Optical spectroscopic observations of low-energy counterparts of Fermi-LAT γ-ray sources

Abstract

Context. A significant fraction of all γ-ray sources detected by the Large Area Telescope aboard the Fermi satellite is still lacking a low-energy counterpart. In addition, there is still a large population of γ-ray sources with associated low-energy counterparts that lack firm classifications. In the last 10 years we have undertaken an optical spectroscopic campaign to address the problem of unassociated or unidentified γ-ray sources (UGSs), mainly devoted to observing blazars and blazar candidates because they are the largest population of γ-ray sources associated to date. Aims. Here we describe the overall impact of our optical spectroscopic campaign on sources associated in Fermi-LAT catalogs, coupled with objects found in the literature. In the literature search we kept track of efforts by different teams that presented optical spectra of counterparts or potential counterparts of Fermi-LAT catalog sources. Our summary includes an analysis of additional 30 newly collected optical spectra of counterparts or potential counterparts of Fermi-LAT sources of a previously unknown nature. Methods. New spectra were acquired at the Blanco 4 m and OAN-SPM 2.1 m telescopes, and those available in the Sloan Digital Sky Survey (data release 15) archive. Results. All new sources with optical spectra analyzed here are classified as blazars. Thanks to our campaign, altogether we discovered and classified 394 targets with an additional 123 objects collected from a literature search. We began our optical spectroscopic campaign between the release of the second and third Fermi-LAT source catalogs (2FGL and 3FGL, respectively), classified about 25% of the sources that had uncertain nature and discovered a blazar-like potential counterpart for ∼10% of UGSs listed therein. In the 4FGL catalog, about 350 Fermi-LAT sources have been classified to date thanks to our campaign. Conclusions. The most elusive class of blazars are found to be BL Lacs since the largest fraction of Fermi-LAT sources targeted in our observations showed a featureless optical spectrum. The same conclusion applied to the literature spectra. Finally, we confirm the high reliability of mid-IR color-based methods to select blazar-like candidate counterparts of unassociated or unidentified γ-ray sources.

Optical spectroscopy of blazars for the Cherenkov Telescope Array

“…With respect to the first one, our sample contains brighter sources with harder gamma-ray spectra, but it contains fewer targets, while our spectra have on average higher and more uniform S/N. Regarding the campaign by Peña-Herazo et al (2020), we target brighter objects with harder gamma-ray spectra and with better determined classifications; we have fewer targets and we have much higher S/N spectra. With respect to both of these campaigns we can thus expect to have a higher redshift detection efficiency.…”

“…It has been shown for a sample of 100 X-ray detected BL Lacs that to date 90% of the 62 targets with a detected host galaxy (Nilsson et al 2003) have a spectroscopic redshift from spectroscopic programmes (see e.g. Peña-Herazo et al 2020;Paiano et al 2020), while more than 80% of the unresolved sources still do not have redshift values. On the other hand, spectra at S /N ∼ 100 and resolution ∼1000 allow the detection of weak host-galaxy features with EWs smaller than 5 Å and of intervening absorption systems (see e.g.…”

“…We cannot present an exhaustive literature review here, but we briefly compare the first results from our programme to those of the main previous and ongoing spectroscopic observations aimed at measuring the redshifts of gamma-ray blazars. While we present here observations of 19 sources, our campaign targets 165 sources; we thus limit our comparison to campaigns targeting extensive samples with about 100 targets or more: the Shaw et al (2013) campaign, the campaign described by Peña-Herazo et al (2020), and the recently undertaken campaign by Paiano et al (2020). With respect to the first one, our sample contains brighter sources with harder gamma-ray spectra, but it contains fewer targets, while our spectra have on average higher and more uniform S/N.…”

“…Other less extensive campaigns with similar aims, which have targeted smaller samples, include those of Masetti et al (2013), Marchesini et al (2016), and Klindt et al (2017). A synthesis of these recent observations has been presented by Peña-Herazo et al (2020) for a total of 416 BL Lacs and BCUs, 311 taken from their own observations and 105 from recent literature including the ones cited above. Only about 30% of these objects have spectroscopic redshift values, and their median redshift is z med = 0.285.…”

Identifying the 3FHL catalog: V. Results of the CTIO-COSMOS optical spectroscopy campaign 2019

“…To this end, spectroscopic campaigns of blazars have been effectively conducted with 4 m, 8 m and 10 m class telescopes to investigate the nature of these sources as well as to provide accurate redshift measurements (e.g., Sbarufatti et al 2005;Shaw et al 2012;Massaro et al 2014;Paggi et al 2014;Landoni et al 2015;Ricci et al 2015;Álvarez Crespo et al 2016a,b;Paiano et al 2017;Peña-Herazo et al 2017;Marchesi et al 2018;Paiano et al 2019Paiano et al , 2020Peña-Herazo et al 2020). The successes of these campaigns prove that the listed classes of telescopes are capable of distinguishing the two blazar subclasses, namely, BL Lacertae objects (BL Lacs) and Flat Spectrum Radio Quasars (FSRQs).…”