ASCA spectroscopy of IRAS 23060 + 0505: penetrating the torus of a type 2 quasar with X-rays

We report on a pointed ASCA observation of the powerful (Lbol ≈ 1.3 × 1046 erg s−1) type 2 infrared luminous galaxy IRAS 23060 + 0505. This galaxy has been the subject of detailed spectroscopic and spectropolarimetric studies which have revealed a hidden broad-line region and have been used to construct a detailed model of the light paths in this source. Our observation detects IRAS 23060 + 0505 in the X-ray band for the first time and complements the optical and infrared studies. We detect hard X-ray emission that can be modelled as a power law with a 2–10 keV luminosity of 1.5 × 1044 erg s−1, thereby verifying the existence of an accreting supermassive black hole in IRAS 23060 + 0505. The obscuring torus of IRAS 23060 + 0505 appears to be Compton thin along our line of sight, and we measure a column to the nuclear X-ray source of about 8 × 1022 cm−2. Comparison of the X-ray column with the Av=30 ± 4 mag extinction measurement by Young et al. suggests that the torus may have a dust-to-gas ratio rather similar to that in the interstellar medium of the Milky Way. We also find some evidence for iron K-line emission. The apparent X-ray to bolometric luminosity ratio for IRAS 23060 + 0505 is ≾ 1 per cent, a rather low value, and we consider possible explanations for this low ratio.