  Detection of the IceCube-170922A neutrino coincident with the flaring blazar
TXS 0506+056, the first and only 3-sigma high-energy neutrino source
association to date, offers a potential breakthrough in our understanding of
high-energy cosmic particles and blazar physics. We present a comprehensive
analysis of TXS 0506+056 during its flaring state, using newly collected Swift,
NuSTAR, and X-shooter data with Fermi observations and numerical models to
constrain the blazar's particle acceleration processes and multimessenger
(electromagnetic and high-energy neutrino) emissions. Accounting properly for
electromagnetic cascades in the emission region, we find a
physically-consistent picture only within a hybrid leptonic scenario, with
gamma-rays produced by external inverse-Compton processes and high-energy
neutrinos via a radiatively-subdominant hadronic component. We derive robust
constraints on the blazar's neutrino and cosmic-ray emissions and demonstrate
that, because of cascade effects, the 0.1-100keV emissions of TXS 0506+056
serve as a better probe of its hadronic acceleration and high-energy neutrino
production processes than its GeV-TeV emissions. If the IceCube neutrino
association holds, physical conditions in the TXS 0506+056 jet must be close to
optimal for high-energy neutrino production, and are not favorable for
ultra-high-energy cosmic-ray acceleration. Alternatively, the challenges we
identify in generating a significant rate of IceCube neutrino detections from
TXS 0506+056 may disfavor single-zone models. In concert with continued
operations of the high-energy neutrino observatories, we advocate regular X-ray
monitoring of TXS 0506+056 and other blazars in order to test single-zone
blazar emission models, clarify the nature and extent of their hadronic
acceleration processes, and carry out the most sensitive possible search for
additional multimessenger sources.
