2 BL Lac candidate selection

In the beginning the HRX-BL Lac sample originated from the Hamburg-RASS Bright X-ray AGN sample (HRX), which was created by identification of the ROSAT All-Sky Survey (RASS) with the aid of objective prism plates of the Hamburg Quasar Survey (HQS; Hagen et al. 1995). The BL Lac subsample was selected on an area of $1687 ~{\rm deg}^2$ with a count-rate limit of $hcps \ge 0.075 ~{\rm s}^{-1}$ and on additional $1150 ~{\rm deg}^2$with a limit of $hcps \ge 0.15 ~{\rm s}^{-1}$. This sample was analyzed by Bade et al. (1998) and is referred here as the HRX-BL Lac core sample. It consists of 39 BL Lacs, 34 of which are also part of the present sample.

The fraction of BL Lacs in the HRX was $\sim$10%. Therefore, an increase of the sample size based on optical identification alone is rather inefficient. This can be alleviated using radio information, as all BL Lacs from the core sample were detected as radio sources in the NRAO VLA Sky Survey (NVSS, Condon et al. 1998). Also, to the authors knowledge, all known BL Lac objects do have radio counterparts down to the $\sim$2.5 mJy level, which is similar to the detection limit of the NVSS. We concluded therefore that for the high X-ray count-rates used we can include radio detection in the NVSS as selection criterium without loosing BL Lac objects. As X-ray input we used the ROSAT Bright Source Catalog (RASS-BSC; Voges et al. 1999) with a count-rate limit ( $0.5 {-} 2.0 ~{\rm keV}$) of $hcps \ge 0.09 ~{\rm s}^{-1}$. We cross-correlated this catalogue with the NVSS adopting an error circle of 30 $^{\prime\prime}$ around the X-ray position. We extended the sky area studied to $4768 ~{\rm deg}^2$ encompassing the area of $2837 ~{\rm deg}^2$ studied by Bade et al. (1998), and we applied a unique limit of $hcps \ge 0.09 ~{\rm s}^{-1}$. The boundaries of the area are given in Table 1.

The cross-correlation yielded 223 matches between X-ray and radio sources. The complete list of these objects is given in Table 8 (this table is only availlable in electronic form). The coordinates listed are the X-ray positions (J2000.0). More than 99.9% of the sources have a positioning error $\Delta \le$ 25 $^{\prime\prime}$ (Voges et al. 1999). The column "Name'' lists alternative names to the ROSAT designation, when available. Redshifts and classification are taken from the NED or SIMBAD database or were determined on the base of own follow-up observations. All objects, for which we obtained own data are marked.

The cross correlation might be incomplete for lobe-dominated radio sources, as in those cases the radio emission will consist of more than one component offset from the X-ray position. However, for none of the X-Ray BSC sources we found multiple radio sources within the search radius, and as BL Lacs are core-dominated radio sources no selection biases are expected.