Searching for γ-ray signature in WHSP blazars
Fermi-LAT detection of 150 excess signal in the 0.3−500 GeV band
1 ASI Science Data Center, ASDC, Agenzia Spaziale Italiana, via del Politecnico snc, 00133 Roma, Italy
2 Sapienza Università di Roma, Dipartimento di Fisica, Piazzale Aldo Moro 5, 00185 Roma, Italy
3 ICRANet-Rio, CBPF, rua Dr. Xavier Sigaud 150, 22290-180 Rio de Janeiro, Brazil
Received: 11 April 2016
Accepted: 1 September 2016
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 νf(ν) ≥ 10-12.1 erg/cm2/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.
Key words: galaxies: active / BL Lacertae objects: general / radiation mechanisms: non-thermal / gamma rays: diffuse background / gamma rays: general
© ESO, 2017