by fitting these spectra using a non-equilibrium ionisation plasma model and a power law, improve our knowledge about chemical and physical parameters of the Cas A SuperNova Remnant: (i) a single thermal component is sufficient to fit all the spectra; (ii) kT is rather uniformly distributed with a minimum in the east and a maximum in the west, and no evidence is found for high kT expected from the interaction of the main shock with the ISM; (iii) from the distribution of the values of the ionisation parameter $n_{\rm e}t$ we infer the presence of two distinct components: the first $(a)$ with n e in the range 1-10 cm -3 , the second ( b ) with values ten times higher; if we associate component a to the CSM and component b to the ejecta, the mass ratio $M(a)/M(b)\leq1/10$ indicates a progenitor star that lost only a small fraction of the envelope during its pre-SN life. In this hypothesis the distribution of component b across the remnant suggests that the explosion was not spherically symmetric; (iv) the distribution of abundances indicates that we are detecting a CSM component with almost solar composition, and an ejecta component enriched in heavier elements. Abundances found for α -elements are consistent with the current view that Cas A was produced by the explosion of a massive star. A low $\element[][]{Fe}$ overabundance can be an indication that at the moment of the explosion the mass-cut was rather high, locking most of the produced $\element[][]{Ni}$ 56 into the stellar remnant.