About 80000 X-ray sources with a detection likelihood were found during the ROSAT All-Sky Survey (Voges et al. 1996b, 1997a), from which the 18811 brightest sources were compiled in the ROSAT Bright Source Catalogue (RASS-BSC) (Voges et al. 1996a,b). Cross-correlation with Galactic and extragalactic catalogues compiled in the SIMBAD and NED database showed that more than of the objects are objects previously unknown. The identification of these objects, and more to come with the release of additional catalogues, is a challenge, as the X-ray data alone provide only limited insight into their nature. Digitized optical sky surveys play a major role in this effort to provide candidates for optical counterparts to the X-ray sources. Despite the relatively small error radii () of the RASS positions, often more than one candidate is present and follow-up optical spectroscopy is needed to determine the nature of the candidates.
The need of ample telescope time for follow-up spectroscopy can be alleviated considerably by using the information provided by (digitized) objective prism plates. The low-dispersion spectra allow to draw conclusions on the nature of the optical candidates and provide in many cases unambiguous identifications of the X-ray sources. Therefore, digitized plates of the objective prism survey for bright QSOs and direct plates at the Hamburger Sternwarte (Hagen et al. 1995) were used to provide identifications for X-ray sources from the RASS. The identification process covers currently deg2 of the high galactic latitude northern sky (337 fields from the Hamburg Quasar Survey (HQS, Hagen et al. (1995), each with an area of ), and with the present paper we release a first catalogue containing optical counterparts for 3847 X-ray sources from the RASS-BSC.
Use of the identifications has been made already to study the RASS content of AGN. From follow-up spectroscopy of 550 counterparts classified as AGN candidates we found a confirmation rate 95% (Bade et al. 1992a). Subsequently, samples of new ROSAT detected emission-line AGN (Bade et al. 1995; Cordis et al., in preparation), and BL Lac objects (Bade et al. 1994; Nass et al. 1996) were analyzed.
In the present paper we describe the identification technique (Sect. 2 (click here)), we discuss the classification criteria (Sect. 3 (click here)) and present the Hamburg/RASS Catalogue of optical identifications (HRC) (Sect. 4 (click here)). In the remaining sections the reliability of the identifications and the completeness of the catalogue are discussed, and statistics on its content are presented.