Gamma-ray Bursts

TL;DR

GRBs are catastrophic, relativistic explosions whose prompt emission and afterglows probe jet physics under extreme conditions. The paper synthesizes the leading models: prompt emission from internal shocks or magnetic dissipation in a relativistic outflow powered by a black-hole accretion or magnetar engine; afterglows arise from external forward shocks enabling precise localization and redshift measurements, revealing long GRBs from collapsing massive stars and short GRBs from compact-object mergers. Energetics are shown to be highly beamed, with $E_{iso}$ up to $10^{55}$ erg and jet-corrected energies around $10^{51}$ erg, and correlations like the Amati, Yonetoku, and Ghirlanda relations discussed, though their universality remains debated. Beyond GRB physics, GRBs serve as cosmological probes of distant galaxies, the intergalactic medium, and multi-messenger physics, including gravitational waves and Lorentz-invariance tests, positioning them as essential laboratories for high-energy astrophysics.

Abstract

Gamma-ray bursts are flashes of high-energy radiation lasting from a fraction of a second to several hours. Military satellites made the first detections of GRBs in the late 1960s. The $γ$-ray emission forms from shocks in a relativistic jet launched from a compact central engine. In addition to the emission of $γ$-rays, the interaction of the jet with the surrounding medium yields afterglow emission that can be observed across the electromagnetic spectrum. Redshift measurements from these afterglows place GRBs from the local to the distant Universe. The central engines of GRBs are thought to be either a hyperaccreting black hole or a highly magnetized neutron star (magnetar). There is now strong observational evidence that this central engine is created either in the core collapse of a rapidly rotating massive star or via the merger of two compact objects (neutron stars or a neutron star with a black hole). The combination of stellar scale events with extreme energies and luminosities makes GRBs powerful probes of the extreme physics involved in their production and of other areas of astrophysics and cosmology. These include as the electromagnetic counterparts of gravitational wave sources, the production and acceleration of relativistic jets, the synthesis of heavy elements, the study of the interstellar and intergalactic medium, and the identification of the collapse of early generations of stars.

Figures (6)

• Figure 1: A sample of gamma-ray burst lightcurves observed by the Swift BAT. These are all events which have led to substantial breakthroughs in our understanding of the GRB phenomena (see Table \ref{['tab:bursts']}).
• Figure 2: The hardness-duration diagram for GRBs observed by the BATSE instrument Kouveliotou+93, which was the first to obtain a large sample of both long and short-GRBs and conclusively demonstrate differences in duration and spectral hardness. The histograms show the collapsed distributions in each axis, while in the hardness panel the red histogram represents the bursts with $T_{90} >2$s and the purple $T_{90} <2$s.
• Figure 3: The observed properties of a GRB, demonstrated with the bright bursts GRB 080319B racusin08 and GRB 130427A perley14. The burst begins with the prompt emission, detected primarily in $\gamma$-rays, but on rare occasions also seen in optical light. This is followed by a much longer-lived afterglow which dominates the emission beyond a few hundred seconds in each case. The optical and radio light is dominated by the afterglow emission, but the observed frequencies lie on different parts of the synchrotron spectrum, which can yield varied temporal behavior for the different wavelength regimes. Ultimately, mapping the evolving multi-wavelength afterglow enables the full properties of the evolving spectrum to be mapped and hence the details of the explosion to be probed.
• Figure 4: The physical processes underlying the production of a GRB. The system begins with the formation of a progenitor. Or the many progenitors suggested in the literature several remain viable, although only the collapse of massive stars and the merger of two neutron stars have been observationally confirmed. This progenitor proceeds to produce a central engine that is either an accreting black hole or a highly magnetised neutron star. The central engine powers the jet, and interactions both inside (prompt) and external to the jet (afterglow) are responsible for the observed emission.
• Figure 5: The observational differences between supernovae and kilonova associated with GRBs showing the lightcurves (left) and spectra (right). We compare the best-sampled kilonova AT2017gfo with well-sampled GRB supernovae, in particular, SN 1998bw, although we also show the X-shooter spectrum of SN 2010bh to demonstrate the IR behavior of broad-lined SN Ic. Supernova lightcurves are approximately 100 times brighter at peak than kilonova and reach an optical peak on timescales of 10-20 days compared to $<1$ day for kilonova. However, supernovae remain dominated by optical light for most of their evolution, while the spectra of kilonovae show a clear shift into the infrared and peak beyond one micron on timescales of only a few days.