Constraints on The Hadronic Content of Gamma Ray Bursts

TL;DR

This paper uses the lack of neutrino detections from Gamma Ray Bursts to place constraints on the hadronic content of their jets, challenging models that predict high PeV proton presence.

Contribution

It provides the first constraints on the hadronic content of GRB jets based on neutrino non-detections and gamma-ray observations, independent of jet model parameters.

Findings

The ratio of pion to electron energy is constrained to be less than 0.24 at 95% confidence.

Gamma-ray data suggests the pion to electron energy ratio is less than 0.3, possibly as low as 0.03 in some bursts.

Results challenge the presence of PeV protons in GRB jets.

Abstract

The IceCube high-energy neutrino telescope has been collecting data since 2006. Conversely, hundreds of Gamma Ray Bursts (GRBs) have been detected by the GBM 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 $E^{-\alpha}$ with $\alpha = 2$, we find that the ratio of the energy carried by pions to that in electrons has to be small $f_\pi / f_e \lesssim 0.24$ at 95\% confidence level. A distribution of spectral slopes can lower $f_\pi / f_e$ by orders of magnitude. Another limit, independent of neutrinos, is obtained if one ascribes the measured Fermi/LAT 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 $\approx 10$, we show that $f_\pi / f_e \lesssim 0.3$. In some bursts, where this ratio is as low as unity, $f_\pi / f_e \lesssim 0.03$. These findings add to the mounting doubts regarding the presence of PeV protons in GRB jets.