The REX survey: A search for radio-emitting X-ray sources

We present the scientific goals, strategy, and first results of the REX pro ject, an effort aimed at creating a sizable and statistically complete samp le of radio-emitting X-ray sources (REXs) using the available data from a V LA survey (NVSS) and the ROSAT PSPC archive. Through a positional cross-cor relation of the two data sets, we have derived a sample of about 1600 REXs. Among the 393 REXs identified so far (either from literature or from our o wn spectroscopic observations) a high fraction are active galactic nuclei ( AGNs; about 60%-80%), typically radio-loud QSOs, and BL Lac objects. The re maining sources are galaxies, typically radio galaxies isolated or in clust ers. Thanks to the low flux limits in the radio (5 mJy at 1.4 GHz) and in t he X-ray band (similar to 5 x 10(-14) ergs s(-1) cm(-2), 0.5-2.0 keV) and t he large area of sky covered by the survey (2183 deg(2)), we intend to deri ve a new complete and unbiased sample of BL Lac objects that will contain b oth radio-selected (RBL) and X-ray-selected (XBL) objects. In this way, the apparent dichotomy resulting from the current samples of BL Lac objects ca n be directly analyzed in a unique sample. Moreover, the high number of BL Lac objects expected in the REX sample (similar to 200) will allow an accur ate estimate of their statistical properties, such as the X-ray, radio, and optical luminosity functions and the cosmological evolution. For these rea sons, the REX sample will be a powerful tool to accurately test the current theoretical models proposed for BL Lac objects. To date, we have discovere d 15 new BL Lac objects and 11 BL Lac candidates with optical properties in termediate between those of a typical elliptical galaxy and those of a typi cal BL Lac object. These objects could harbor weak sources of nonthermal co ntinuum in their nuclei, and if confirmed they could represent the faint ta il of the BL Lac population. The existence of such "weak" BL Lac objects ha s been matter of discussion in recent literature and could lead to a reasse ssment of the defining criteria of a BL Lac object, and consequently to a r evision of their cosmological and statistical properties. Finally, the samp le of similar to 800 emission-line AGNs generated by the REX survey will be useful in addressing many of the open questions regarding AGN phenomenolog y, such as the relationship between radio-loud and radio-quiet AGNs.