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Up: High X-ray-to-optical flux ratio


1 Introduction

The RASS-BSC contains 18811 X-ray sources (Voges et al. 1996a) with a count rate above 0.05 ctss-1. These sources have a detection likelihood of $\geq \!15$ and contain at least 15 source photons. Of RASS sources about 65 percent still remain unidentified (Voges et al. 1996b). Several programs have been carried out to identify RASS sources with particular properties.

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 ${\rm log}\,(f_{\rm X}/f_{\rm B}) \gt 1.3$ 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 $(N_{\rm H})$ 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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Up: High X-ray-to-optical flux ratio

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