Gamma-ray bursts

Gamma-ray bursts (GRBs) are the most luminous explosions in the universe, releasing isotropic-equivalent energies up to ~10⁵⁴ erg in seconds. Long GRBs (> 2 s) arise from the collapsar scenario: the core of a rapidly rotating, stripped massive star (a Wolf–Rayet star of ≳ 20–30 M☉) collapses to a black hole, which drives a jet that drills through the stellar envelope and breaks out with a bulk Lorentz factor Γ ≈ 100–1000. Short GRBs (< 2 s) come from neutron-star mergers, as confirmed by GW170817.

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The fireball and its shocks

In the standard fireball model, the central engine ejects a series of shells with different Lorentz factors:

• Internal shocks — faster shells catch up with slower ones at R ~ 10¹³–10¹⁵ cm, producing the erratic, spiky prompt gamma-ray light curves. With B ~ 10³–10⁶ G in the shocked regions, the Hillas condition allows proton acceleration to ~10²⁰ eV — the classic proposal by Waxman (1995) and Vietri (1995) that made GRBs one of the first quantitative UHECR source candidates.
• External shock — the ejecta finally decelerate against the circumburst medium, producing the long-lived, fading afterglow from X-rays to radio. The forward shock can further accelerate particles, and late-time afterglow shocks are an alternative UHECR site.

Observational status

• GRB 221009A (the “BOAT” — brightest of all time) was so luminous that its photons above 10 TeV challenged standard emission models, renewing interest in hadronic processes in GRB jets.
• IceCube stacking searches find no neutrinos coincident with thousands of GRBs, constraining the prompt-phase proton content: standard internal-shock scenarios can supply at most ~1–10% of the UHECR flux unless the emission region is larger or the baryon loading is tuned.
• The local GRB rate (~1 Gpc⁻³ yr⁻¹ beamed) is low; each burst must then eject ~10⁵³ erg in cosmic rays to sustain the observed flux — energetically demanding, but not excluded, especially if low-luminosity GRBs and trans-relativistic supernovae (far more numerous) contribute.

Challenges

The neutrino non-detection is the strongest constraint on GRBs as the dominant UHECR source. Low-luminosity GRBs, choked jets, and afterglow acceleration remain viable variants — and, like TDEs, GRBs are transients, so magnetic time delays hide the association between bursts and arriving particles.

Related

• Blazars & AGN jets — persistent relativistic jets
• Tidal disruption events — jetted transients from supermassive black holes
• Open questions