The Hamburg/RASS catalogue of optical identifications covers about 8480deg2\ of the extragalactic northern sky and should therefore provide a representative overview of the RASS-BSC content in the optical brightness region accessible by the HQS objective prism and direct plates. The distribution of the offsets between the optical and the X-ray positions shows that in most cases () the offsets are below 20'' (see Fig. 4 (click here)). The mean offset is 10.2'' (median 9'') which is considerably below the value of 20'' given by Voges (1994). However, it should be pointed out that Fig. 4 (click here) contains only the X-ray sources with a proposed optical counterpart. The positional offset is also a classification criterion, and possibly for some X-ray sources the optical counterpart was not found because of a comparatively large offset.
Figure 4: Histogram of the offset between the RASS II X-ray position and the position of the counterpart for all identified X-ray sources
The completeness of the catalogue can be judged from the class of still unidentified objects (Codes "8'' and "0''). These classes comprise together of the catalogue. In the vast majority of these cases the optical counterpart must be faint () and we do not expect a significant population of stars at these faint magnitudes. As shown in Fig. 5 (click here) very few M-stars were found at B > 17 and they already have the largest log(/fB) -ratios among main-sequence stars. Thus we expect the catalogue to be fairly complete for stellar sources from the main sequence.
Figure 5: Distribution of optical brightness B for AGN candidates and M dwarfs (hashed). B was derived from the objective prism plates for all plotted objects. Optical brightness values B > 20 are probably erroneous. On average AGN are weaker than M dwarfs but there is a small overlap
Figure 6: Number of three object classes in relation to the total number of X-ray sources above a given X-ray count rate limit
The optical counterparts to the unidentified sources are therefore almost certainly extragalactic objects, and among these mostly AGN. If the relation of the is taken and folded with the log(/fB) distribution from Bade et al. (1995) AGN with B > 18.5 and with B > 20 are expected. The optically weak galaxy clusters and BLLac objects are not included in this estimate. Further support for the proposed association of the bulk of unidentified sources with optically faint extragalactic objects is illustrated in Fig. 6 (click here). The fraction of unidentified X-ray sources or sources with empty error circles on the HQS Schmidt plates decreases with increasing X-ray count rate limit, and the unidentified X-ray sources disappear for high count rates. The portion of AGN candidates remains nearly constant, only for the highest count rates the galactic X-ray emitters are more abundant. This flat distribution stands in contrast to observational results (Stocke et al. 1991) that the portion of AGN rises continuously with decreasing count rate limit. In addition X-ray selected AGN show cosmological evolution in the sense that they are more abundant in ancient times (Hasinger et al. 1993; Boyle et al. 1994) which would even increase the number of X-ray faint AGN.
The HRC allows a convenient compilation of X-ray emitting object samples with substantial size for subsequent follow-up studies. Since the HRC is limited in its completeness (mainly due to its optical brightness limit) investigations aiming at flux-limited samples, cannot depend solely on the HRC. The current version of the HRC was produced by correlation of RASSI identifications with the RASS-BSC. The identification of RASS sources with objective prism plates is an ongoing project. Updates resulting from this work will be posted on our WEB page (http://www.hs.uni-hamburg.de).
The ROSAT project is supported by the Ministerium für
Bildung, Wissenschaft, Forschung und Technologie (BMBF/DARA) and by the
Gesellschaft (MPG). We thank Hans Hagen for contributions to the software development of this project. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. D.R. wishes to thank in particular the former President of the University of Hamburg, Dr. Peter Fischer-Appelt, without whose support in a critical phase of the HQS the present project would never have become real. This work has been funded by the Deutsche Forschungsgemeinschaft under Re 353/22-1 to 4 and by the BMBF under DARA 500R96016.