The ROSAT bright survey: I. Identification of an AGN sample with hard ROSAT X-ray spectra

The ROSAT Bright Survey (RBS) aims to completely optically identify the mor e than 2000 brightest sources detected in the ROSAT all-sky survey at galac tic latitudes /b/ > 30 degrees (excluding LMC, SMC, Virgo cluster). This pa per presents a subsample of 66 bright point-like ROSAT survey sources with almost hard PSPC spectra, the hardness ratio HR1 is > 0.5 for most of the s ources. The subsample could be nearly completely identified by low-resoluti on optical spectroscopy with the following breakdown into object classes: 3 1 Seyfert galaxies, 22 BL Lac candidates, 5 clusters of galaxies, 1 catacly smic variable, and 5 bright stars. Only one object remained unidentified an d one X-ray source was a spurious detection. The redshift distribution peak s around 0.06 for the Seyferts and around 0.13 for the BL Lac candidates. O bservations with medium spectral resolution were obtained for most of the n ew Seyfert galaxies. A large fraction (20 objects) are type I Seyfert galax ies, the other fraction includes Seyfert galaxies of type 1.5 - 1.8 (5 obje cts), two LINERs, and 4 possible narrow-line Seyfert 1 galaxies (NLS1). Abo ut one third of the new Seyfert's have nearby companion galaxies displaying either emission or absorption lines at the same redshift. Among them are a couple of systems showing direct morphological evidence for interaction. T he large fraction of interacting galaxies among our sample suggests a scena rio where interaction is the main trigger of AGN activity.