OPTICAL SPECTROSCOPIC OBSERVATIONS OF BLAZARS AND γ-RAY BLAZAR CANDIDATES IN THE SLOAN DIGITAL SKY SURVEY DATA RELEASE NINE

We present an analysis of the optical spectra available in the Sloan Digital Sky survey data release nine (SDSS dr9) for the blazars listed in the ROMA-BZCAT and for the γ-ray blazar candidates selected in our previous investigations that lying within the survey footprint have still an uncertain classification. First, we present a new statistical method to derive the best association radius to find the optical counterparts for the ROMA-BZCAT blazars in the SDSS photometric catalog. Then, adopting such value of the association radius, we crossmatched the SDSS spectroscopic catalog with our selected samples of blazars and γ-ray blazar candidates to search for those having an optical spectrum available. SDSS spectrahavebeen usedboth toclassifyourblazar-likesourcesand whenever possible to confirm their redshift. On the basis of the SDSS spectra we have been able to classify 35 BL Lac candidates listed in the ROMA-BZCAT with a redshift estimate for 9 of them in addition to 3 flat spectrum radio quasars. In particular, 20 out of the 85 ROMA-BZCATsources are also detected in the γ-ray and listed in the 2FGL. On the other hand, for the γ-ray blazar candidates, we found that 8 out the 11 analyzed having a blazar-like optical nature, so confirming the reliability of our procedures to search for potential counterparts of the Fermi sources.