Search

Gamma-ray bursts (GRBs) are immensely powerful explosions, originating at cosmological distances, whose outbursts persist for durations ranging from milliseconds to tens of seconds or more. In these brief moments, the explosions radiate more energy than the Sun will release in its entire 10Gyr lifetime. Current theories attribute these phenomena to the final collapse of a massive star, or the coalescence of a binary system induced by gravity wave emission. New results from Swift and related programmes offer fresh understanding of the physics of GRBs, and of the local environments and host galaxies of burst progenitors. Bursts found at very high red shifts are new tools for exploring the intergalactic medium, the first stars and the earliest stages of galaxy formation. This Royal Society Discussion Meeting has brought together leading figures in the field, together with young researchers and students, to discuss and review the latest results from NASA's Swift Gamma-ray Burst Observatory and elsewhere, and to examine their impact on current understanding of the observed phenomena.

A thermal radiative component is likely to accompany the first stages of the prompt emission of gamma-ray bursts (GRBs) and X-ray flashes. We analyse the effect of such a component on the observable spectrum, assuming that the observable effects are due to a dissipation process occurring below or near the thermal photosphere. For comparable energy densities in the thermal and leptonic components, the dominant emission mechanism is Compton scattering. This leads to a nearly flat energy spectrum (nuFnu proportional, 0) above the thermal peak at approximately 10-100 keV and below 10-100 MeV, for a wide range of optical depths 0.03 less, similar tau less, similar 100, regardless of the details of the dissipation mechanism or the strength of the magnetic field. For higher values of the optical depth, a Wien peak is formed at 100 keV to 1 MeV. In particular, these results are applicable to the internal shock model of GRBs, as well as to slow dissipation models, e.g. as might be expected from reconnection, if the dissipation occurs at a sub-photospheric radii. We conclude that dissipation near the thermal photosphere can naturally explain (i) clustering of the peak energy at sub-MeV energies at early times, (ii) steep slopes observed at low energies, and (iii) a flat spectrum above 10 keV at late times. Our model thus provides an alternative scenario to the optically thin synchrotron-synchrotron self-Compton model.

It is embarrassing that 95% of the Universe is unaccounted for. Galaxies and larger-scale cosmic structures are composed mainly of "dark matter", whose nature is still unknown. Favoured candidates are weakly interacting particles that have survived from the very early Universe, but more exotic options cannot be excluded. (There are strong arguments that the dark matter is not composed of baryons.) Intensive experimental searches are being made for the "dark" particles (which pervade our entire Galaxy), but we have indirect clues to their nature too. Inferences from galactic dynamics and gravitational lensing allow astronomers to "map" the dark-matter distribution; comparison with numerical simulations of Galaxy formation can constrain (for example) the particle velocities and collision cross-sections; and, of course, progress in understanding the extreme physics of the ultra-early Universe could offer clues to what particles might have existed then, and how many would have survived. The mean cosmic density of dark matter (plus baryons) is now pinned down to be only ca.30% of the so-called critical density corresponding to a "flat" Universe. However, other recent evidence-microwave background anisotropies, complemented by data on distant supernovae-reveals that our Universe actually is "flat", but that its dominant ingredient (ca.70% of the total mass energy) is something quite unexpected: "dark energy" pervading all space, with negative pressure. We now confront two mysteries. (i) Why does the Universe have three quite distinct basic ingredients-baryons, dark matter and dark energy-in the proportions (roughly) 5%, 25% and 70%? (ii) What are the (almost certainly profound) implications of the "dark energy" for fundamental physics?