Constraints on the hadronic content of gamma ray bursts

The IceCube High-energy Neutrino Telescope has been collecting data since 2006. Conversely, hundreds of gamma-ray bursts (GRBs) have been detected by the GRB Monitor on board Fermi since its launch in 2008. So far no neutrino event has been associated with a GRB, despite many models predicting the generation of high-energy neutrinos through GRB photon interaction with PeV protons in the GRB jet. We use the non-detection of neutrinos to constrain the hadronic content of GRB jets independent of jet model parameters. Assuming a generic particle spectrum of E ^–α with α = 2, we find that the ratio of the energy carried by pions to that in electrons has to be small f _π/f_e ≲ 0.24 at 95% confidence level. A distribution of spectral slopes can lower f _π/f_e by orders of magnitude. Another limit, independent of neutrinos, is obtained if one ascribes the measured Fermi/Large Area Telescope GeV gamma-ray emission to pair-photon cascades of high-energy photons resulting from (the same photon-hadronic interactions and subsequent) neutral pion decays. Based on the generally observed MeV-to-GeV GRB fluence ratio of ≈10, we show that f _π/f_e ≲ 0.3. In some bursts, this ratio is as low as unity, f _π/f_e ≲ 0.03. These findings add to mounting doubts regarding the presence of PeV protons in GRB jets.