New Results from the REX Survey

Abstract

Abstract. The REX survey is an ongoing project aimed at the selection of new samples of QSOs and BL Lac objects. Spectroscopic identification of the ~1600 sources in the survey is in progress reaching, so far, about 40% of complete identification level. In this paper, the most recent results derived from the REX survey are briefly summarized. The REX Survey: Sampling the BL Lac PopulationThe main scientific goal of the REX survey is the selection of new samples of AGNs, like radio-loud QSOs and BL Lac objects. One of the most interesting characteristics of the REX survey is that it has been designed to sample the BL Lac population more homogeneously than previous radio or X-ray surveys. The EMSS survey (Morris et al. 1991), for instance, has selected objects with a flat X-ray-to-radio spectral index (ctRx <0.7, defined as High Energy Peaked BL Lacs, HBL) while the radio selected 1 Jy sample (Stickel et al. 1991) has found sources with a^x >0.7 (Low Energy peaked BL Lacs, LBLs). In Figure 1 the OLRX distribution of the BL Lacs discovered so far in the REX survey is compared to that of the BL Lacs in the EMSS and the 1 Jy samples. The REX survey is clearly finding many objects with intermediate properties between HBLs and LBLs, thus suggesting that the presence of a dichotomy in the BL Lac class, often claimed in the literature, could be artificially created by selection effects. The XB-REX Sample: Studying the Cosmological EvolutionA first completely identified sub-sample of 237 REXs has already been selected by imposing an X-ray flux limit greater than 4x 10 -1 3 erg s _ 1 c m -2 and a magnitude (B) brighter than 20.5. The resulting sample (the X-ray Bright REX sample, XB-REX) has been used to estimate the cosmological evolution of emission 257 terms of use, available at https://www.cambridge.org/core/terms. https://doi

Radio spectra of a sample of X-ray selected BL Lacs

“…The picture is however far from clear: while surveys like the Deep X-ray Radio Blazar Survey (DXRBS; Perlman et al 1998;Padovani et al 2002;Giommi et al, 2002), the "sedentary" survey (Giommi et al 1999) and the ROSAT -All Sky Survey (RASS; Bade et al 1998;Beckmann et al 2003) found very low values of V e /V a for XBLs, other surveys like the Radio Emitting X-ray survey (REX; Caccianiga et al 1999, Caccianiga et al, 2002 did not find the same result, possibly indicating that the negative evolution in this class of objects is not so strong as was previously measured.…”

“…Since BL Lacs are both radio-loud and X-ray-loud, surveys in these frequency bands discover them with high efficiency (e.g. the "1 Jansky" sample: Stickel et al, 1991; the Einstein Extended Medium-Sensitivity Survey (EMSS) sample: Morris et al, 1991;Rector et al, 2000; the Deep X-ray Radio Blazar Survey (DXRBS) sample: Perlman et al, 1998, Landt et al 2001; the ROSAT All Sky Survey (RASS) sample: Bade et al, 1998; the "sedentary" survey: Giommi, Menna & Send offprint requests to: Anna Wolter, anna@brera.mi.astro.it Padovani, 1999; the Radio Emitting X-ray (REX) survey sample: Caccianiga et al, 1999, Caccianiga et al, 2002. This splits BL Lacs into two empirical subclasses, namely radio-selected BL Lacs (RBLs) and X-ray-selected BL Lacs (XBLs).…”