X-ray and Gamma-ray Follow-ups of IceCube Neutrino Alerts: The Case of PKS 0735+178

Follow-up observations of IceCube neutrino alerts across the electromagnetic spectrum remain a promising approach to search for astrophysical neutrino sources, despite the challenges in the association between a known source and a single neutrino event. Hard X-ray and TeV gamma-ray bands are particularly interesting because they offer strong constraints on the hadronic processes that also produce neutrinos. We present on a multi-wavelength target-of-opportunity program that includes data from the NuSTAR and VERITAS observatories. We focus on the observations of the blazar PKS 0735+178 triggered by the IceCube neutrino event IceCube-211208A, announced in December 2021. The blazar was observed at the highest X-ray and gamma-ray fluxes within days after the neutrino alert. The broadband SED observed with Swift, NuSTAR, Fermi, VERITAS, and H.E.S.S. captures a spectral upturn in the X-ray band and a spectral cutoff in the gamma-ray band, constraining the emission models of the blazar. Both leptonic and lepto-hadronic models, with the existence of external photons, can describe the observed SED. We show a numerical lepto-hadronic model that could potentially describe both the electromagnetic and neutrino emissions. Multi-messenger observations of correlated neutrino and gamma-ray events are important for the identification of astrophysical neutrino and ultra-high-energy cosmic-ray sources.