Gamma-ray bursts (GRB) are sudden, intense flashes of gamma-rays which, for a few blinding seconds, light up in an otherwise fairly dark gamma-ray sky. (You can see a short video overview of GRB and Swift). GRB are detected at the rate of about once a day, and while they are on, they outshine every other gamma-ray source in the sky, including the sun. Major advances have been made in the last eight years, including the discovery of slowly fading x-ray, optical and radio afterglows of GRBs, the identification of host galaxies at cosmological distances, and finding evidence for many of them being associated with star forming regions, and in some cases with supernovae , the most recent example being GRB060218/SN2006aj . Progress has been made in understanding how the GRB and afterglow radiation arises in terms of a relativistic fireball shock model. Non-specialist overviews are in Science and in my Gamma ray bursts theory review (2008) in Scholarpedia, 3(3):4337. A more technical discussion is in my GRB review in Rep. Prog. Phys. . There is also a summary of some of the specific research activities dealing with GRB at Penn State . My Astrophysical Data System (ADS) electronical archive reprints are here , and my arXiv:astro-ph preprints are here. Other Penn State faculty working primarily on GRBs include Dr. Derek Fox, Dr. John Nousek and Dr. David Burrows, with at least five or six other faculty involved part-time. The nucleus of the observational activity centers on the Swift team, in close interaction with the Penn State GRB theory team. The rest of this page gives a general overview of GRB.
Until a decade ago, GRB were thought to be just that, bursts of gamma-rays which were largely devoid of any observable traces at any other wavelengths. GRBs were first reported in 1973, based on 1969-71 observations by the Vela military satellites monitoring for nuclear explosions in verification of the Nuclear Test Ban Treaty. When these mysterious gamma-ray flashes were first detected, which did not come from Earth's direction, the first suspicion (quickly abandoned) was that they might be the product of an advanced extraterrestrial civilization. Soon, however, it was realized that this was a new and extremely puzzling cosmic phenomenon. A major advance occurred in 1991 with the launch of the Compton Gamma-Ray Observatory (CGRO), whose results have been summarized Fishman & Meegan 1995. The all-sky survey from the Burst and Transient Experiment (BATSE) onboard CGRO, which measured about 3000 bursts, showed that they were isotropically distributed, suggesting a cosmological distribution, with no dipole and quadrupole components. Some of the related work at Penn State on the cosmological GRB distribution is in the previous link. This isotropic distribution and the brightness distribution (log N- log P) provided strong support for a cosmological origin, and the detailed gamma-ray spectra and time histories imposed significant constraints on viable models, which led to the development of the fireball shock model.
A dramatic development starting in 1997 was the measurement and localization of fading x-ray signals in a number of GRBs by the Beppo-SAX satellite , starting with the February 28 burst GRB970228. These afterglows, whose existence and properties had been theoretically predicted, decay as a power law in time typically for weeks. This made possible also optical and radio detections, which, as fading beacons, pinpoint the location of the GRB event. These afterglows, in turn, enabled the measurement of redshift distances, the identification of host galaxies, and the confirmation that GRB were, as suspected, at cosmological distances of the order of billions of light-years, similar to those of the most distant galaxies and quasars. Even at those distances they appear so bright that their energy output during its brief peak period has to be larger than that of any other type of source, of the order of a solar rest-mass if isotropic, or some percent of that if collimated. This energy output rate is comparable to burning up the entire mass-energy of the sun in a few tens of seconds, or to emit over that same period of time as much energy as our entire Milky Way does in a hundred years.
The energy density in a GRB event is so large that an optically thick pair/photon
fireball is expected to form, which will expand carrying with itself some fraction
of baryons. The main challenge in the early 90's was not so much the ultimate energy
source, but how to turn this energy into predominantly gamma rays with the right
nonthermal broken power law spectrum with the right temporal behavior.
To explain the observations, the relativistic fireball shock
model was proposed by Rees and Meszaros in
(1992) and
(1994), following pioneering earlier
earlier work by Cavallo & Rees, Paczynski, Goodman and Shemi & Piran. This model
has been quite succesful in explaining the various features of GRB, and a general
discussion of it is given, e.g.
here.
Much of the recent work has concentrated on GRB afterglows, a highlight of which was the prediction (Meszaros & Rees, (1997) of the general X-ray and optical behavior of burst afterglows, confirmed afterwards by Beppo-SAX observations of GRB 970228. Since then more than 160 afterglows have been studied in detail. With the demise of Beppo-SAX in 2002, continued localizations of GRB were made, albeit at a slower rate, by the HETE-2 satellite. Prompt optical flashes, which had also been expected from theory , were found in several bursts; many afterglows were found to be collimated, easing the energy constraints; and a new variety of softer bursts dubbed "X-ray flashes" was identified, which are very similar to classical GRB but have a softer spectrum. The shape of the jet, and how this affacted the GRB vs. XRF properties and statistics was investigated. Other work concentrated on identifying the stellar and galactic progenitors of GRB. Many of the afterglows identified by Beppo-SAX and HETE-2 (all belonging to the class of "long" bursts, >10 s duration) were shown to be associated with massive young stars, and in some cases with a type Ic supernova ; a supernova association had been previously advocated by Woosley and Paczynski. These discoveries led to work on jets and cocoons from GRB in massive progenitors, and on modeling the central engine resposible for the energy release. The main ideas invoke the formation of a several solar mass black hole with a disrupted debris torus which is rapidly accreted, which feeds an MHD or electron-positron-baryon jet. The relativistic fireball or jet can result from either the merger of a compact binary, such as a double neutron star (which might be responsible for short bursts (< 10 s), or from the collapse of the fast-rotating core of a massive star, dubbed a collapsar, which leads to long bursts (>10 s).
A new era of large numbers of precise afterglow detections, localizations and follow-ups started with the dedicated multi-wavelenght GRB satellite Swift (picture above), launched in November 20, 2004, in which Penn State is playing a major role. This resulted in a number of interesting developments, opening a series of new questions. (Here is a short video overview of GRB and Swift). Swift achieved the long-awaited goal of accurately localizing afterglows starting a minute or so after the burst trigger, at gamma-ray, X-ray and optical wavelenghts. This revealed the hitherto unexplored afterglow behavior between minutes to hours, enabling a study of the transition from the prompt emission and the subsequent long term afterglow, and revealing a rich range of early X-ray behavior. It also achieved the long-awaited discovery of the afterglows of ``short" gamma-ray bursts (whose hard gamma-ray emission is briefer than 2 s), many of which are in elliptical host galaxies. It furthermore broke through the symbolic redshift z=6 barrier, beyond which very few objects of any kind have been measured. And it confirmed the GRB/SN association with a very nearby long burst, GRB060218/SN2006aj, which in addition showed for the first time a supernova shock break-out. This is a link to a brief review of the first scientific results of Swift and its theoretical implications . A discussion of the standard modeld of GRB and afterglows, as well as a more detailed discussion of Swift observations and the associated theoretical developments as of Feb. 2006 is in my recent Rep. Prog. Phys. GRB review.
A recent emphasis on GRB research at Penn State and elsewhere has been the possibility of ultra-high energy neutrino production in GRB and AGN, measurable with ICECUBE . This is related to the possibility that GRB are sources of ultra-high energy cosmic rays , up to about the "GZK" limit of 1E20 eV, which are the target for large cosmic ray detectors such as the Pierre Auger Observatory , currently nearing completion in Argentina. Penn State is substantially involved in both ICECUBE and AUGER. Associated with these processes, one also expects high (GeV) and very high (>TeV) energy photon production, which may either be produced by the leptonic or the hadronic component (or both), of GRB, AGN and related sources. Such high photons will be measurable with GLAST . The other interesting possibilty is that GRB are sources of gravitational waves (GW). This is especially exciting now that Swift is beginning to test the paradigm of short bursts as binary neutron star (or neutron star-black hole) mergers, since these are prime candidates for generating GWs, which are being searched for with the LIGO Laser Interferometric Gravitational Observatory.
Research sponsors: NASA, NSF