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Subsections

2 Flux ratio criterion construction and the sample

2.1 Criterion construction

The alternative flux ratio constraint stems from the statistics to known X-ray sources. Here known sources are the correlation results between RASS-BSC and several catalogs of known objects available in computer-readable format, including catalogs of AGN (Véron-Cetty & Véron 1996), white dwarfs and NGC, the Bright Star Catalog, NED[*] and SIMBAD.

To evaluate X-ray flux for every source in RASS-BSC will cost much time. B magnitude of counterparts is not available either[*]. We establish restrict on X-ray count-rate to E magnitude to do preselection from RASS-BSC instead, where E-magnitude can be derived from the Digitized Sky Survey (DSS) image (Cao et al. 1997). We should point out that not taking Galactic absorbing material into account will reduce precision of the flux ratio constraint and then the efficiency of this preselection method.

We all know that the relationship between magnitude (m) and flux (f) is:
\begin{eqnarray}
m = -2.5\,{\rm log}\, f + {\rm constant} . \end{eqnarray} (1)
For E magnitude, the relation can be written as:
   \begin{eqnarray}
E = -2.5\,{\rm log}\, f_{\rm E} + {\rm constant} .\end{eqnarray} (2)
Despite Galactic absorption material $(N_{\rm H})$ count rate (c) is roughly proportional to X-ray flux $(f_{\rm X})$, so with Eq. (2):

\begin{displaymath}
{\rm log}\, (f_{\rm X}/f_{\rm E}) 
= {\rm log}\, f_{\rm X} -...
 ..., f_{\rm E} \propto
 {\rm log}\,\, c + 0.4\,E + {\rm constant}.\end{displaymath}

We can see that for fixed ${\rm log}\, (f_{\rm X}/f_{\rm E})$, ${\rm log}\, c+ 0.4\, E$ = constant.

B or V magnitudes of known X-ray sources can be drawn from those catalogs that we have mentioned. They must be changed to E magnitude at first. A relation was derived by Humphreys et al. (1991) between the Johnson BVR photometry and the E photographic magnitude on the Palomar plates:
\begin{eqnarraystar}
{ E - R = -0.011 + 0.148\,(V-R) + 0.058\,(V-R)^4 }\end{eqnarraystar}
\begin{eqnarraystar}
{\rm for} -0.2 < V-R < 1.7 \end{eqnarraystar}
then
\begin{eqnarray}
&& \hspace*{-3mm}E - V = -0.011 - 0.852\,(V-R) + 0.058\,(V-R)^4 
\nonumber\\ && \hspace*{-3mm}{\rm for} -0.2 < V-R < 1.7 . \nonumber\end{eqnarray}
Typical color indexes, (V-R) and (B-V), for various types of objects can be drawn from literatures. Now for a known X-ray source, its count rate and E magnitude are available. A plot of ${\rm log}\, c+ 0.4\, E$, versus apparent optical magnitude, E magnitude, is shown in Fig. 1. Only O-M spectral type stars, white dwarfs, emission line AGN and BL Lacertae objects are shown in this graph.

 
\begin{figure}
\psfig {figure=ds7948f1.ps,width=17.6cm,height=10.0cm,clip=true}
\end{figure} Figure 1: Plot of ${\rm log}\,\,c + 0.4\;E\;$vs. E-magnitude for known sources

There is an apparent gap between Galactic stars and extragalactic objects except white dwarfs. Emission line AGN concentrate on the discrete area between line A (${\rm log}\, c+ 0.4\, E$ = 4.9) and line B (${\rm log}\, c+ 0.4\, E$ = 6.4). It looks like that BL Lacertae objects have been scatted between $E= 14^{\rm m}$ and $E = 20^{\rm m}$ randomly. Nearly half number of BL Lacertae objects (45 to 105) located at the region above line B, however emission line AGN in this region are only about $3.6\%$ of its total amount included in the statistic. So taking ${\rm log}\, c+ 0.4\, E$ $ \geq 6.4$ as high X-ray-to-optical flux ratio criteria can avoid too much normal emission line AGN being included in the sample.

2.2 The sample

The high X-ray-to-optical flux ratio sample is defined by following criteria:

1.
Exclude known objects. The known objects mean the same as that in Sect. 2.1.
2.
Located in the northern hemisphere. In practice we choose $\delta \geq 3\hbox{$^\circ$}$ (epoch 1950), because we can evaluate E magnitude from DSS images only for those objects with declination larger than 3$^\circ$.
3.
High Galactic latitude $(\vert b\vert \geq 20\hbox{$^\circ$})$, in order to keep away from low Galactic sky area where mainly stars are located.
4.
There are counterparts within the circle with radius D=D1 + 5'', where D1 is the ROSAT X-ray position error given in the RASS-BSC.
5.
The E magnitude of the counterpart is less than $18.0^{\rm m}$, because it is hard to get high enough signal to noise ratio spectrum with a 2 m telescope for objects fainter than that limit, and meets the high X-ray-to-optical ratio criterion:
\begin{eqnarraystar}
{\rm log}\,c + 0.4\,E \geq 6.4.\end{eqnarraystar}
Thirty-four X-ray sources with Right Ascension between $23^{\rm h}$ and $13^{\rm h}$ were selected from RASS-BSC. Table 1 lists the X-ray information for the X-ray sources. There are several cases which have two or more counterparts in the cross-circle. Only those satisfying the above criteria are listed in the Table 1[*]. Objects are identified by ROSAT name (Col. 1). The optical position (epoch 2000.0, Col. 2 and Col. 3) are derived from the DSS image and should be accurate better than 2''. The X-ray information (e.g. position error, count-rate, hardness 1, and extent in arcsecond) is given in Col. 4 to Col. 7 respectively. These parameters are all given in RASS-BSC and useful for identification procedure. The E magnitude, derived from the DSS images, is given in Col. 8.


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