Searching forγ-ray signature in WHSP blazars

Abstract

Aims. A direct search of γ-ray emission centered on multifrequency selected candidates is a valuable complementary approach to the standard search adopted in current γ-ray Fermi-LAT catalogs. Our sources are part of the 2WHSP sample that was assembled with the aim of providing targets for Imaging Atmospheric Cherenkov Telescopes (IACT). A likelihood analysis based on their known position enabled us to detect 150 γ-ray excess signals that have not yet been reported in previous γ-ray catalogs (1FGL, 2FGL, 3FGL). By identifying new sources, we solve a fraction of the extragalactic isotropic γ-ray background (IGRB) composition, improving the description of the γ-ray sky. Methods. We perform data reduction with the Fermi Science Tools using positions from 400 high synchrotron peaked (HSP) blazars as seeds of tentative γ-ray sources; none of them have counterparts from previous 1FGL, 2FGL and 3FGL catalogs. Our candidates are part of the 2WHSP sample (currently the largest set of HSP blazars). We focus on HSPs characterized by bright synchrotron component with peak flux ν f (ν) ≥ 10 −12.1 erg/cm 2 /s, testing the hypothesis of having a γ-ray source in correspondence to the WHSP positions. Our likelihood analysis considers the 0.3−500 GeV energy band, integrating over 7.2 yr of Fermi-LAT observation and making use of the Pass 8 data release. Results. From the 400 candidates tested, a total of 150 2WHSPs showed excess γ-ray signature: 85 high-significance detections with test statistic (TS) > 25, and 65 lower-significance detections with TS between 10 to 25. We assume a power law spectrum in the 0.3−500 GeV band and list the spectrum parameters describing all 150 new γ-ray sources. We study the γ-ray photon spectral index distribution, the likelihood of detection according to the synchrotron peak brightness (figure of merit parameter), and plot the measured γ-ray LogN-LogS of HSP blazars, also discussing the portion of the IGRB that has been resolved by the present work. We also report on four cases where we could resolve source confusion and find counterparts for unassociated 3FGL sources with the help of high-energy TS maps together with multifrequency data. The 150 new γ-ray sources are named with the acronym 1BIGB for the first version of the Brazil ICRANet Gamma-ray Blazar catalog, in reference to the cooperation agreement supporting this work.

Hunting extreme BL Lacertae blazars with Fermi-Large Area Telescope

“…To do so, we cross-match the entire BCU list from 4LAC-DR2 with the 2BIGB catalog (Arsioli et al 2020). 2BIGB, built on top of another catalog named 3HSP (Chang et al 2019), and other previous releases (Arsioli & Chang 2017;Arsioli et al 2018). The 3HSP catalog includes a new analysis of 𝛾-ray data from LAT, taken over the first 11 years of mission, at locations of infrared sources with similar properties to HSP objects.…”

The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope

“…The 4LAC includes 2863 sources, with 650 FSRQs, 1052 BL Lacs, 1092 BCUs, and 68 other AGNs. Some 1347 sources were not reported in the previous 3LAC catalog, but some of them may have been reported elsewhere, e.g., Arsioli & Chang (2017); Arsioli & Polenta (2018). These new sources comprise 195 FSRQs, 276 BL Lacs, 836 BCUs, and 40 non-blazar AGNs.…”

A search for new γ-ray blazars from infrared selected candidates

“…In 3FGL and 4FGL catalogues, sources were detected at 4σ significance (Acero et al 2015;Abdollahi et al 2020, respectively). However, in this analysis, sources with √ T S ∼ 3σ are considered to be γ-ray detections with low significance following the discussion from Arsioli et al (2017Arsioli et al ( , 2018. All cases with TS between 9 and 25 are considered to be a relevant excess signature, given that those cases actually have a multi-frequency counterpart (radio to X-rays) as expected from blazars.…”

“…A complete description of the EGB is still an open issue in gamma-ray astronomy. Therefore, searching for new gamma-ray sources from low energy blazar candidates can improve the description of the gamma-ray sky (Arsioli et al 2017).…”