Optical spectroscopic observations of gamma-ray blazars candidates I: preliminary results

Abstract: A significant fraction ($\sim 30$ %) of the gamma-ray sources listed in the second $\textit{Fermi}$ LAT (2FGL) catalog is still of unknown origin, being not yet associated with counterparts at lower energies. Using the available information at lower energies and optical spectroscopy on the selected counterparts of these gamma-ray objects we can pinpoint their exact nature. Here we present a pilot project pointing to assess the effectiveness of the several classification methods developed to select gamma-ray blazar candidates. To this end, we report optical spectroscopic observations of a sample of 5 gamma-ray blazar candidates selected on the basis of their infrared WISE colors or of their low-frequency radio properties. Blazars come in two main classes: BL Lacs and FSRQs, showing similar optical spectra except for the stronger emission lines of the latter. For three of our sources the almost featureless optical spectra obtained confirm their BL Lac nature, while for the source WISEJ022051.24+250927.6 we observe emission lines with equivalent width $EW\sim 31$ $\AA$, identifying it as a FSRQ with $z = 0.48$. The source WISEJ064459.38+603131.7, although not featuring a clear radio counterpart, shows a blazar-like spectrum with weak emission lines with $EW \sim 7$ $\AA$, yielding a redshift estimate of $z=0.36$. In addition we report optical spectroscopic observations of 4 WISE sources associated with known gamma-ray blazars without a firm classification or redshift estimate. For all of these latter sources we confirm a BL Lac classification, with a tentative redshift estimate for the source WISEJ100800.81+062121.2 of $z = 0.65$.