VERITAS observations of the BL Lac object TXS 0506+056

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 ($\ddot{I}ƒ$) 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 $4.8\pm 1.3$.