Uhecr Escape Mechanisms for Protons and Neutrons From Gamma-Ray Bursts, and the Cosmic-Ray-Neutrino Connection

Abstract

The paradigm that gamma-ray burst (GRB) fireballs are the sources of the ultra-high energy cosmic rays (UHECRs) is being probed by neutrino observations. Very stringent bounds can be obtained from the cosmic ray (proton)-neutrino connection, assuming that the UHECRs escape as neutrons. In this study, we identify three different regimes as a function of the fireball parameters: the standard "one neutrino per cosmic ray" case, the optically thick (to neutron escape) case, and the case where leakage of protons from the boundaries of the shells (direct escape) dominates. In the optically thick regime, photomeson production is very efficient, and more neutrinos will be emitted per cosmic ray than in the standard case, whereas in the direct escape-dominated regime, more cosmic rays than neutrinos will be emitted. We demonstrate that, for efficient proton acceleration, which is required to describe the observed UHECR spectrum, the standard case only applies to a very narrow region of the fireball parameter space. We illustrate with several observed examples that conclusions on the cosmic rayneutrino connection will depend on the actual burst parameters. We show that the definition of the pion production efficiency currently used by the IceCube collaboration underestimates the neutrino production in the optically thick case. Finally, we point out that the direct escape component leads to a spectral break in the cosmic ray spectrum emitted from a single source. The resulting "twocomponent model" can be used to even more strongly pronounce the spectral features of the observed UHECR spectrum than the dip model.

Multi-messenger Light Curves from Gamma-Ray Bursts in the Internal Shock Model

“…Another important aspect is that the relationship between neutrino and cosmic-ray emission depends on the UHECR escape mechanism (Baerwald et al 2013). It has been shown that, depending on the parameter values, a fraction of cosmic rays must directly escape from the sources without neutrino production (Baerwald et al 2013).…”

“…The maximum proton energy ¢ E p,max is computed by balancing the acceleration rate, and the synchrotron, adiabatic, and photohadronic energy loss rates (Baerwald et al 2013).…”

“…The latter situation occurs if the proton Larmor radius-determined by the physical conditions in the shellexceeds the shell width at the highest energies; see Baerwald et al (2013) for a detailed discussion. This direct escape produces a hard proton spectrum (it is dubbed a "high-pass filter" in Globus et al 2015).…”

“…,  = 5 6 p , and h = 1.0 (acceleration efficiency; Baerwald et al 2013). See Table 3 for an explanation of each parameter.…”

“…We compute secondary particle production using the NeuCosmA software Hümmer et al 2012;Baerwald et al 2013). This assumes a proton density…”