VERITAS observations of the BL Lac object TXS 0506+056

TL;DR

This paper reports the detection of VHE gamma-ray emission from the blazar TXS 0506+056 by VERITAS following a neutrino alert, providing insights into the source's high-energy behavior and its potential link to astrophysical neutrinos.

Contribution

First VERITAS detection of VHE gamma rays from TXS 0506+056 during a neutrino event, establishing a connection between gamma-ray emission and neutrino production.

Findings

TXS 0506+056 was detected with 5.8σ significance in 35 hours.

The source's flux was 1.6% of the Crab Nebula flux above 110 GeV.

The spectral index of the emission was soft, at 4.8 ± 1.3.

Abstract

On 2017 September 22, the IceCube Neutrino Observatory reported the detection of the high-energy neutrino event \icnu, of potential astrophysical origin. It was soon determined that the neutrino direction was consistent with the location of the gamma-ray blazar \txs~(3FGL J0509.4+0541), which was in an elevated gamma-ray emission state as measured by the \emph{Fermi} satellite. VERITAS observations of the neutrino/blazar region started on 2017 September 23 in response to the neutrino alert and continued through 2018 February 6. While no significant very-high-energy (VHE; E $>$ 100 GeV) emission was observed from the blazar by VERITAS in the two-week period immediately following the IceCube alert, TXS 0506+056 was detected by VERITAS with a significance of 5.8 standard deviations ($\sigma$) in the full 35-hour data set. The average photon flux of the source during this period was $(8.9 \pm 1.6) \times 10^{-12} \; \mathrm{cm}^{-2} \, \mathrm{s}^{-1}$, or 1.6\% of the Crab Nebula flux, above an energy threshold of 110 GeV, with a soft spectral index of $4.8 \pm 1.3$.