The brightest electromagnetic explosions in the universe — collapsars, relativistic jets, and afterglows that light up the gamma-ray sky for seconds and the radio sky for years.
Scroll into the collapsarLong-duration GRBs trace back to the collapse of massive, rapidly rotating stars — typically stripped-envelope Wolf–Rayet progenitors with M > 25 M☉. After millions of years of fusion, the iron core can no longer be supported.
The angular momentum, magnetic geometry, and stripping history of this progenitor decide whether the resulting compact object can launch a relativistic jet — or whether the collapse will fizzle into an ordinary supernova.
In less than a second, the iron core implodes to nuclear densities. A proto-neutron star forms; if rotation is high enough and accretion continues, it collapses further into a stellar-mass black hole surrounded by an accretion torus.
The shock launched by the bounce stalls inside the envelope. Energy must escape somehow — and that's where the central engine takes over.
Two narrow, ultra-relativistic jets are launched along the rotation axis, drilling through the stellar envelope. Bulk Lorentz factors of Γ ≳ 100–1000 are required to avoid catastrophic pair-production opacity — the famous "compactness problem."
If the jets succeed in piercing the star within the collapse timescale, we see a GRB. If they stall, we see a low-luminosity event or a failed-jet supernova instead.
Internal shocks between shells of different Lorentz factors in the jet — or magnetic reconnection in a Poynting-flux-dominated outflow — convert bulk kinetic energy into gamma-rays at MeV energies. This is what triggers Swift/Fermi.
The light curve is famously erratic, lasting from a fraction of a second to several minutes, with millisecond variability — a direct fingerprint of the central engine activity time and inner-jet physics.
As the jet decelerates against the circumburst medium, a forward shock accelerates electrons that synchrotron-radiate from X-ray to radio. The afterglow chronicles the jet's geometry, the surrounding density profile, and the microphysics of the shock.
My doctoral work with the GROWTH-India Telescope focused on rapid optical follow-up of Swift and Fermi triggers, mapping the early afterglow phase that constrains the reverse shock and the deceleration radius.