Dissecting the region around IceCube-170922A: the blazar TXS 0506+056 as
  the first cosmic neutrino source
  We present the dissection in space, time, and energy of the region around the
IceCube-170922A neutrino alert. This study is motivated by: (1) the first
association between a neutrino alert and a blazar in a flaring state, TXS
0506+056; (2) the evidence of a neutrino flaring activity during 2014 - 2015
from the same direction; (3) the lack of an accompanying simultaneous
$\gamma$-ray enhancement from the same counterpart; (4) the contrasting flaring
activity of a neighbouring bright $\gamma$-ray source, the blazar PKS 0502+049,
during 2014 - 2015. Our study makes use of multi-wavelength archival data
accessed through Open Universe tools and includes a new analysis of Fermi-LAT
data. We find that PKS 0502+049 contaminates the $\gamma$-ray emission region
at low energies but TXS 0506+056 dominates the sky above a few GeV. TXS
0506+056, which is a very strong (top percent) radio and $\gamma$-ray source,
is in a high $\gamma$-ray state during the neutrino alert but in a low though
hard $\gamma$-ray state in coincidence with the neutrino flare. Both states can
be reconciled with the energy associated with the neutrino emission and, in
particular during the low/hard state, there is evidence that TXS 0506+056 has
undergone a hadronic flare with very important implications for blazar
modelling. All multi-messenger diagnostics reported here support a single
coherent picture in which TXS 0506+056, a very high energy $\gamma$-ray blazar,
is the only counterpart of all the neutrino emissions in the region and
therefore the most plausible first non-stellar neutrino and, hence, cosmic ray
source.
