An exploration of hadronic interactions in blazars using IceCube

Context. Hadronic models, involving matter (proton or nuclei) acceleration in blazar jets, imply high energy photon and neutrino emissions due to interactions of high-energy protons with matter and/or radiation in the source environment. Aims. This paper shows that the sensitivity of the IceCube neutrino telescope in its 40-string configuration (IC-40) is already at the level of constraining the parameter space of purely hadronic scenarios of activity of blazars. Methods. Assuming that the entire source power originates from hadronic interactions, and assuming that the model describe the data, we estimate the expected neutrino flux from blazars based on the observed gamma-ray flux by Fermi, simultaneously with IC-40 observations. We consider two cases separately to keep the number of constrainable parameters at an acceptable level: proton-proton or proton-gamma interactions are dominant. Comparing the IC-40 sensitivity to the neutrino flux expected from some of the brightest blazars, we constrain model parameters characterizing the parent high-energy proton spectrum. Results. Using together the Fermi photon observations and the IC-40 neutrino sensitivities, we find that when pp interactions dominate, some constraints on the primary proton spectrum can be imposed. For instance, for the tightest constrained source 3C 454.3, the very high energy part of the spectra of blazars is constrained to be harder than E-2 with cut-off energies in the range of E-cut >= 10(18) eV. When interactions of high-energy protons on soft photon fields dominate, we can find similarly tight constraints on the proton spectrum parameters if the soft photon field is in the UV range or at higher energy, e.g. if it originates from the big blue bump produced by the accretion disk. The tightest constraint is for 3C 273, with the cut-off energy constrained to be E-cut greater than or similar to 10(18) eV for any spectral index and different soft photon fields, including the radiation from the accretions disk, broad line region or the synchrotron radiation from the jet. Conclusions. We have adopted a simplified approach using the IC-40 sensitivity (about one half of the full detector) to constrain parameters of blazar hadronic models. Data of the full detector are already available and corresponding constraints should be about a factor of 3 better than what discussed here.