Absorption and scattering by interstellar dust: XMM-Newton observation of Cyg X-2

Abstract

We present results of the XMM-Newton observation on the bright X-ray binary Cyg X-2. In our analysis we focus upon the absorption and scattering of the X-ray emission by interstellar dust. The scattering halo around Cyg X-2, observed with the CCD detector EPIC-pn, is well detected up to ~7 arcmin and contributes ~5-7% to the total source emission at 1 keV, depending on the dust size distribution model considered. For the first time spatially resolved spectroscopy of a scattering halo is performed. In the halo spectrum we clearly detect the signature of the interstellar dust elements: O, Mg, and Si. The spectral modeling of the halo shows a major contribution of silicates (olivine and pyroxene). The spatial analysis of the halo shows that the dust is smoothly distributed toward Cyg X-2 at least for ~60% of the path to the source. Within the instrumental limits, the data do not show preference for a specific dust size distribution; namely the Mathis, Rumpl & Nordsieck (1977) or the Weingartner & Draine (2001) model. We used the Mie theory to compute the differential scattering cross section. The RGS data were used to investigate the ISM absorption, in particular the region of the oxygen edge. Combining the RGS results with the information on dust grains provided by the EPIC-pn spectrum of the halo we estimate a column density for dust absorption by oxygen, provided that it is locked in silicate grains (abridged).