A survey of active galaxies at TeV photon energies with the HAWC gamma-ray observatory

TL;DR

This paper presents a comprehensive survey of TeV gamma-ray emission from active galaxies using four and a half years of HAWC data, detecting persistent signals from key blazars and setting upper limits on others.

Contribution

It provides the first extensive analysis of TeV emission from a large sample of active galaxies with the HAWC observatory, including detections and upper limits.

Findings

Detected persistent TeV emission from Mkn 421 and Mkn 501.

Found evidence for emission from M87, VER J0521+211, and 1ES 1215+303.

Excluded collective emission from most sources except Mkn 421 and Mkn 501.

Abstract

The High Altitude Water Cherenkov Gamma-Ray Observatory (HAWC) continuously detects TeV photons and particles within its large field-of-view, accumulating every day a deeper exposure of two thirds of the sky. We analyzed 1523~days of HAWC live data acquired over four and a half years, in a follow-up analysis of {138} nearby ($z<0.3$) active galactic nuclei from the {\em Fermi} 3FHL catalog culminating within $40^\circ$ of the zenith at Sierra Negra, the HAWC site. This search for persistent TeV emission used a maximum-likelihood analysis assuming intrinsic power-law spectra attenuated by pair production of gamma-ray photons with the extragalactic background light. HAWC clearly detects persistent emission from Mkn~421 and Mkn~501, the two brightest blazars in the TeV sky, at 65$\sigma$ and 17$\sigma$ level, respectively. {Weaker evidence for long-term emission is found for three other known very-high energy emitters:} the radiogalaxy M87 and the BL Lac objects VER~J0521+211 and 1ES~1215+303, the later two at $z\sim 0.1$. We find evidence for collective emission from the set of 30 previously reported very high-energy sources that excludes Mkn~421 and Mkn~501 with a random probability $\sim 10^{-5}$. Upper limits are presented for the sample under the power-law assumption and in the predefined (0.5-2.0), (2.0-8.0) and (8.0-32.0) TeV energy intervals.