The Hamburg / RASS optical identification project

We use digitized Schmidt direct and prism plates taken for the northern hemisphere Hamburg Quasar Survey (HQS) to obtain optical identifications for all high galactic latitude X-ray sources in the ROSAT Bright Source Catalogue (RASS-BSC) at |b| ≥ 30 • and δ ≥ 0 •. In this part of the sky the RASS-BSC contains about 5300 X-ray sources. Presently, identifications exist for ∼ 4100 RASS-BSC sources in this ga-lactic latitude range. The ROSAT Bright Source Catalogue (1RXS-B) contains a total of 18 811 ROSAT all-sky survey sources with a count-rate limit of 0.05 cts s −1 in the 0.1-2.4 keV energy range and with a typical positional accuracy of ∼ 30 ′′ (Voges et al. 1999). The goal of our project is to obtain optical identifications for all 1RXS-B sources in the northern hemisphere with δ ≥ 0 • and galactic latitudes | b |≥ 30 •. In this part of the sky the 1RXS-B catalogue contains 5341 sources. 2. Method of identification For the identification of the optical counterparts we make use of the digitized objective prism and direct Schmidt plates of the Hamburg Quasar Survey (Hagen et al. 1995). A description of the digital data base is given by Engels et al. (this conference). The identification procedure is as follows: 1. We first search on the digitized direct plates for all possible counterparts in the X-ray error circle. A minimum error circle of 40 ′′ is adopted. 2. Then spectra are extracted from the data base of the digitized spectral plates at the positions of the objects. The limiting magnitude of the spectral plates is B ≈ 18.5. 3. Using these spectra the objects are classified interactively. Object categories used are listed in Tab. 1. 4. Finally, all possible counterparts are entered into a catalogue and the most likely optical counterpart is identified by a flag (cf. Fig. 1).