Astronomy & Astrophysics manuscript no. ROXA˙astroph c  ESO 2008 February 5, 2008 ROXA: a new multi-frequency selected large sample of blazars with SDSS and 2dF optical spectroscopy Sara Turriziani 1, 2 , Elisabetta Cavazzuti 2, 3 and Paolo Giommi 2, 3 1 Universit` a degli studi di Roma, Tor Vergata, Dip. Fisica, via della ricerca scientifica 1, 00133 Roma, Italy. 2 ASI Science Data Center, ASDC c/o ESRIN, via G. Galilei 00044 Frascati, Italy. 3 Agenzia Spaziale Italiana, Unit` a Osservazione dell’Universo, viale Liegi 26 00198 Roma, Italy Received ....; Accepted .... ABSTRACT Context. Although Blazars are a small fraction of the overall AGN population they are expected to be the dominant population of extragalactic sources in the hard X-ray and gamma-ray bands and have been shown to be the largest contaminant of CMB fluctuation maps. So far the number of known blazars is of the order of several hundreds, but the forthcoming AGILE, GLAST and Planck space observatories will detect several thousand of objects of this type. Aims. In preparation for these missions it is necessary to identify new samples of blazars to study their multi-frequency characteristics and statistical properties. Methods. We compiled a sample of objects with blazar-like properties via a cross-correlation between large radio (NVSS, ATCAPMN) and X- ray surveys (RASS) using the SDSS-DR4 and 2dF survey data to spectroscopically identify our candidates and test the validity of the selection method. Results. We present the Radio - Optical - X-ray catalog built at ASDC (ROXA), a list of 816 objects among which 510 are confirmed blazars. Only 19% of the candidates turned out to be certainly non-blazars demonstrating the high efficiency of our selection method. Conclusions. Our catalog includes 173 new blazar identifications, or about 10% of all presently known blazars. The relatively high flux threshold in the X-ray energy band (given by the RASS survey) preferentially selects objects with high f x / f r ratio leading to the discovery of new High Energy Peaked BL Lac (HBLs). Our catalog therefore includes many new potential targets for GeV-TeV observations. Key words. galaxies: active – galaxies: Blazar: BL Lacertae surveys: 1. Introduction Blazars are the rarest (∼ 5%) and most extreme type of Active Galactive Nuclei (AGN) known. Historically, the classification of AGN has been largely based on observational characteristics leading to the proliferation of different classes of objects. All sources of this type, however, can be seen as part of a general paradigm in which AGN are divided into Thermal Emission Dominated (TED) AGN, where the emitted radiation is mostly generated through the accretion process onto a super-massive black hole, and Non-Thermal Emission Dominated (NTED) AGN, where the observed emission is mostly non-thermal and is generated in a jet of material moving away from the nucleus at relativistic speeds (e.g. Giommi & Colafrancesco 2006). Within this framework blazars are the small subset of NTED AGN in which the jet is closely aligned to the line of sight causing their emission to be strongly amplified by relativistic effects (as originally proposed by Blandford & Rees 1978). We recognize a source as a blazar if it shows the properties usually associated to aligned beamed emission such as strong Send offprint requests to: S. Turriziani, turriziani@asdc.asi.it and rapidly variable emission in all energy bands, from radio to GeV, sometimes TeV energies, core dominated radio emis- sion with flat radio spectral index, superluminal motion of ra- dio compact regions, the presence of one sided jets (a jet on the other side is thought to exist, but with emission that is de- amplified by relativistic effects) and high brightness tempera- tures (T b ∼ 10 11 − 10 18 K), close to or above the Compton limit (T b ≈ 10 12 ). Sources that initially show only some of these properties, in later observations often also show the others, strengthening the hypothesis that these are basically equivalent and insep- arable features related to the same underlying physical process. Blazars include BL Lacertae objects (BL Lacs) where we observe a non-thermal optical continuum with no or very weak emission lines, and Flat Spectrum Radio Quasars (FSRQs) which exhibit both strong narrow and broad emission lines. Furthermore, BL Lacs can be distinguished by the peak of the synchrotron emission in their Spectral Energy Distributions (SEDs). Objects with synchrotron peak at low energy (typi- cally in the Infra-Red) are generally found in radio surveys arXiv:0705.1498v1 [astro-ph] 10 May 2007