1999A&AS..135..243C

We have used log c+ 0.4 E (where c is count rate, and E stands for E magnitude from POSS) as an alternative expression for log(f_X/f_opt) to study the X-ray-to-optical flux ratio distribution for known sources in ROSAT All Sky Survey Bright Source Catalog (RASS-BSC). Attention focuses on which classes of X-ray emitters have the highest X-ray-to-optical flux ratios. Then a high X-ray-to-optical flux ratio criterion is set and used to select a sample with 34 unidentified X-ray sources from the RASS-BSC for optical spectroscopy. The optical identifications of the sample are presented. Those 34 X-ray sources have been identified in the following classes: 10 emission line active galactic nuclei, 7 BL Lacertae objects and 3 BL Lac candidates, 7 clusters of galaxies, 4 Galactic objects and 3 objects remain unidentified due to low signal to noise ratio of the spectra. Identification plausibility is based upon the optical spectroscopy, morphology in the DSS image and the X-ray properties of the sources. We find that preliminary classification can be achieved while extent and hardness ratio in X-ray band, optical morphology and X-ray-to-optical flux ratio are concerned. All of the X-ray sources in the sample are identified with previously known classes of X-ray emitters, and the result is in accord with the E magnitude vs. (log c+ 0.4 E) diagram. The proportions of WD and CVs are much higher than that in the EMSS sample. We consider it is due to the lower bandpass of the ROSAT PSPC and the softness of the energy spectrum of these objects. AGN detected in this sample have extremely soft X-ray emission, which infers the presence of a soft X-ray excess in the AGN of this sample. The optical identification results also prove that to find BL Lacertae objects by high X-ray-to-optical flux ratio is an efficient way.