VERITAS Results from Deep Exposure on the Distant FSRQ 4C +55.17

TL;DR

This study reports on deep VERITAS observations of the distant FSRQ 4C +55.17, aiming to detect VHE gamma-ray emission and constrain emission regions and EBL models, but results show no detection, supporting absorption within the broad-line region.

Contribution

First deep VHE observational constraints on the distant FSRQ 4C +55.17, testing emission location hypotheses and EBL models at high redshift.

Findings

No VHE gamma-ray detection was made.

The upper limits support gamma-ray absorption within the broad-line region.

Results help constrain EBL models at z=0.89.

Abstract

We present results from a deep VERITAS exposure of the distant (z=0.89) flat-spectrum radio quasar (FSRQ) 4C +55.17. The high flux, hard index and steady emission found by Fermi LAT observations make this blazar a promising very-high-energy (VHE; E>100 GeV) candidate, offering a possibility to clarify the location of FSRQ VHE emission. Non-detection supports the hypothesis that any VHE gamma-rays are produced within and absorbed by the broad-line region while VHE detection would support an emission region outside the broad line region and far from the base of the jet. This FSRQ additionally provides the possible means, by photon-photon pair production, to constrain the currently available extragalactic background light (EBL) models out to the groundbreaking redshift of z=0.89. The log-parabolic model that is fitted to the LAT photons allows an extrapolation of the fit up to VHE while accounting for the gamma-ray absorption by the EBL. The VERITAS upper limit derived from the deep exposure is compared to this extrapolated VHE flux.