The ROSAT All-Sky Survey Bright Source Catalogue (RASS-BSC)

We have cross-correlated our list of 1103 UV-excess objects with the ROSAT All-Sky Survey bright source catalogue [56]. There are 2225 X-ray sources in the area of interest. We have found 57 X-ray sources within 4$^\prime$ of a FBS source, while we expected eight chance coincidences. Therefore most of them are probably real associations. We have measured the accurate optical position of the 33 non-QSO coincidences (Table 1); the differences between the X-ray and accurate optical positions are smaller than 40$^{\prime\prime}$ except for eight objects, confirming the reality of the associations.

The source RX J17173+4227, which is associated with the radiosource B3 1715+425, has been identified with the Zwicky cluster of galaxies Zw 8193 at z = 0.183 [13]; the X-ray and radio positions are in good agreement within the error limits, while the optical position of FBS 1715+424 is about 42$^{\prime\prime}$ away from the X-ray position; this is therefore a chance coincidence. Within 1$^\prime$, we found 50 coincidences: 21 QSOs, 22 stars and seven unidentified objects listed in Table 2; recently, one of these objects (FBS 0950+664) has been identified on objective prism plates as an AGN [14].