The obscured QSO 1SAX J1218.9+2958

Abstract

We present results from XMM-Newton observations of the obscured QSO 1SAX J1218.9+2958. We find that the previously reported optical and soft X-ray counterpart positions are incorrect. However we confirm the spectroscopic redshift of 0.176. The optical counterpart has a K magnitude of 13.5 and an R-K colour of 5.0 and is therefore a bright extremely red object (ERO). The X-ray spectrum is well described by a power-law ($\Gamma =2.0\pm 0.2$) absorbed by an intrinsic neutral column density of $8.2_{-0.7}^{+1.1}\times 10^{22}c{m}^{-2}$. We find that any scattered emission contributes at most 0.5 percent to the total X-ray flux. From the optical/near-IR colour we estimate that the active nucleus must contribute at least 50 percent of the total flux in the K band and that the ratio of extinction to X-ray absorption is 0.1-0.7 times that expected from a Galactic dust-gas ratio and extinction curve. If 1SAX J1218.9+2958 were 100 times less luminous it would be indistinguishable from the population responsible for most of the 2-10 keV X-ray background. This has important implications for the optical/IR properties of faint absorbed X-ray sources.

arXiv comment6 pages, 4 figures, MNRAS accepted

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