Astron. Astrophys. Suppl. Ser. 139, 575-599

An AGN sample with high X-ray-to-optical flux ratio from RASS

I. The optical identification

J.Y. Wei^1,2 - D.W. Xu^1,2 - X.Y. Dong^1,2 - J.Y. Hu^1,2

1 - Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100012, PR China
2 - National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China

Received June 21; accepted August 26, 1999

Abstract:

We present the first results of a program to identify the unknown bright active galactic nuclei (AGN) in the ROSAT All Sky Survey Bright Source Catalog (RASS-BSC). We have used $\log C + 0.4\,R$ as an alternative expression for $\log (f_{\rm X}/f_{{\rm opt}})$, where C is X-ray count rate and R stands for R magnitude. Then a high X-ray-to-optical flux ratio criterion has been used to select an AGN sample with 165 unidentified X-ray sources for optical spectroscopy. Those 165 X-ray sources have been identified in the following classes: 115 emission line AGN (QSOs and Seyferts), 2 BL Lac objects and 4 BL Lac candidates, 22 clusters of galaxies, 12 Galactic stars and 10 objects remain unidentified. This represent a success rate of about 73% for detecting AGN using our selection criteria. Plausibility is based upon the optical classification and X-ray characteristics of the sources.

Key words: surveys -- X-rays: general -- X-rays: galaxies -- galaxies: active