On 2017 September 22 , the IceCube Neutrino Observatory reported the detection of the high-energy neutrino event IC 170922A , of potential astrophysical origin .
It was soon determined that the neutrino direction was consistent with the location of the gamma-ray blazar TXS 0506+056 ( 3FGL J0509.4+0541 ) , which was in an elevated gamma-ray emission state as measured by the 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 .