  Individual astrophysical sources previously detected in neutrinos are limited
to the Sun and the supernova 1987A, whereas the origins of the diffuse flux of
high-energy cosmic neutrinos remain unidentified. On 22 September 2017 we
detected a high-energy neutrino, IceCube-170922A, with an energy of
approximately 290 TeV. Its arrival direction was consistent with the location
of a known gamma-ray blazar TXS 0506+056, observed to be in a flaring state. An
extensive multi-wavelength campaign followed, ranging from radio frequencies to
gamma-rays. These observations characterize the variability and energetics of
the blazar and include the first detection of TXS 0506+056 in very-high-energy
gamma-rays. This observation of a neutrino in spatial coincidence with a
gamma-ray emitting blazar during an active phase suggests that blazars may be a
source of high-energy neutrinos.
