Previous optical identifications of X-ray sources detected with the Einstein
Observatory (EMSS, Stocke et al. 1991) and the EXOSAT satellite (HGLS,
Giommi et al. 1991) have shown that different
classes X-ray sources occupy apparently different X-ray-to-optical
flux ratio range, and those with the highest X-ray-to-optical flux ratio
are all spectacular objects such as: BL Lacertae objects, clusters
of galaxies, white dwarfs and cataclysmic variable stars etc.
Nass et al. (1996) also pointed out that can be used as an efficient,
radio-independent means for selecting candidate BL Lacertae objects while
carrying out their RASS sources identification program.
Although to identify high X-ray-to-optical flux ratio sources should have been
fruitful, a flux ratio criterion was seldom used at any preselection
procedures. The reason for this situation is that there are always several
optical objects inside the error circle for an X-ray source, and the flux ratio
for an X-ray source will not be the same if a different counterpart is assumed.
It will take much time to calculate X-ray and optical flux for each possible
counterpart of RASS-BSC Sources if we use a high flux ratio criterion.
To make preselection much easier, we make use of the fact that despite
Galactic absorbing material X-ray count rate is roughly proportional
to X-ray flux. So count rate can replace
X-ray flux in the X-ray-to-optical
flux ratio criterion. For each object inside the X-ray source's error
circle, we evaluate its E magnitude and check whether it satisfies the flux
ratio criterion. Only those that have at least one object inside the error
circle which can fulfil the criterion are included in the sample.
The paper is ordered as follows: Sect. 2 describes how the alternative flux ratio criterion is constructed and how the sample is defined; Sect. 3 gives an overview on the observation and the data reduction, then optical identification procedure is discussed in detail; in Sect. 4, we report the identification results to the sample and discuss the implication of the results. Final conclusion is given in Sect. 5.
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