Ultrahigh Energy Cosmic Rays from Gamma Ray Bursts: Implications of the Recent Observational Results by Milagro

It has been speculated earlier that Gamma Ray Bursts are sources of ultrahigh energy cosmic rays. Recently, the search for high energy photons from Gamma Ray Bursts by Milagro group has put limits on the isotropic luminosity of these transient sources in very high energy photons. The implications of the results obtained by Milagro to our understanding of the ultrahigh energy cosmic ray spectrum from these sources have been discussed in the present work. PACS: 98.70.Rz; 96.40.-z; 95.85.Pw Gamma Ray Bursts (GRBs) are likely to be excellent sites for particle acceleration and are expected to emit ultrahigh energy cosmic rays (UHECR) [1, 2]. Earlier, several groups have investigated the possibility of TeV photon emission from GRBs [3, 4] in the burst phase. Mechanisms have been invoked which can give rise to TeV energy photons inside GRBs. They are (i) synchrotron self-compton (SSC) process, which involves inverse compton scattering of sufficiently high energy electrons on the ambient synchrotron photons within the source, the synchrotron photons being radiated by non-thermal population of electrons inside the GRB, (ii) photo-pion production by ultrahigh energy protons within the source and the subsequent decay of the neutral pions to photons, (iii) synchrotron radiation by ultrahigh energy protons having energies above 10eV in the magnetic field within the source. TeV photons would interact with the low energy photons inside the source and also with the photons of the intergalactic medium. Their flux would be diminished due to their annihilation with the low energy photons before reaching the Earth. Milagro telescope located in northern New Mexico observes the northern sky for 100GeV to 100TeV γ-ray emission. The Milagro group searched for 40s−3h transient very high energy emission [5, 6]. They have obtained flux limits and also constrained the absolute luminosity of GRBs in very high energy γ-rays. Their limits on very Email address: [email protected]