High-Energy Particles from γ-Ray Bursts

Abstract

The widely accepted interpretation of the phenomenology of γ-ray bursts (GRBs), bursts of 0.1 MeV-1 MeV photons lasting for a few seconds (see [1] for a review), is that the observable effects are due to the dissipation of the kinetic energy of a relativistically expanding wind, a “fireball,” whose primal cause is not yet known (see [2],[3] for reviews). The recent detection of “afterglows,” delayed low energy (X-ray to radio) emission of GRBs (see [4] for review), confirmed the cosmological origin of the bursts, through the redshift determination of several GRB host-galaxies, and confirmed standard model predictions of afterglows that result from the collision of an expanding fireball with its surrounding medium (see[5] for review). In this review, the production in GRB fireballs of γ-rays, high-energy cosmic-rays and neutrinos is discussed in the light of recent GRB and ultra-high-energy cosmic-ray observations.

The fireball model is described in detail in Sect.2. We do not discuss in this section the issue of GRB progenitors, i.e. the underlying sources producing the relativistic fireballs. At present, the two leading progenitor scenarios are collapses of massive stars [6],[7], and mergers of compact objects [8],[9]. As explained in Sect.2, the evolution of the fireball and the emission of γ-rays and afterglow radiation (on time scale of a day and longer) are largely independent of the nature of the progenitor. Thus, although present observations provide stringent constraints on the fireball model, the underlying progenitors remain unknown (e.g. [10]; see [4],[5] for discussion). In Sect.3, constraints imposed on the fireball model by recent afterglow observations are discussed, which are of importance for high energy particle production.

The association of GRBs and ultra-high energy cosmic-rays (UHECRs) is discussed in Sect.4. Recent afterglow observations strengthen the evidence for GRB and UHECR association, which is based on two key points (see [11] for recent review). First, the constraints imposed on fireball model parameters by recent observations imply that acceleration of protons is possible to energy higher than previously assumed, 10²¹ eV. Second, the inferred local (z = 0) GRB energy generation rate of γ-rays, 10⁴⁴erg/Mpc³yr, is remarkably similar to the local generation rate of UHECRs implied by cosmic-ray observations.

Based on lectures given at the ICTP Summer School on Astroparticle Physics and Cosmology (ICTP, Trieste Italy, June 2000), and at the VI Gleb Wataghin School on High Energy Phenomenology (UNICAMP, Campinas Brazil, July 2000).