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5 Conclusion

This is a rather small sample compared with 18811 entries in RASS-BSC. However it gives us a view into what kind of objects occupy the highest X-ray-to-optical flux ratio regime. We can draw following conclusion from the sample identification results:

1.
Finding BL Lac objects by identifying X-ray sources with very high X-ray-to-optical flux ratio is proved to be fruitful.
2.
Those X-ray parameters listed in RASS-BSC can give a clue to the optical counterparts of X-ray sources. Pre-identification can be achieved if the combination properties in X-ray band (count-rate, hardness ratio and extent) and optical band (visual magnitude and morphology) are concerned.
3.
The peculiar HR1 distribution of AGN predicts that there is a soft X-ray excess over the flat hard X-ray continuum of AGN in the sample.

Acknowledgements

We thank an anonymous referee for valuable comments and suggestions. Dr. D.-W. Xu, X.-J. Jiang and Dr. Z.-H. Zhu give us much help in preparing the manuscript. This work is partly supported by the Pandeng Project of the Chinese Scientific Committee and the Chinese National Natural Science Foundation.


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