The Detection of Teraelectronvolt Radiation from a Flat Spectrum Radio Quasar

TL;DR

This paper reports the first detection of very high-energy gamma-ray emission from a flat spectrum radio quasar, revealing that such objects can accelerate particles to TeV energies despite dense photon fields.

Contribution

It presents the first VHE detection of an FSRQ, S5 1027+74, including the first TeV emission from this class, challenging previous assumptions about their opacity.

Findings

First >7σ VHE detection of an FSRQ.

Detection of a spectral break with a rising shape above 10 GeV.

Evidence of a third emission bump peaking at multi-TeV energies.

Abstract

The very high-energy (VHE; $>$100 GeV) radiation carries the signatures of the matter-energy interaction in some of the most extreme astrophysical environments. Considering broad emission line blazars, i.e., flat spectrum radio quasars (FSRQs), the dense photon fields surrounding the relativistic jet can prohibit the particle population from accelerating to very high energies and producing VHE radiation. They can also possibly make the environment opaque for the VHE $\gamma$ rays due to $\gamma\gamma$ pair production, thus explaining the paucity of VHE-detected FSRQs and non-detection of TeV radiation ($>$1 TeV) from them. Here we report, for the first time, a $>$7$\sigma$ detection of an FSRQ, S5 1027+74 ($z=0.123$), in the VHE band, including the first ever detection of TeV emission from an object of this class, using the Fermi Large Area Telescope observations. Its $\gamma$-ray spectrum covering the 100 MeV to 2 TeV band revealed a prominent spectral break with a flat, rising shape above $\sim$10 GeV, a feature never detected from other VHE-detected FSRQs. The radio-to-$\gamma$-ray spectral energy distribution of S5 1027+74 provides strong evidence of a third bump peaking at multi-TeV energies. These enigmatic findings imply that FSRQ jets can accelerate particles to extremely high energies and provide tantalizing clues about the complex radiative environment of relativistic jets.