Open AccessThe obscured QSO 1SAX J1218.9+2958
Author(s) -
Loaring N. S.,
Page M. J.,
Ramsay G.
Publication year - 2003
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2003.07016.x
Subject(s) - physics , astrophysics , extinction (optical mineralogy) , redshift , flux (metallurgy) , absorption (acoustics) , infrared , population , magnitude (astronomy) , astronomy , galaxy , optics , medicine , materials science , metallurgy , environmental health
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 × 10 22 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.