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Subsections

3 Optical identification procedure

3.1 The optical spectroscopy and data reduction

The low resolution spectra of objects in the sample were taken with the 2.16 m telescope at Xinglong station of Beijing Astronomical Observatory from Dec. 1996 to Mar. 1998. We used OMR spectrograph at the cassegrain focus and a TEK 1024 CCD camera as detector. Two gratings of 200 Å/mm and 400 Å/mm were employed in order to get large specral coverage (>4800 Å), which is broad enough to span all of the gaps between the bright emission-line pairs common in AGN e.g., MgII and H$_\gamma$ or ] and MgII, and to cover identical features for normal galaxies.

He-Ar comparison lamp was used to get wavelength calibration. One to three KPNO standard stars (Massey & Strobel 1988) were observed to flux calibrate the spectra at every observing night. Because of changing weather conditions, an absolute flux calibration could not be achieved. All of the data were reduced with standard IRAF procedures. For a few objects, observations were performed twice, and two spectra were combined in order to get a higher S/N ratio spectrum. The spectra are shown in Fig. 2 with ROSAT name as labels.

 
\begin{figure}
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\psfig {figure=ds7948f2s10.ps,width=88mm,height=45mm,clip=}
}

 \end{figure} Figure 2: Optical spectra of the counterparts to the X-ray sources. $f_\lambda$ in units of 10-15 erg cm-2 s-1 Å-1 is plotted against wavelength in Å

 
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\psfig {figure=ds7948f2s20.ps,width=88mm,height=45mm,clip=}
}\end{figure*} Figure 2: continued

 
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\psfig {figure=ds7948f2s30.ps,width=88mm,height=45mm,clip=}
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3.2 Spectroscopic and imaging criteria for classification

We classify the optical counterparts as Galactic stars or extragalactic sources by means of whether spectral features present redshifts. Only four objects are Galactic stars. Two are white dwarfs with very broad Balmer series in absorption, and a very blue continuum. The other two are cataclysmic variable stars. Their blue nonthermal continuum are distinct. Balmer series emission lines are present in the two spectra.

Extragalactic sources in this sample are classified with emission line AGN, clusters of galaxies, BL Lacterae objects and their candidates. If an extragalactic object possesses broad (FWHM > 1000 kms-1) and strong ($W_\lambda$$\gg 5$ Å) emission lines, we classify it as an AGN. BL Lac objects are those with featureless spectra: (i) any emission line that is present, must have $W_\lambda$< 5 Å; (ii) if a CaII "break'' is present due to starlight in the BL Lac object host galaxy, it must have a "contrast'' $\leq 
25\% $ (Morris et al. 1991). Those with spectrum of a normal galaxy are classified either as clusters of galaxies if a clustering morphology presents in the DSS image, or as BL Lac object candidates (The reason for classifying them as BL Lac object candidates but not normal galaxies are illustrated in Sect. 4.4.) if no more objects are located inside or just outside the error circle.

3.3 Plausibility of identification based on X-ray information

RASS-BSC contains information on X-ray extent, extent likelihood, hardness ratio and other parameters besides count-rate. We use this information to determine whether the proposed optical counterpart to the X-ray source is a plausible one. The X-ray properties of WDs (very soft X-ray colours) and nearby galaxy clusters (hard and extended X-ray emission) have already been noted and used to determine the most plausible counterpart to an X-ray source by Bade et al. (1994, 1995).

According to Fig. 1, Emission line AGN, BL Lac objects and white dwarfs are suitable optical counterparts for objects in this sample. Clusters of galaxies and X-ray binaries are also X-ray luminous objects according to Bade et al. (1994). Normal galaxies are not plausible candidates for this sample's counterparts.

X-ray extent given in RASS-BSC can give a rough view on the size of X-ray emitter. We can use it to determine whether the X-ray source is an extent source or a point-like one. If we use a conservative extent criterium (extent-likelihood > 10 and extent > 30) (W. Pietsch et al. 1998) seven sources in the sample are X-ray extended. There are also celestial object groups appearing at the DSS images of these 7 X-ray extended sources. They are all classified as clusters of galaxies by us. Other sources in this sample are neither extent at X-ray band nor in group in optical images.

Two hardness ratios, HR1 and HR2, and their errors are defined in the RASS-BSC:
\begin{eqnarraystar}
{HR1 =\frac{H-S}{H+S}}\end{eqnarraystar}
\begin{eqnarraystar}
{HR2=\frac{H1-H2}{H1+H2}}\cdot\end{eqnarraystar}
Where H, S, H1 and H2 are the count-rate in the hard H-band (0.5-2.0 keV), the soft S-band (0.1-0.4 keV), the hard H1-band (0.5-0.9 keV) and the hard H2-band (0.9-2.0 keV) respectively. Because H is the sum of H1 and H2, and error for HR2 is much higher than for HR1, we only make comparision on HR1 among X-ray sources. In Fig. 3 we give the HR1 distribution of BL Lac objects, Seyferts, QSOs and WDs from which understandings on the hardness ratio for these four classes objects can be constructed. The HR1 for WDs is extremely concentrated between -0.9 and -1.0. On the contrary, BL Lac objects' X-ray emission is very hard, they seldom occupy the range where HR1 is lower than -0.5. Of 75 BL Lacertae objects only 2 have HR1 value smaller than -0.5. In general, AGN don't have very soft X-ray colour.

 
\begin{figure}
\includegraphics [width=8cm,height=5.5cm,clip]{ds7948f3_a.ps}

\i...
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\includegraphics [width=8cm,height=5.5cm,clip]{ds7948f3_d.ps}

 \end{figure} Figure 3: Hardness 1 for BL Lacertae Objects, QSOs, Seyferts and White Dwarfs

The spectral classification results are in accord with the statistics. Still, there are some points that are of consideration. In this sample emission line AGN are generally soft (HR1 < 0). This situation may have connection with their higher X-ray-to-optical flux ratio. We have no statistical information on the HR1 distribution for CVs and clusters of galaxies, but clusters of galaxies are generally accepted as possessing hard X-ray colour. Both CVs in this sample have HR1 smaller than -0.8, but it may be not the common sense for CVs. According to Richman's (1996) research, only about half number of CVs have soft X-ray excess.


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