Observational Study of High X‐Ray–to–Optical Flux Ratio RASS Sources

Abstract

We have performed a program to identify the unknown bright high Galactic latitude sources in the ROSAT All Sky Survey Bright Source Catalog (RASS-BSC) which have the highest X-rayEtoEoptical Nux ratios. Our goal is to establish a new bright X-rayEselected sample of active galactic nuclei (AGNs) for statistical and individual study. The results of the Einstein Medium Sensitivity Survey have shown that di†erent classes of X-ray sources represent di†erent narrow ranges in the X-rayEtoEoptical Nux ratios. We used log C] 0.4R as an alternative expression for where C is the X-ray count rate and R replog ( fX/fopt), resents the R magnitude from USNO-A1.0.1 According to our statistical analysis of known RASS-BSC X-ray sources, we found an apparent gap between Galactic stars and extragalactic objects except in the case of white dwarfs (Fig. 1) ; emission-line AGNs are concentrated in the region log Co[0.4R] 4.9. Then this high X-rayEtoEoptical Nux ratio criterion was used to select a sample with 165 unidentiÐed X-ray sources from RASS-BSC for optical spectroscopy. At present, over 93% of the X-ray sources in the sample have been fully identiÐed in the following proportions : emission-line AGNs (QSOs and Seyferts), 69.7%; BL Lacertae objects and their candidates, 3.6%; clusters of galaxies, 12.7%; Galactic stars, 7.3%; and 6.7% remain unidentiÐed because the proposed stellar counterparts are too faint to be the likely X-ray emitters. The plausibility of identiÐcation is based upon the optical classiÐcation, the