Upper limits from five years of blazar observations with the VERITAS Cherenkov telescopes

TL;DR

This study analyzes five years of VERITAS observations of 114 blazars, setting upper limits on their VHE gamma-ray fluxes, performing optical spectroscopy for redshift estimation, and conducting a stacked analysis revealing a 4 sigma excess.

Contribution

It provides the first comprehensive upper limits for a large sample of undetected blazars and includes optical spectroscopy to estimate redshifts, enhancing understanding of their VHE emission.

Findings

No significant VHE gamma-ray detection for individual sources.

A stacked analysis shows a 4 sigma excess, indicating potential collective emission.

Optical spectroscopy constrains redshifts for several blazars.

Abstract

Between the beginning of its full-scale scientific operations in 2007 and 2012, the VERITAS Cherenkov telescope array observed more than 130 blazars; of these, 26 were detected as very-high-energy (VHE; E>100 GeV) {\gamma}-ray sources. In this work, we present the analysis results of a sample of 114 undetected objects. The observations constitute a total live-time of ~570 hours. The sample includes several unidentified Fermi-Large Area Telescope (LAT) sources (located at high Galactic latitude) as well as all the sources from the second Fermi-LAT catalog which are contained within the field of view of the VERITAS observations. We have also performed optical spectroscopy measurements in order to estimate the redshift of some of these blazars that do not have a spectroscopic distance estimate. We present new optical spectra from the Kast instrument on the Shane telescope at the Lick observatory for 18 blazars included in this work, which allowed for the successful measurement or constraint on the redshift of four of them. For each of the blazars included in our sample we provide the flux upper limit in the VERITAS energy band. We also study the properties of the significance distributions and we present the result of a stacked analysis of the data-set, which shows a 4 {\sigma} excess.