The obscured QSO 1SAX J1218.9+2958

We present results from XMM-Newton observations of the obscured quasi-stellar object 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. 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^+1.1_-0.7× 10²² cm^-2. We find that any scattered emission contributes at most 0.5 per cent to the total X-ray flux. From the optical/near-infrared colour we estimate that the active nucleus must contribute at least 50 per cent 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/infrared properties of faint absorbed X-ray sources.