The obscured QSO 1 SAX J 1218 . 9 + 2958

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 (Γ = 2.0±0.2) absorbed by an intrinsic neutral column density of 8.2 −0.7 × 10 22 cm. 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× 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.