Searching for $\gamma$-ray signature in WHSP blazars: Fermi-LAT detection of 150 excess signal in the 0.3-500 GeV band

Abstract

A direct search of $\gamma$-ray emission centered on multi-frequency selected candidates is a valuable complementary approach to the standard search adopted in current $\gamma$-ray Fermi-LAT catalogs. Our candidates are part of the 2WHSP sample, that was assembled with the aim of providing targets for Imaging Atmospheric Cherenkov Telescopes (IACT), and is currently the largest set of high synchrotron peaked (HSP) blazars. We perform a likelihood analysis with the Fermi Science Tools using positions from 400 2WHSP blazars as seeds of tentative $\gamma$-ray sources. This enabled us to detect 150 $\gamma$-ray excess signals that have not yet been reported in previous $\gamma$-ray catalogs (1FGL, 2FGL, and 3FGL). By identifying new sources, we solve a fraction of the extragalactic isotropic $\gamma$-ray background (IGRB) composition, improving the description of the $\gamma$-ray sky. Our analysis considers the 0.3-500 GeV energy band, integrating over 7.2 yrs of Fermi-LAT observation and making use of the Pass 8 data release. Within the 150 2WHSPs that showed excess $\gamma$-ray signature: 85 are high-significance detections with test statistic (TS)$>$25, and 65 are lower-significance detections with TS between 10 to 25. We study the $\gamma$-ray photon spectral index distribution, the likelihood of detection according to the synchrotron peak brightness, and plot the measured $\gamma$-ray LogN-LogS of HSP blazars, also discussing the portion of the IGRB that has been resolved by this work. We 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 multi-frequency data. The 150 new $\gamma$-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.