4 Criteria for classification

The optical spectra are classified in a manner similar to related optical identification programs of X-ray sources (Stocke et al. 1991; Appenzeller et al. 1998). In the present investigation the plausibility is based on the optical classification and X-ray characteristics like hardness ratios and extension parameter. In many cases an unambiguous identification of an object as optical counterpart is possible using only the spectroscopy and imaging information from the POSS. However, we found a few cases where the optical classification alone does not allow a decision whether the proposed optical counterpart to the X-ray source is a plausible one. In these special cases additional information on the nature of the X-ray sources derived from RASS-BSC was used to check the plausibility. Bade et al. (1994) discussed the X-ray properties of white dwarfs (very soft X-ray colors) and nearby clusters of galaxies (hard and extended X-ray emission). Pietsch et al. (1998) also used a conservative extent criterion (extent likelihood > 10 and extent $> 30\hbox{$^{\prime\prime}$ }$) as an indicator that the X-ray emission does not originate from a nuclear source. The redshifts of the spectral features are used to discriminate between X-ray sources with Galactic and extragalactic counterparts. Since our spectral resolution is insufficient to detect emission components in the Ca II resonance lines, we use Balmer emission components as tracers for coronal activity. Among 12 Galactic stars, 3 are M stars with molecular absorption bands, 3 are white dwarfs with very blue continuum and broad Balmer lines in absorption, and 6 are cataclysmic variable stars with Balmer emission lines present in the spectra.

Extragalactic counterparts in the sample are classified with emission line AGN, clusters of galaxies, BL Lac objects and their candidates. Generally, normal galaxies are not expected to be strong X-ray emitters and plausible candidates for the sample's counterparts. Objects with broad (FWHM> 1000kms^-1) and strong ( $W_\lambda\,\gg\,5$ Å) permitted emission lines are classified as emission line AGN. The spectroscopic and imaging criteria required to identify an X-ray source as a cluster of galaxies involve the spectrum of a normal galaxy and visual evidence for the cluster on the POSS.

Indications pointing to a BL Lac nature of a specific source are: absence of emission lines with $W_\lambda\,\ge\,5$ Å, contrast of the Ca II break from the host galaxy small than 25%. Four objects with seemingly normal galaxy spectra are classified as BL Lac candidates due to their centimeter radio fluxes, high X-ray luminosities and consistent ( $\alpha_{{\rm ox}}, \alpha_{{\rm ro}}$) values. The use of the $\alpha_{{\rm ox}} - \alpha_{{\rm ro}}$ diagram for classification was discussed by Stocke et al. (1991) and Nass et al. (1996). It was noticed that X-ray selected BL Lac objects contain a greater percentage of starlight due to the underlying galaxy than radio-selected BL Lac objects. Browne & Marchã (1993) also pointed out the possibility of misclassified X-ray sources as elliptical galaxies or clusters of galaxies. We tried without success to find other plausible counterparts since other objects visible on the POSS within the error circle are too faint to be observed with the 2.16 m telescope. However, we can not exclude that the counterparts are among these faint objects. Further observations, imaging and spectroscopy, are needed in these fields. Table 1 gives the X-ray parameters for these four BL Lac candidates. The X-ray flux is the unabsorbed flux in units of 10^-12 ergs cm^-2 s^-1 and has been computed assuming a power law spectrum and Galactic absorption. The radio fluxes at 1.4 GHz are taken from the NRAO VLA Sky Survey (NVSS, Condon et al. 1998). The two-point spectral indices $\alpha_{{\rm ox}} = -{\rm log}(S_{2{\rm keV}}/S_{2500\,\mbox{\tiny\AA}})/2.605$ and $\alpha_{{\rm ro}} = {\rm log}(S_{5\,{\rm GHz}}/S_{2500\,\mbox{\tiny\AA}})/5.38$ (Stocke et al. 1985; Tananbaum et al. 1979) have been K-corrected for the redshift of the object by assuming spectral slopes within each band of $\alpha_{\rm r} = -0.7$; $\alpha_{\rm x} = -1$ and $\alpha_{\rm o} = -1.4$ (Ghisellini et al. 1986).

X-ray photon index for a power law fit with Galactic absorption.