Multiwavelength spectral modelling of the candidate neutrino blazar PKS 0735+178

The BL Lac object PKS 0735+178 was in its historic γ-ray brightness state during 2021 December. This period also coincides with the detection of a neutrino event IC 211208A, which was localized close to the vicinity of PKS 0735+178. We carried out detailed γ-ray timing and spectral analysis of the source in three epochs: (a) quiescent state (E₁), (b) moderate-activity state (E₂), and (c) high-activity state (E₃) coincident with the epoch of neutrino detection. During the epoch of neutrino detection (E₃), we found the largest variability amplitude of 95 per cent. The γ-ray spectra corresponding to these three epochs are well fit by the power-law model and the source is found to show spectral variations with a softer when brighter trend. In epoch E₃, we found the shortest flux doubling/halving time of 5.75 h. Even though the spectral energy distribution in the moderate-activity state and in the high-activity state could be modelled by the one-zone leptonic emission model, the spectral energy distribution in the quiescent state required an additional component of radiation over and above the leptonic component. Here, we show that a photomeson process was needed to explain the excess γ-ray emission in the hundreds of GeV that could not be accounted for by the synchrotron self-Compton process.