EDP Sciences
First Results from XMM-Newton
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Volume 365, Number 1, January I 2001
First Results from XMM-Newton
Page(s) L225 - L230
DOI http://dx.doi.org/10.1051/0004-6361:20000048

A&A 365, L225-L230 (2001)
DOI: 10.1051/0004-6361:20000048

Cassiopeia A: On the origin of the hard X-ray continuum and the implication of the observed O VIII Ly-$\alpha$/Ly-$\beta$ distribution

J. A. M. Bleeker1, R. Willingale2, K. van der Heyden1, K. Dennerl3, J. S. Kaastra1, B. Aschenbach3 and J. Vink4,5,6.

1  SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
2  Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
3  Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85740 Garching, Germany
4  Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
5  Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027, USA
6  Chandra Fellow

(Received 6 October 2000 / Accepted 24 October 2000 )

We present the first results on the hard X-ray continuum image (up to 15 keV) of the supernova remnant Cas A measured with the EPIC cameras onboard XMM-Newton. The data indicate that the hard X-ray tail, observed previously, that extends to energies above 100 keV does not originate in localised regions, like the bright X-ray knots and filaments or the primary blast wave, but is spread over the whole remnant with a rather flat hardness ratio of the 8-10 and 10-15 keV energy bands. This result does not support an interpretation of the hard X-radiation as synchrotron emission produced in the primary shock, in which case a limb brightened shell of hard X-ray emission close to the primary shock front is expected. In fact a weak rim of emission near the primary shock front is discernable in the hardest X-ray image but it contains only a few percent of the hard X-ray emissivity. The equivalent width of the Fe-K line blend varies by more than an order of magnitude over the remnant, it is hard to explain this as Fe-emission from the reverse shock heated ejecta given the ejecta temperature and the age of the remnant. The uniquely high wavelength-dispersive RGS-spectrometer has allowed, for the first time, to extract monochromatic images in several highly ionised element species with high spectral resolution. We present here a preliminary result on the measurement of the OVIII Ly-$\alpha$ and Ly-$\beta$ brightness distribution and brightness ratios. The large observed decrease of the Ly-$\alpha$/Ly-$\beta$ ratio going from the N to the SE can be explained by small-scale $(10\hbox{$^{\prime\prime}$ })$ variations in the $N_{\rm H}$ column over the remnant and the potential presence of resonance scattering of the OVIII Ly-$\alpha$ photons in the limb brightened shell.

Key words: ISM: supernova remnants -ISM: individual objects: Cas A -X-rays: ISM

Offprint request: J. A. M. Bleeker, J.A.M.Bleeker@sron.nl

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