The Lockman Hole: Radio Identification of Very Faint Rosat Sources

Abstract

The ROSAT satellite has carried out a very deep X-ray survey of a particular region of the sky, the Lockman Hole, known for its exceptionally low HI column density. The field was also observed at 20 cm with the C configuration of the Very Large Array. We made four observations of three hours each, in different pointing positions, and combined the four resulting images to form one mosaic effectively representing one single 12 hour observation and covering the whole ROSAT field. The radio image has an RMS noise of 29 µJy in the central part, degrading to ~50 µJy at the outer 20 arcmin radius. A complete radio catalogue was constructed: it contains 150 sources with peak flux density e120 µJy. In addition we searched for radio sources with peak flux > 3σ at the positions of X ray sources.A comparison of the radio and X ray source lists yielded 22 radio-X ray (RX) pairs. As the number of X ray sources is around 130, this corresponds to a radio identification rate of ~ 17%. Two or at most three of the radio identifications are expected to be spurious. It is well-known that most sub-mJy radio sources are associated with galaxies, as is confirmed by a preliminary search for optical counterparts on POSS prints. However, this is not true if the radio sources also have an X ray counterpart. Using deep CCD images, which are available for the entire Lockman Hole field, most RX sources (at least two thirds and probably more) turn out to be associated with quasars or AGNs. In fact in the αOX — αRO spectral index diagram our proposed AGN identifications of RX sources fall in the strip normally populated by radio-loud as well as radio-quiet AGNs. The possible galaxy identifications are somewhat more problematical, as they fall outside the “normal” galaxy region in the αOX — αRO diagram. Probably a number of these should be considered cluster X ray sources.The surface density of bright RX quasars (B < 19.5) is quite high and may be even close to the overall density of optical quasars. Beyond B ~ 20 the RX quasar density quickly remains behind that of optical quasars; this increasing gap is clearly due to the radio cutoff. The Lockman Hole radio survey will be discussed in detail elsewhere.Key wordsLockman HoleROSAT surveyradio-X identifications