We report on multiwavelength target-of-opportunity observations of the blazar
PKS 0735+178, located 2.2 degrees away from the best-fit position of the
IceCube neutrino event 211208A. The source was in a high-flux state in the
optical, ultraviolet, X-ray, and GeV gamma-ray bands around the time of the
neutrino event, exhibiting daily variability in the soft X-ray flux. The X-ray
data from Swift-XRT and NuSTAR characterize the transition between the
low-energy and high-energy components of the broadband spectral energy
distribution, and the gamma-ray data from Fermi-LAT, VERITAS, and H.E.S.S.
require a spectral cut-off near 100 GeV. Both measurements provide strong
constraints on leptonic and hadronic models. We analytically explore a
synchrotron self-Compton model, an external Compton model, and a lepto-hadronic
model. Models that are entirely based on internal photon fields face serious
difficulties in matching the observed spectral energy distribution (SED). The
existence of an external photon field in the source would instead explain the
observed gamma-ray spectral cut-off in both leptonic and lepto-hadronic models,
and it would allow a proton jet power that marginally agrees with the Eddington
limit in the lepto-hadronic model. A numerical lepto-hadronic model with
external target photons reproduces the observed SED and is reasonably
consistent with the neutrino event despite requiring a high jet power.Comment: Proceedings paper presented at the 38th International Cosmic Ray
Conference (ICRC2023), held 26 July - 3 August, 2023, in Nagoya, Japa