Flux upper limits from the multiwavelength observations. The confidence level varies between the different observations as indicated in the legend and some limits depend on the assumed source spectrum (Swift XRT and BAT Γ = −2 and Fermi LAT Γ = −2.1; see Sect. 4). For the optical telescopes, the limit corresponding to the deepest observation is shown, while for the other instruments, all analyzed data were combined. The limit for the Swift BAT is purely based on the observation taken 100 s after the detection of the first neutrino (compare Sect. 4.2.1) and hence applies to prompt gamma-ray emission. Follow-up observations were triggered 22 h after the detection of the neutrino triplet.