COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. the Brightest Explosions in the Universe Every Time a Gamma-Ray Burst Goes Off, a Black Hole Is Born

Home , Neil Gehrels

A PICTURE LIKE THIS could not have been drawn with any confidence a decade ago, because no one had yet figured out what causes gamma-ray bursts—flashes of high-energy radiation that light up the sky a couple of times a day. Now astronomers think of them as the ultimate stellar swan song. A black hole, created by the implosion of a giant star, sucks in debris and sprays out some of it. A series of shock waves emits radiation.

By Neil Gehrels, Luigi Piro and Peter J. T. Leonard

Early in the morning of January 23, 1999, a robotic telescope in New Mexico picked up a faint flash of light in the constellation Corona Borealis. Though just barely visible through binoculars, it turned out to be the most brilliant explosion ever witnessed by humanity. We could see it nine billion light-years away, more than halfway across the observable universe. If the event had instead taken place a few thousand light-years away, it would have been as bright as the midday sun, and it would have dosed Earth with enough radiation to kill off nearly every living thing. The flash was another of the famous gamma-ray bursts, which in recent decades have been one of astronomy’s most intriguing mysteries. The first sighting of a gamma-ray burst (GRB) came on July 2, 1967, from military satellites watch- ing for nuclear tests in space. These cosmic explosions proved to be rather different from the man-made explosions that the

COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. A (VERY) WARM AFTERGLOW X-RAYS: Eight hours after a burst went off on February 28, 1997, VISIBLE LIGHT: A comparably quick reaction by astronomers on astronomers using the BeppoSAX satellite—including one of the La Palma in the Canary Islands allowed the same afterglow to be authors (Piro)—saw an x-ray afterglow for the first time. The seen in visible light. Over the next week, the light dimmed to one second image was taken a couple days later, by which time the sixth its original brightness, and as it did so, the surrounding x-rays had faded by a factor of 20. galaxy slowly became apparent.

X-RAYS VISIBLE LIGHT

Eight hours Three days 21 hours Eight days

satellites were designed to detect. For most last longer—the majority—are “long.” rays alone did not provide enough infor- of the 35 years since then, each new burst The two categories differ spectroscopi- mation to settle the question for sure. Re- merely heightened the puzzlement. When- cally, with short bursts having relatively searchers would need to detect radiation ever researchers thought they had the ex- more high-energy gamma rays than long from the bursts at other wavelengths. Vis- planation, the evidence sent them back to bursts do. The January 1999 burst emit- ible light, for example, could reveal the square one. ted gamma rays for a minute and a half. galaxies in which the bursts took place, The monumental discoveries of the Arguably the most important result allowing their distances to be measured. past several years have brought astrono- from BATSE concerned the distribution Attempts were made to detect these burst mers closer to a definitive answer. Before of the bursts. They occur isotropically— counterparts, but they proved fruitless. 1997, most of what we knew about that is, they are spread evenly over the en- GRBs was based on observations from tire sky. This finding cast doubt on the A BURST OF PROGRESS the Burst and Transient Source Experi- prevailing wisdom, which held that bursts THE FIELD TOOK a leap forward in ment (BATSE) onboard the Compton came from sources within the Milky 1996 with the advent of the x-ray space- Gamma Ray Observatory. BATSE re- Way; if they did, the shape of our galaxy, craft BeppoSAX, built and operated by vealed that two or three GRBs occur or Earth’s off-center position within it, the Italian Space Agency with the partic- somewhere in the observable universe on should have caused them to bunch up in ipation of the Netherlands Space Agency. ) a typical day. They outshine everything certain areas of the sky. The uniform dis- BeppoSAX was the first satellite to local- right else in the gamma-ray sky. Although each tribution led most astronomers to con- ize GRBs precisely and to discover their x- ( is unique, the bursts fall into one of two clude that the instruments were picking ray “afterglows.” The afterglow appears rough categories. Bursts that last less than up some kind of event happening through- when the gamma-ray signal disappears. It two seconds are “short,” and those that out the universe. Unfortunately, gamma persists for days to months, diminishing with time and degrading from x-rays into Institute of Space Astrophysics and Cosmic Physics, CNR less potent radiation, including visible University of Amsterdam Overview/Gamma-Ray Bursts light and radio waves. Although Bep- ■ For three decades, the study of gamma-ray bursts was stuck in first gear— poSAX detected afterglows for only long

astronomers couldn’t settle on even a sketchy picture of what sets off these bursts—no counterparts of short bursts ); ENRICO COSTA cosmic fireworks. have yet been identiﬁed—it made follow- ); PAUL J. GROOT ■ Over the past five years, however, observations have revealed that bursts are up observations possible at last. Given the left the birth throes of black holes. Most of the holes are probably created when a positional information from BeppoSAX,

massive star collapses, releasing a pulse of radiation that can be seen billions optical and radio telescopes were able to preceding pages of light-years away. identify the galaxies in which the GRBs ■ Now the research has shifted into second gear—fleshing out the theory and took place. Nearly all lie billions of light- probing subtle riddles, especially the bursts’ incredible diversity. years away, meaning that the bursts must MARK A. GARLICK ( be enormously powerful [see “Gamma- AND THE BEPPOSAX TEAM (

86 SCIENTIFIC AMERICAN DECEMBER 2002 COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. Ray Bursts,” by Gerald J. Fishman and much energy, most of that energy escapes proximately in proportion to the square Dieter H. Hartmann; Scientific Ameri- as neutrinos, and the remainder leaks out of the jet angle. For example, if the jet can, July 1997]. Extreme energies, in more gradually than in a GRB. Conse- subtends 10 degrees, it covers about one turn, call for extreme causes, and re- quently, the luminosity of a supernova at 500th of the sky, so the energy require- searchers began to associate GRBs with any given moment is a tiny fraction of ment goes down by a factor of 500; more- the most extreme objects they knew of: that of a GRB. Even quasars, which are over, for every GRB that is observed, an- black holes. famously brilliant, give off only about other 499 GRBs go unseen. Even after Among the first GRBs pinpointed by 1040 watts. taking beaming into account, however, BeppoSAX was GRB970508, so named If the burst beamed its energy in par- the luminosity of GRB990123 was still an because it occurred on May 8, 1997. Ra- ticular directions rather than in all direc- impressive 1043 watts. dio observations of its afterglow provid- tions, however, the luminosity estimate ed an essential clue. The glow varied er- would be lower. Evidence for beaming GRB-SUPERNOVA CONNECTION ratically by roughly a factor of two dur- comes from the way the afterglow of ONE OF THE MOST interesting dis- ing the first three weeks, after which it GRB990123, among others, dimmed coveries has been the connection between stabilized and then began to diminish. over time. Two days into the burst, the GRBs and supernovae. When telescopes The large variations probably had noth- rate of dimming increased suddenly, went to look at GRB980425, they also ing to do with the burst source itself; which would happen naturally if the ob- found a supernova, designated SN1998- rather they involved the propagation of served radiation came from a narrow jet bw, that had exploded at about the same the afterglow light through space. Just as of material moving at close to the speed time as the burst. The probability of a Earth’s atmosphere causes visible star- of light. Because of a relativistic effect, the chance coincidence was one in 10,000 light to twinkle, interstellar plasma caus- observer sees more and more of the jet as [see “Bright Lights, Big Mystery,” by es radio waves to scintillate. For this pro- it slows down. At some point, there is no George Musser; News and Analysis, Sci- cess to be visible, the source must be so more to be seen, and the apparent bright- entific American, August 1998]. small and far away that it appears to us as ness begins to fall off more rapidly [see il- A link between GRBs and supernovae a mere point. Planets do not twinkle, be- lustration on next page]. For GRB990123 has also been suggested by the detection cause, being fairly nearby, they look like and several other bursts, the inferred jet- of iron in the x-ray spectra of several disks, not points. opening angle is a few degrees. Only if the bursts. Iron atoms are known to be syn- Therefore, if GRB970508 was scintil- jet is aimed along our line of sight do we thesized and dumped into interstellar lating at radio wavelengths and then see the burst. This beaming effect reduces space by supernova explosions. If these stopped, its source must have grown from the overall energy emitted by the burst ap- atoms are stripped of their electrons and a mere point to a discernible disk. “Dis- cernible” in this case means a few light- weeks across. To reach that size, the source FADING AWAY must have been expanding at a consider- BRIGHTEST GAMMA-RAY BURST 16 days 59 days able rate—close to the speed of light. yet recorded went off on January The BeppoSAX and follow-up obser- 23, 1999. Telescopes tracked its

AND NASA vations have transformed astronomers’ brightness in gamma rays (blue view of GRBs. The old concept of a sud- in graph), x-rays (green), visible den release of energy concentrated in a few light (orange) and radio waves brief seconds has been discarded. Indeed, (red). At one point, the rate of Time (minutes) even the term “afterglow” is now recog- dimming changed abruptly—a 012 nized as misleading: the energy radiated 10–9 telltale sign that the radiation Gamma rays during both phases is comparable. The was coming from narrow jets of spectrum of the afterglow is characteris- Robotic-telescope measurement high-speed material. About two 10–12 Space Telescope Science Institute tic of electrons moving in a magnetic ﬁeld weeks into the burst, after the at or very close to the speed of light. visible light had dimmed by a X-rays The January 1999 burst (GRB990123) factor of four million, the Hubble 10–15 was instrumental in demonstrating the Space Telescope took a picture Visible Change in light immense power of the bursts. If the burst and found a severely distorted –18 dimming rate ); ANDREW FRUCHTER radiated its energy equally in all direc- galaxy. Such galaxies typically 10 tions, it must have had a luminosity of a graph have high rates of star Radio

45 19 meter) square per (watts Intensity few times 10 watts, which is 10 times formation. If bursts are the 10–21 as bright as our sun. Although the other explosions of young stars, they 0.1 1 10 well-known type of cosmic cataclysm, a should occur in just such a place. Time (days)

CORNELIA BLIK ( supernova explosion, releases almost as

www.sciam.com SCIENTIFIC AMERICAN 87 COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. later hook up with them again, they give ting observers see both. This slight mis- BEAM LINES off light at distinctive wavelengths, re- alignment would explain GRB980425. ferred to as emission lines. Early, margin- Whereas this hypothesis supposes that RELATIVITY PLAYS TRICKS on observers’ al detections of such lines by BeppoSAX most or all GRBs might be related to su- view of jets from gamma-ray bursts. and the Japanese x-ray satellite ASCA in pernovae, a slightly different scenario at- 1997 have been followed by more solid tributes only a subset of GRBs to super- Moving at close to the speed of light, measurements. Notably, NASA’s Chandra novae. Roughly 90 of the bursts seen by 1the jet emits light in narrow beams. X-ray Observatory detected iron lines in BATSE form a distinct class of their own, Some beams bypass the observer. GRB991216, which yielded a direct dis- defined by ultralow luminosities and long tance measurement of the GRB. The fig- spectral lags, meaning that the high- and ure agreed with the estimated distance of low-energy gamma-ray pulses arrive sev- LIGHT BEAM the burst’s host galaxy. eral seconds apart. No one knows why Additional observations further sup- the pulses are out of sync. But whatever port the connection between GRBs and the reason, these strange GRBs occur at OBSERVER supernovae. An iron-absorption feature the same rate as a certain type of super- appeared in the x-ray spectrum of GRB- nova, called Type Ib/c, which occurs JET 990705. In the shell of gas around an- when the core of a massive star implodes. CENTRAL ENGINE other burst, GRB011211, the European Space Agency’s X-ray Multi-Mirror satel- GREAT BALLS OF FIRE As the jet slows, the beams widen, so lite found evidence of emission lines from EVEN LEAVING ASIDE the question of 2fewer of them bypass the observer. silicon, sulfur, argon and other elements how the energy in GRBs might be gener- More of the jet comes into view. commonly released by supernovae. ated, their sheer brilliance poses a para- Although researchers still debate the dox. Rapid brightness variations suggest matter, a growing school of thought that the emission originates in a small re- holds that the same object can produce, gion: a luminosity of 1019 suns comes in some cases, both a burst and a super- from a volume the size of one sun. With nova. Because GRBs are much rarer than so much radiation emanating from such supernovae—every day a couple of GRBs a compact space, the photons must be so go off somewhere in the universe, as op- densely packed that they should interact posed to hundreds of thousands of su- and prevent one another from escaping. pernovae—not every supernova can be The situation is like a crowd of people associated with a burst. But some might who are running for the exit in such a Eventually beams from the edges be. One version of this idea is that super- panic that that nobody can get out. But if 3reach the observer. The entire jet is nova explosions occasionally squirt out the gamma rays are unable to escape, now visible. Data reveal this transition. jets of material, leading to a GRB. In most how can we be seeing GRBs? of these cases, astronomers would see ei- The resolution of this conundrum, de- ther a supernova or a GRB, but not both. veloped over the past several years, is that If the jets were pointed toward Earth, the gammas are not emitted immediately. light from the burst would swamp light Instead the initial energy release of the ex- from the supernova; if the jets were plosion is stored in the kinetic energy of aimed in another direction, only the su- a shell of particles—a fireball—moving at pernova would be visible. In some cases, close to the speed of light. The particles however, the jet would be pointed just include photons as well as electrons and slightly away from our line of sight, let- their antimatter counterpart, positrons. This fireball expands to a diameter of 10 NEIL GEHRELS, LUIGI PIRO and PETER J. T. LEONARD bring both observation and theory to billion to 100 billion kilometers, by which the study of gamma-ray bursts. Gehrels and Piro are primarily observers—the lead scien- point the photon density has dropped tists, respectively, of the Compton Gamma Ray Observatory and the BeppoSAX satellite. enough for the gamma rays to escape un- Leonard is a theorist, and like most theorists, he used to think it unlikely that the bursts hindered. The fireball then converts some were bright enough to be seen across the vastness of intergalactic space. “I have to admit of its kinetic energy into electromagnetic that the GRBs really had me fooled,” he says. Gehrels is head of the Gamma Ray, Cosmic radiation, yielding a GRB. THE AUTHORS Ray and Gravitational Wave Astrophysics Branch of the Laboratory for High Energy Astro- The initial gamma-ray emission is physics at the NASA Goddard Space Flight Center. Piro is a member of the Institute of Space most likely the result of internal shock Astrophysics and Cosmic Physics of the CNR in Rome. Leonard works for Science Systems waves within the expanding fireball.

and Applications, Inc., in support of missions at Goddard. Those shocks are set up when faster blobs JUAN VELASCO

88 SCIENTIFIC AMERICAN DECEMBER 2002 COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. in the expanding material overtake slow- Although the fireball can transform merge into one. Just as in the hypernova er blobs. Because the fireball is expanding the explosive energy into the observed ra- scenario, the result is the formation of a so close to the speed of light, the timescale diation, what generates the energy to be- single black hole surrounded by a disk. witnessed by an external observer is vast- gin with? That is a separate problem, and Many celestial phenomena involve a ly compressed, according to the principles astronomers have yet to reach a consen- hole-disk combination. What distinguish- of relativity. So the observer sees a burst sus. One family of models, referred to as es this particular type of system is the of gamma rays that lasts only a few sec- hypernovae or collapsars, involves stars sheer mass of the disk (which allows for a onds, even if it took a day to produce. The born with masses greater than about 20 gargantuan release of energy) and the lack fireball continues to expand, and eventu- to 30 times that of our sun. Simulations of a companion star to resupply the disk ally it encounters and sweeps up sur- show that the central core of such a star (which means that the energy release is a rounding gas. Another shock wave forms, eventually collapses to form a rapidly ro- one-shot event). The black hole and disk this time at the boundary between the tating black hole encircled by a disk of have two large reservoirs of energy: the fireball and the external medium, and per- leftover material. gravitational energy of the disk and the sists as the fireball slows down. This ex- A second family of models invokes bi- rotational energy of the hole. Exactly how ternal shock nicely accounts for the GRB nary systems that consist of two compact these would be converted into gamma ra- afterglow emission and the gradual degra- objects, such as a pair of neutron stars diation is not fully understood. It is pos- dation of this emission from gamma rays (which are ultradense stellar corpses) or a sible that a magnetic field, 1015 times to x-rays to visible light and, finally, to ra- neutron star paired with a black hole. The more intense than Earth’s magnetic field, dio waves. two objects spiral toward each other and builds up during the formation of the BURSTING OUT

MERGER SCENARIO FORMATION OF A GAMMA-RAY BURST could begin either with the merger of two neutron stars or with the collapse of a massive star. Both these events create a black hole with a disk of material around it. The hole-disk system, in X-RAYS, NEUTRON STARS turn, pumps out a jet of material at close to the speed of VISIBLE LIGHT, light. Shock waves within this material give off radiation. JET COLLIDES WITH RADIO AMBIENT MEDIUM WAVES (external shock wave)

GAMMA RAYS BLOBS COLLIDE SLOWER (internal shock wave) FASTER BLOB BLACK HOLE DISK BLOB

CENTRAL ENGINE

PREBURST

GAMMA-RAY EMISSION

MASSIVE STAR AFTERGLOW

HYPERNOVA SCENARIO JUAN VELASCO

www.sciam.com SCIENTIFIC AMERICAN 89 COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. disk. In so doing, it heats the disk to such galaxy. If compact objects were the cul- swering some of the most fundamental high temperatures that it unleashes a fire- prit, GRBs should not occur preferential- questions in astronomy: How do stars ball of gamma rays and plasma. The fire- ly in star-forming regions. end their lives? How and where are black ball is funneled into a pair of narrow jets Although hypernovae probably ex- holes formed? What is the nature of jet that flow out along the rotational axis. plain most GRBs, compact-star coales- outflows from collapsed objects? Because the GRB emission is equally cence could still have a place in the big pic- well explained by both hypernovae and ture. This mechanism may account for the BLASTS FROM THE PAST compact-object mergers, some other qual- poorly understood short-duration GRBs. ONE OUTSTANDING question concerns ities of the bursts are needed to decide be- Moreover, additional models for GRBs the dark, or “ghost,” GRBs. Of the rough- tween these two scenarios. The associa- are still in the running. One scenario pro- ly 30 GRBs that have been localized and tion of GRBs with supernovae, for exam- duces the fireball via the extraction of en- studied at wavelengths other than gamma ple, is a point in favor of hypernovae, ergy from an electrically charged black rays, about 90 percent have been seen in which, after all, are essentially large su- hole. This model suggests that both the x-rays. In contrast, only about 50 percent pernovae. Furthermore, GRBs are usual- immediate and the afterglow emissions have been seen in visible light. Why do ly found just where hypernovae would be are consequences of the fireball sweeping some bursts fail to shine in visible light? expected to occur—namely, in areas of re- up the external medium. Astronomers One explanation is that these GRBs cent star formation within galaxies. A have come a long way in understanding lie in regions of star formation, which massive star blows up fairly soon (a few gamma-ray bursts, but they still do not tend to be filled with dust. Dust would million years) after it is born, so its know precisely what causes these explo- block visible light but not x-rays. Anoth- deathbed is close to its birthplace. In con- sions, and they know little about the rich er intriguing possibility is that the ghosts trast, compact-star coalescence takes variety and subclasses of bursts. are GRBs that happen to be very far away. much longer (billions of years), and in the All these recent findings have shown The relevant wavelengths of light pro- meantime the objects will drift all over the that the field has the potential for an- duced by the burst would be absorbed by intergalactic gas. To test this hypothesis, measurement of the distance via x-ray The Destinies of Massive Stars spectra will be crucial. A third possibility is that ghosts are optically faint by nature. STARS SPEND MOST OF THEIR LIVES in the relatively unexciting main-sequence Currently the evidence favors the dust ex- evolutionary phase, during which they casually convert hydrogen into helium in their planation. High-sensitivity optical and ra- cores via nuclear fusion. Our sun is in this phase. According to basic stellar theory, dio investigations have identified the stars more massive than the sun shine more brightly and burn their fuel more quickly. probable host galaxies of two dark GRBs, A star 20 times as massive as the sun can keep going for only a thousandth as long. and each lies at a fairly moderate distance. As the hydrogen in the core of a star runs out, the core Another mystery concerns a class of contracts, heats up and starts to fuse heavier elements, such Main-sequence events known as the x-ray-rich GRBs, or phase as helium, oxygen and carbon. The star thus evolves into a simply the x-ray flashes. Discovered by giant and then, if sufficiently massive, a supergiant star. If Supergiant BeppoSAX and later confirmed by re- phase the initial mass of the star is at least eight times that of the sun, analysis of BATSE data, these bursts are the star successively fuses heavier and heavier elements now known to represent 20 to 30 percent in its interior until it produces iron. Iron fusion does not Explosion of GRBs. They give off more x-radiation release energy—on the contrary, it uses up energy. So than gamma radiation; indeed, extreme the star suddenly finds itself without any useful fuel. cases exhibit no detectable gamma radi- The result is a sudden and catastrophic collapse. The ation at all. core is thought to turn into a neutron star, a stellar corpse One explanation is that the fireball is that packs at least 40 percent more mass than the sun Black loaded with a relatively large amount of into a ball with a radius of only 10 kilometers. The hole baryonic matter such as protons, making remainder of the star is violently ejected into space in a for a “dirty fireball.” These particles in- powerful supernova explosion. crease the inertia of the fireball, so that it There is a limit to how massive a neutron star can moves more slowly and is less able to be—namely, two to three times as massive as the sun. If it is boost photons into the gamma-ray range. any heavier, theory predicts it will collapse into a black hole. It can be pushed over Alternatively, the x-ray flashes might the line if enough matter falls onto it. It is also possible that a black hole can be come from very distant galaxies—even formed directly during the collapse. Stars born with masses exceeding roughly 20 more distant than the galaxies proposed solar masses may be destined to become black holes. The creation of these holes to explain the ghost GRBs. Cosmic ex- provides a natural explanation for gamma-ray bursts. —N.G., L.P. and P.J.T.L. pansion would then shift the gamma rays

into the x-ray range, and intergalactic gas CORNELIA BLIK

90 SCIENTIFIC AMERICAN DECEMBER 2002 COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. Classes of Gamma-Ray Bursts TYPICAL PERCENTAGE DURATION OF INITIAL AFTERGLOW AFTERGLOW HYPOTHETICAL EXPLANATION BURST CLASS OF ALL INITIAL EMISSION GAMMA-RAY X-RAY VISIBLE CENTRAL FOR PECULIAR (SUBCLASS) BURSTS (SECONDS) EMISSION EMISSION EMISSION ENGINE PROPERTIES

Long 25 20 Energetic Not applicable (normal) explosion of massive star

Long 30 20 Energetic Extremely distant, (ghosts or explosion of obscured by dust, dark) massive star or intrinsically faint

Long 25 30 Energetic Extremely distant (x-ray-rich or explosion of or weighed down x-ray flashes) massive star by extra particles

Short 20 0.3 Merger of pair Does not occur in of compact a star-forming objects region, so ambient ?? gas is less dense and external shocks are weaker

would block any visible afterglow. In fact, Another goal is to probe extreme gam- October 17, is expected to detect 10 to 20 none of these x-ray flashes has a de- ma-ray energies. GRB940217, for exam- GRBs a year. The Energetic X-ray Imag- tectable visible-light counterpart, a find- ple, emitted high-energy gamma rays for ing Survey Telescope, planned for launch ing that is consistent with this scenario. more than an hour after the burst, as ob- a decade from now, will have a sensitive If either x-ray flashes or ghost GRBs are served by the Energetic Gamma Ray Ex- gamma-ray instrument capable of detect- located in extremely distant galaxies, they periment Telescope instrument on the ing thousands of GRBs. could illuminate an era in cosmic history Compton Gamma Ray Observatory. As- The field has just experienced a series that is otherwise almost invisible. tronomers do not understand how such of breakthrough years, with the discovery The next step for GRB astronomy is extensive and energetic afterglows can be that GRBs are immense explosions occur- to flesh out the data on burst, afterglow produced. The Italian Space Agency’s ring throughout the universe. Bursts pro- and host-galaxy characteristics. Observers AGILE satellite, scheduled for launch in vide us with an exciting opportunity to need to measure many hundreds of bursts 2004, will observe GRBs at these high en- study new regimes of physics and to learn of all varieties: long and short, bright and ergies. The supersensitive Gamma-Ray what the universe was like at the earliest faint, bursts that are mostly gamma rays, Large Area Space Telescope mission, ex- epochs of star formation. Space- and bursts that are mostly x-rays, bursts with pected to launch in 2006, will also be key ground-based observations over the com- visible-light afterglows and those without. for studying this puzzling phenomenon. ing years should allow us to uncover the Currently astronomers are obtaining burst Other missions, though not designed detailed nature of these most remarkable positions from the second High Energy solely for GRB discovery, will also con- beasts. Astronomers can no longer talk of Transient Explorer satellite, launched in tribute. The International Gamma-Ray bursts as utter mysteries, but that does not October 2000, and the Interplanetary Astrophysics Laboratory, launched on mean the puzzle is completely solved. Network, a series of small gamma-ray de- tectors piggybacking on planetary space- MORE TO EXPLORE craft. The Swift mission, scheduled for Gamma-Ray Bursts of Doom. Peter J. T. Leonard and Jerry T. Bonnell in Sky & Telescope, Vol. 95, launch next fall, will offer multiwave- No. 2, pages 28–34; February 1998. length observations of hundreds of GRBs Observation of X-ray Lines from a Gamma-Ray Burst (GRB991216): Evidence of Moving Ejecta from the Progenitor. Luigi Piro et al. in Science, Vol. 290, pages 955–958; November 3, 2000. and their afterglows. On discovering a Preprint available at arXiv.org/abs/astro-ph/0011337 GRB, the gamma-ray instrument will trig- Gamma-Ray Bursts: Accumulating Afterglow Implications, Progenitor Clues, and Prospects. ger automatic onboard x-ray and optical Peter Mészáros in Science, Vol. 291, pages 79–84; January 5, 2001. arXiv.org/abs/astro-ph/0102255 observations. A rapid response will de- Blinded by the Light. Stan Woosley in Nature, Vol. 414, pages 853–854; December 20, 2001. termine whether the GRB has an x-ray or The Biggest Bangs: The Mystery of Gamma-Ray Bursts, the Most Violent Explosions visible afterglow. The mission will be sen- in the Universe. Jonathan I. Katz. Oxford University Press, 2002. sitive to short-duration bursts, which have Flash! The Hunt for the Biggest Explosions in the Universe. Govert Schilling. barely been studied so far. Cambridge University Press, 2002.