  On February 17 2016, the IceCube real-time neutrino search identified, for
the first time, three muon neutrino candidates arriving within 100 s of one
another, consistent with coming from the same point in the sky. Such a triplet
is expected once every 13.7 years as a random coincidence of background events.
However, considering the lifetime of the follow-up program the probability of
detecting at least one triplet from atmospheric background is 32%. Follow-up
observatories were notified in order to search for an electromagnetic
counterpart. Observations were obtained by Swift's X-ray telescope, by ASAS-SN,
LCO and MASTER at optical wavelengths, and by VERITAS in the very-high-energy
gamma-ray regime. Moreover, the Swift BAT serendipitously observed the location
100 s after the first neutrino was detected, and data from the Fermi LAT and
HAWC observatory were analyzed. We present details of the neutrino triplet and
the follow-up observations. No likely electromagnetic counterpart was detected,
and we discuss the implications of these constraints on candidate neutrino
sources such as gamma-ray bursts, core-collapse supernovae and active galactic
nucleus flares. This study illustrates the potential of and challenges for
future follow-up campaigns.
