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We report on multiwavelength target-of-opportunity observations of the blazarPKS 0735+178, located 2.2 degrees away from the best-fit position of theIceCube neutrino event 211208A. The source was in a high-flux state in theoptical, ultraviolet, X-ray, and GeV gamma-ray bands around the time of theneutrino event, exhibiting daily variability in the soft X-ray flux. The X-raydata from Swift-XRT and NuSTAR characterize the transition between thelow-energy and high-energy components of the broadband spectral energydistribution, 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 strongconstraints on leptonic and hadronic models. We analytically explore asynchrotron self-Compton model, an external Compton model, and a lepto-hadronicmodel. Models that are entirely based on internal photon fields face seriousdifficulties in matching the observed spectral energy distribution (SED). Theexistence of an external photon field in the source would instead explain theobserved 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 Eddingtonlimit in the lepto-hadronic model. A numerical lepto-hadronic model withexternal target photons reproduces the observed SED and is reasonablyconsistent with the neutrino event despite requiring a high jet power.

Multiwavelength Observations of the Blazar PKS~0735+178 in Spatial and Temporal Coincidence with an Astrophysical Neutrino Candidate IceCube-211208A