We present spectroscopic identification for a sample of 3LAC \(\gamma\)-ray sources, classifed as Blazars of Unknown type (BCU). The 3LAC catalogue had originally 460 BCUs out of 1591 objects. The low resolution spectroscopy was carried out using 2-4 metre class telescopes. Our selection is biased towards high Comton Dominance objects, as the initial idea was search for potential TeV-sources. All 34 observed targets have weak emission lines and the one (?) low redshift object shows only absorption lines of the host galaxy. Our
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The third Fermi-LAT \(\gamma\)-ray Catalog (3FGL; (Ackermann 2015)) has 3055 objects. The majority of these (1591, 52%) are Active Galactic Nuclei (AGN) and specifically blazars (3LAC; Acero et al. 2015). The 3LAC catalogue includes 632 BL Lacs (BLL), 467 FLat Spectrum Radio Quasars (FSRQ), 460 Blazars of Unknown type (BCU), and 32 non-blazar AGNs. A large fraction (29%) of AGNs are classified as BCU because they lack optical spectroscopic information. This lack of spectroscopy prevents the full exploitation of the data, since the spectral classification and the distance of the source (redshift) are crucial to infer their intrinsic emission and to probe the cosmic extragalactic background light.
Due to the large number of BCUs a preselection of targets for the optical follow-up observations is mandatory. The selection can be e.g. multi frequncey matching, like radio, IR, optical selection or radio, optical \(\gamma\)-ray properties. The optical selection is Typically the targets need to be brighter than V-magnitude \(\sim\) 19.
Massaro et al ApJ 740, L48 (2011) found that blazars populate a distinct region with respect to the other extragalactic sources in the IR-colour-colour diagramme. Using WISE and radio catalogues D’Abrusco et al ApJS, 215, 14 (2014) compiled a \(\gamma\)-ray blazar canididate catalogue. A fllow-up spectroscopic identificatins are presented by Ricci et al. AJ 149, 160 (2015) and Alvarez Crespo et al. AJ 151, 32, (2016). They found that out of 22 BCU objects 18 are classified as BL Lac objects (WISE BLL) and four have broad emission lines (WISE FSRQ).
Our sample, 34 targets, NTAGU (NOT TNG Active Galaxy of Uncertain type), had initial selection biases towards BCU II and high synchrotron peak frequency, and high 50GeV counts , however the final NTAGU sample has observed properties closer to the full 3LAC BL Lac sample than the TeV BL Lacs from the 3LAC catalogue (Table 1). All but three are from the 3LAC Clean Sample.
We carried out low resolution spectroscopic observations using Nordic Optical Telescope (NOT) and Telescope National Galileo (TNG) at La Palma. At NOT we performed the observations during service nights (“filler” targets) whereas at TNG we had dedicated observation time (PI Tescaro). An example of our data, we show a spectrum of 3LAC J1418.5+3543 with S/N \(\sim\)22 in Figure 1.
The preliminary analysis suggests that all but two of the targets show a featureless spectrum, suggesting a “BL Lac” classification (NTAGU BLL). Two TNG targets with absorption lines are probably low-luminosity BL Lacs whose light is swamped by the host galaxy (Browne, & Marchã, 1993 MNRAS 261 795).