Two search strategies are commonly used to find BL Lacs. The first is a search for strong X-ray sources with a high ratio of X-ray to optical flux, yielding X-ray selected BL Lacs (XBL). The second is to search among flat spectrum radio sources to find radio selected ones (RBL). As the radio and X-ray surveys got more and more sensitive, the properties of both groups started to overlap, raising the question of how they are related. Padovani & Giommi (1995) noticed that the spectral energy distribution (quantified by $\log \nu L_\nu$ ) of radio and X-ray selected BL Lacs shows peaks at different frequencies, and suggested that this is the basic difference between the two classes of BL Lacs. They introduced the notation of high-energy cutoff BL Lacs (HBLs) and low-energy cutoff BL Lacs (LBLs). Most, but not all, XBLs are HBLs, while the group of LBLs is preferentially selected in the radio region.

In general, BL Lacs are considered as part of a larger class of objects, the blazars, which have similar properties but show emission lines in addition, and for which this scenario applies as well. Ghisellini et al. (1998) proposed that the range of peak frequencies observed is governed primarily by the efficiency of radiative cooling, and that the other physical parameters strongly depend on it. They found an inverse correlation between the energy of the Lorentz factor of particles emitting at the peaks of the SED ( $\gamma_{\rm peak}$ ) and the energy density of the magnetic and radiation field of $\gamma_{\rm peak} \propto U^{-0.6}$ . This correlation was extended later on for low power (high peaked) BL Lacs by taking into account the finite time for the injection of particles in the jet (Ghisellini et al. 2002). Combined modelling of the time-dependent electron injection and the self consistent radiation transport in jets of high peaked blazars lead to the conclusion that differences in the appearance can be explained by either self-synchrotron or external Compton dominated processes (Böttcher & Chiang 2002). Other studies focussed on the importance of shock events in the blazar jets to explain variability on short timescales (e.g. Bicknell & Wagner 2002). Based on these models the most important factor in the appearance of blazars seems nowadays the energy density of the jet. Maraschi & Tavecchio (2002) showed that this energy density is related to the accretion rate in the AGN disk and proposed that all blazar types have similar black hole masses but that the low power blazars exhibit lower accretion rates.

Unlike all other AGNs, and different from RBLs also, the space density or the luminosity of XBLs showed an increase with time (e.g. Rector et al. 2000). This is called negative evolution. Bade et al. (1998) probed this property with a new ROSAT selected XBL sample and confirmed the negative evolution only for extreme XBLs, e.g. HBLs with very high energy cutoffs ( $\log \nu_{\rm peak} > 16.5$ ). The result for less extreme XBLs, called intermediate-energy cutoff BL Lacs (IBLs) by Bade et al., was compatible with no evolution. This difference in evolutionary behaviour indicated the presence of a smooth transition between HBLs and LBLs. However, these findings were based on only 39 BL Lacs, prompting us to increase the size of this sample considerably. The results of this effort, the HRX-BL Lac sample presented here, comprises now 77 BL Lacs and is the largest complete XBL sample so far.

Recent models tried to explain the different evolutionary behaviour of HBLs and LBLs by assuming that BL Lacs start as LBLs and evolve into HBLs as they grow older (Georganopoulos & Marscher 1998; Cavaliere & D'Elia 2002). As described by e.g. Padovani & Urry (1990) the spectral energy distributions (SED) of BL Lacs are characterized by two components, both consisting of beamed continuum emission from the plasma of the jets. The first component is synchrotron emission, peaking in the mm to far IR for LBLs. The second component is inverse Compton (IC) emission peaking at MeV energies. HBLs have SEDs peaking in the keV and in the GeV-TeV band respectively. A decrease of power of the jets during the BL Lac evolution would then be accompanied by an increase of the peak frequencies and accordingly a transformation of the LBLs into HBLs (Georganopoulos & Marscher 1998). This model is in fact valid for the whole blazar class: BL Lacs in general show lower power and beaming factors than the Flat Spectrum Radio Quasars (FSRQs), as revealed by e.g. Madau et al. (1987), Padovani (1992), Ghisellini et al. (1993). It naturally explains the different evolution, which is slightly negative for HBLs, slightly positive for LBLs, and clearly positive for the FSRQ/blazar class. This model explains the different types of BL Lac objects only by different global intrinsic power (Maraschi & Rovetti 1994), and not by a different viewing angle. Nevertheless different orientation is probably important as secondary effect necessary to explain the large scatter of observed quantities.

The HRX-BL Lac sample contributes to the discussion with a large and complete sample of X-ray selected BL Lac objects. Previous studies (e.g. Fossati et al. 1998) used a compilation of different BL Lac surveys, like the X-ray selected EMSS (Stocke et al. 1991; Rector et al. 2000), the radio selected 1 Jy BL Lac sample (Stickel et al. 1991; Rector & Stocke 2001), and a FSRQ sample derived from the 2 Jy radio sample of Wall & Peacock (1985) to investigate the overall picture of the blazar class, ranging from the FSRQs to the BL Lac objects. In contrast to this the HRX-BL Lac survey is concentrating on a blazar subclass, the HBLs and IBLs, and is homogeneous in having the same selection criteria for all objects, making it comparable with the REX-survey (Maccacaro et al. 1998; Caccianiga et al. 1999), the DRXBS (Perlman et al. 1998; Landt et al. 2002), and the sedentary multifrequency BL Lac sample (Giommi et al. 1999).

We will describe our selection method of BL Lac candidates in Sect. 2 and the results of the identification process using literature data and own observations in Sect. 3. The spectral energy distribution of the HRX-BL Lac sample is analyzed in Sect. 4, where we demonstrate that for the HBL class the knowledge about the X-ray and optical flux is sufficient to determine the peak frequency of the synchrotron branch. The spatial distribution of the sample is described in Sect. 5. We conclude with a discussion of the compatibility of the results from the HRX BL Lac sample with recent studies.

Throughout the article a cosmology with $H_0 = 50 \rm ~ km ~ s^{-1} ~ Mpc^{-1}$ and a deceleration parameter q₀ = 0.5, assuming a Friedmann universe with $\Lambda = 0$ , has been used.

Table 1: Boundaries of the selection area.
Boundaries (J2000.0) Area

$\alpha$ $\delta$ [ $~{\rm deg}^2$ ]

$7^{\rm h} \le \alpha < 8^{\rm h}$ $30 ^{\circ}< \delta < 85 ^{\circ}$ 426

$8^{\rm h} \le \alpha < 12^{\rm h}$ $20 ^{\circ}< \delta < 85 ^{\circ}$ 2248

$12^{\rm h} \le \alpha < 14^{\rm h}$ $20 ^{\circ}< \delta < 65 ^{\circ}$ 970

$14^{\rm h} \le \alpha \le 16^{\rm h}$ $20 ^{\circ}< \delta < 85 ^{\circ}$ 1124

Boundaries (J2000.0)		Area
$\alpha$	$\delta$	[ $~{\rm deg}^2$ ]
$7^{\rm h} \le \alpha < 8^{\rm h}$	$30 ^{\circ}< \delta < 85 ^{\circ}$	426
$8^{\rm h} \le \alpha < 12^{\rm h}$	$20 ^{\circ}< \delta < 85 ^{\circ}$	2248
$12^{\rm h} \le \alpha < 14^{\rm h}$	$20 ^{\circ}< \delta < 65 ^{\circ}$	970
$14^{\rm h} \le \alpha \le 16^{\rm h}$	$20 ^{\circ}< \delta < 85 ^{\circ}$	1124

1 Introduction