Gamma-ray bursts (GRBs) display a bimodal duration distribution with a separation between the short- and long-duration bursts at about 2 s. The progenitors of long GRBs have been identified as massive stars based on their association with Type Ic core-collapse supernovae (SNe), their exclusive location in star-forming galaxies, and their strong correlation with bright UV regions within their host galaxies. Short GRBs have long been suspected on theoretical grounds to arise from compact object binary mergers (neutron star–neutron star or neutron star–black hole). The discovery of short GRB afterglows in 2005 provided the first insight into their energy scale and environments, as well as established a cosmological origin, a mix of host-galaxy types, and an absence of associated SNe. In this review, I summarize nearly a decade of short GRB afterglow and host-galaxy observations and use this information to shed light on the nature and properties of their progenitors, the energy scale and collimation of the relativistic outflow, and the properties of the circumburst environments. The preponderance of the evidence points to compact object binary progenitors, although some open questions remain. On the basis of this association, observations of short GRBs and their afterglows can shed light on the on- and off-axis electromagnetic counterparts of gravitational wave sources from the Advanced LIGO/Virgo experiments.

Short-Duration Gamma-Ray Bursts

The physics of gamma-ray bursts & relativistic jets

We provide a comprehensive review of major developments in our understanding of gamma-ray bursts, with particular focus on the discoveries made within the last fifteen years when their true nature was uncovered. We describe the observational properties of photons from the radio to multi-GeV bands, both in the prompt emission and the afterglow phases. Mechanisms for the generation of these photons in GRBs are discussed and confronted with observations to shed light on the physical properties of these explosions, their progenitor stars and the surrounding medium. After presenting observational evidence that a powerful, collimated, jet moving at close to the speed of light is produced in these explosions, we describe our current understanding regarding the generation, acceleration, and dissipation of the jet and compare these properties with jets associated with AGNs and pulsars. We discuss mounting observational evidence that long duration GRBs are produced when massive stars die, and that at least some short duration bursts are associated with old, roughly solar mass, compact stars. The question of whether a black-hole or a strongly magnetized, rapidly rotating neutron star is produced in these explosions is also discussed. We provide a brief summary of what we have learned about relativistic collisionless shocks and particle acceleration from GRB afterglow studies, and discuss the current understanding of radiation mechanism during the prompt emission phase. We discuss theoretical predictions of possible high-energy neutrino emission from GRBs and the current observational constraints. Finally, we discuss how these explosions may be used to study cosmology, e.g. star formation, metal enrichment, reionization history, as well as the formation of first stars and galaxies in the universe.

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The Physics of Gamma-Ray Bursts and Relativistic Jets

Long duration gamma-ray bursts (GRBs) originate from the core collapse of massive stars, but the identity of the central engine remains elusive. Previous work has shown that rapidly spinning, strongly magnetized protoneutron stars ('millisecond protomagnetars') produce outflows with energies, time-scales and magnetizations σ 0 (maximum Lorentz factor) that are consistent with those required to produce long duration GRBs. Here we extend this work in order to construct a self-consistent model that directly connects the properties of the central engine to the observed prompt emission. Just after the launch of the supernova shock, a wind heated by neutrinos is driven from the protomagnetar. The outflow is collimated into a bipolar jet by its interaction with the progenitor star. As the magnetar cools, the wind becomes ultrarelativistic and Poynting flux dominated (σ 0 >> 1) on a time-scale comparable to that required for the jet to clear a cavity through the star. Although the site and mechanism of the prompt emission are debated, we calculate the emission predicted by two models: magnetic dissipation and shocks. Magnetic reconnection may occur near the photosphere if the outflow develops an alternating field structure due to e.g. magnetic instabilities or a misalignment between the magnetic and rotation axes. Shocks may occur at larger radii because the Lorentz factor of the wind increases with time, such that the faster jet at late times collides with slower material released earlier. Our results favour magnetic dissipation as the prompt emission mechanism, in part because it predicts a relatively constant 'Band' spectral peak energy E peak with time during the GRB. The baryon loading of the jet decreases abruptly when the neutron star becomes transparent to neutrinos at t = t v-thin ~ 10-100 s. Jets with ultrahigh magnetization cannot effectively accelerate and dissipate their energy, which suggests this transition ends the prompt emission. This correspondence may explain both the typical durations of long GRBs and the steep decay phase that follows. Residual rotational or magnetic energy may continue to power late time flaring or afterglow emission, such as the X-ray plateau. We quantify the emission predicted from protomagnetars with a wide range of physical properties (initial rotation period, surface dipole field strength and magnetic obliquity) and assess a variety of phenomena potentially related to magnetar birth, including low-luminosity GRBs, very luminous GRBs, thermal-rich GRBs/X-ray flashes, very luminous supernovae and short-duration GRBs with extended emission.

The protomagnetar model for gamma-ray bursts

With its rapid-response capability and multiwavelength complement of instruments, the Swift satellite has transformed our physical understanding of γ-ray bursts (GRBs). Providing high-quality observations of hundreds of bursts, and facilitating a wide range of follow-up observations within seconds of each event, Swift has revealed an unforeseen richness in observed burst properties, shed light on the nature of short-duration bursts, and helped realize the promise of GRBs as probes of the processes and environments of star formation out to the earliest cosmic epochs. These advances have opened new perspectives on the nature and properties of burst central engines, interactions with the burst environment from microparsec to gigaparsec scales, and the possibilities for nonphotonic signatures. Our understanding of these extreme cosmic sources has thus advanced substantially; yet, more than 40 years after their discovery, GRBs continue to present major challenges on both observational and theoretical fronts.

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Gamma-Ray Bursts in the Swift Era

Over the past decade, our physical understanding of gamma-ray bursts (GRBs) has progressed rapidly, thanks to the discovery and observation of their long-lived afterglow emission. Long-duration (≳2 s) GRBs are associated with the explosive deaths of massive stars ('collapsars', ref. 1), which produce accompanying supernovae; the short-duration (≲2 s) GRBs have a different origin, which has been argued to be the merger of two compact objects. Here we report optical observations of GRB 060614 (duration ~100 s, ref. 10) that rule out the presence of an associated supernova. This would seem to require a new explosive process: either a massive collapsar that powers a GRB without any associated supernova, or a new type of 'engine', as long-lived as the collapsar but without a massive star. We also show that the properties of the host galaxy (redshift z = 0.125) distinguish it from other long-duration GRB hosts and suggest that an entirely new type of GRB progenitor may be required.

Karen Michelle Lewis

Papers

A novel explosive process is required for the γ-ray burst GRB 060614