Gamma-ray bursts are the brightest and most distant explosions in the Universe. A convincing body of evidence now links these bursts to supernovae, but there is still more to learn about their origins.
Although previously limited to a small pool of specialists, research on γ-ray bursts (GRBs) has now entered the mainstream of astrophysics, as attendance at two recent conferencesFootnote 1 attests. Most progress in the field concerns the most numerous class of GRBs, the so-called long bursts, which last more than two seconds. The brief, paroxysmic emission of γ-rays in a long burst is followed by a long-wavelength, fainter afterglow over much longer timescales1,2,3. We know that the bursts take place in galaxies at cosmological distances4, and they are probably associated with the birth of a black hole following the collapse and explosion of very massive stars. The basic mechanism producing the GRB and its associated afterglow is understood in terms of an energetic, collimated flow of matter, called the fireball model5. But what in turn generated that flow of matter, what is the progenitor of a GRB?
The nature of the progenitor was a focal issue at both conferences. The quest is not easy. The radiation observed in a GRB and its afterglow are produced by shock waves at considerable distances from the central source; the fireball brings with it very little memory of the central engine, depending only on basic parameters such as the total energy or the collimation angle of the flow (T. Piran, Hebrew Univ., Jerusalem). However, by examining the environment surrounding GRBs, astronomers have found some clues.
The connection between long GRBs and certain types of supernova was first suggested by the coincidence of a burst on 25 April 1998 with a supernova (SN1998bw)6,7. A later recovery in the brightness of some GRBs has also been attributed to an underlying supernova component8, but this was not unambiguous proof. However, a bright burst detected on 29 March this year showed, coinciding with re-brightening, broad spectral features that were basically identical to those observed in SN1998bw (J. Hjorth, Univ. Copenhagen). This burst is thus firmly linked to a coincident supernova, SN2003dh.
Radiation from particular atoms — iron9, silicon, sulphur and magnesium — seen in the X-rays associated with some GRBs is a tell-tale sign of a supernova explosion (J. Osborne, Univ. Leicester; D. Watson, Univ. Copenhagen; N. Butler, Massachusetts Inst. Technology). All metals (astronomically speaking, anything heavier than helium) in the Universe are the end result of nucleosynthesis in stars, and are ejected in supernovae. Data also indicate outflowing velocities around GRBs that are consistent with supernova ejecta. Each single measurement is not of overwhelming statistical significance, but I find the body of data rather compelling. In one scheme of events (D. Lazzati, Inst. Astronomy, Cambridge), atomic emission lines are produced when material ejected by the supernova is illuminated by the X-ray component of the GRB. This requires a two-step sequence, with the supernova occurring at least a month before the GRB, as in the 'supranova' model10: in the aftermath of a supernova, a rapidly rotating neutron star is formed, which later collapses into a black hole, powering the GRB explosion.
But the GRB and supernova explosion around 29 March occurred within days of each other. This near simultaneity is more consistent with the 'collapsar' model (S. Woosley, Univ. California at Santa Cruz). It postulates that the core of a massive rotating star collapses into a black hole, and the huge amount of energy released accelerates material along the rotation axis, creating an energetic jet that expands out of the stellar surface. This jet eventually produces the GRB and its afterglow. A substantial fraction of energy is also dumped in the stellar interior, and this drives the supernova explosion. Radiation at optical wavelengths from this so-called engine-driven supernova is generated through the production of nickel (56Ni) atoms and their subsequent decay into iron atoms (56Fe). But, depending on the precise conditions, it may be that stable iron isotopes are synthesized in preference to 56Ni, with the result that the supernova explosion is not visible at optical wavelengths (A. MacFayden, Caltech). This could explain the apparent dichotomy, admittedly based on sparse data, between iron X-ray features and optical supernova signatures in GRBs: usually only one or the other is found. The X-ray lines could be produced near the central engine, powered by the energy wasted as the jet pierces through the stellar interior (P. Mészáros, Pennsylvania State Univ.).
A newly discovered type of cosmic explosion, called X-ray flashes (XRFs), has generated considerable interest. These resemble GRBs in almost every respect11, and they constitute at least one-third of the total population of GRBs. With the measurement of the distance to two XRFs, it has been shown (D. Lamb, Univ. Chicago) that these events follow the same relationship between isotropic energy and peak energy as do GRBs12 — which strongly suggests that GRBs and XRFs share the same origin. One possibility is that an XRF is a GRB seen off-axis, rather than head-on. In the collapsar model, in addition to the collimated, high-energy jet that gives rise to the GRB, a considerable fraction of energy is ejected at wider angles and could produce a burst that peaks at X-ray wavelengths, rather than in γ-rays.
The evidence in favour of a connection between supernovae and GRBs — and probably also XRFs — seems overwhelming. But there are still some inconsistencies in the picture: a radio survey of a particular sample of supernovae (called type Ib and Ic) thought to be connected with GRBs shows that basically none of these objects has properties similar to those of GRB afterglows or of SN1998bw, the first supernova to be linked to a GRB (E. Berger, Caltech); this makes it difficult to understand how there can be simple underlying physics across the board for GRBs and supernovae. And despite the efforts made to assemble the observational evidence into a single model, not all the pieces of the puzzle fit. For instance, the observed evolution of most of the afterglows is awkward to explain in the collapsar model (R. Chevalier, Univ. Virginia), although not a problem for the supranova model (A. Konigl, Univ. Chicago).
GRBs are among the most distant sources that we can observe in the Universe, because they are so bright. For cosmology, this opens the exciting perspective of GRBs as probes of the unobserved history of the Universe, looking back to when the first population of stars and primordial galaxies formed (G. Djorgovsky, Caltech). Present estimates indicate that although a substantial fraction of GRBs actually lie in this region of the Universe (technically, beyond a 'redshift' of five; A. Loeb, Harvard Univ.), they are not visible at optical wavelengths because this radiation is absorbed by hydrogen in the intergalactic medium. Interestingly, several GRBs have no detected optical counterpart13. And because most distance (redshift) measurements are derived from optical data, analyses are at present biased against the most distant GRBs: X-ray and infrared measurements are needed as well.
The SWIFT observatory, launching in May 2004, will deliver hundreds of precise, fast localizations of GRBs at γ-ray wavelengths and track within minutes their X-ray and optical afterglows (N. Gehrels, NASA Goddard Space Flight Center), improving considerably on the pioneering method first used by the BeppoSAX mission (F. Frontera, Univ. Ferrara). In addition, the HETE-2 satellite, currently in orbit, should continue to provide complementary X-ray localizations of GRBs (G. Ricker, Massachusetts Inst. Technology). For the study of GRBs, the future is certainly bright.
*Gamma-ray Bursts: An Enigma and a Tool, Santorini, Greece, 24–27 August 2003; GRB 2003, Santa Fe, New Mexico, USA, 8–12 September 2003.
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