Gamma Ray Bursts : Peter Mészáros

Until a decade and half 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 500 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 Rep. Prog. Phys. GRB review.

This important result was quickly bested by the even more stringent results on GRB 090510 (Abdo et al, 2009b, Nature 462:331). The detection by Fermi of GRB 090510 was momentous for three completely different reasons. First, it was the first short GRB to be clearly detected in the LAT, up to 31 GeV, a record at the time of measurement. This required an even larger lower limit for the bulk Lorentz factor, Gamma > 1200. Second, this burst, in contrast to a number of previous ones, also showed for the first time a clear second spectral component, in addition to the usual Band-type simple broken power law (see fig. on left). However, it is as yet unclear whether this is of leptonic or hadronic origin. Third, this burst also showed a time lag between the high and low energy emission, allowing an even stricter limit on the quantum gravity energy scale. The experimental lower limit for the first order term in this burst exceeds the Planck energy by a factor of 4, so the first order term can be ruled out. It is remarkable that these unimaginably high energies around the Planck scale, which are completely out of reach of even the highest energy particle accelerators such as the LHC at CERN, can nonetheless be probed with GRB observations of photons in the tens of GeV range, which give a set of robust experimental lower limits on this fundamental energy scale.

By now, Fermi has detected many bursts at LAT energies, of which a minor fraction are short bursts. Many show no clear evidence for a second, high energy component, being simple Band spectra. However most of the brightest LAT bursts show a clear second component: e.g. the short burst GRB 090510, the long burst GRB 090902B, etc. show this extra component. Many of the LAT bursts also show a time lag of the hard GeV photons relative to the softer MeV photons, and the spectral properties of long and short bursts do not seem to differ qualitatively at high energies. A recent interpretation for the GeV-MeV time lags based on a nuclear collsional GRB jet modes (Mészáros and Rees, 2011) has been proposed. We have investigates the properties of hypthesized Pop. III GRBs, which might form at redshifts 10-20 from the collapse of putative very massive Pop. III stars. Such collapsar-type GRBs could be powered by the Blandford-Znajek mechanism producing a Poynting dominated jet. If so, they should be bright enough to discover, with a distinctive photospheric X-ray spectrum, a possible GeV extension and also bright L or K-band emission; e.g. Toma, Sakamoto and Mészáros, 2011.

Then, on August 17, 2017, LIGO discovered its first binary neutron star (BNS) merger, GW170817, which was followed, 1.7 seconds later, by a short gamma-ray burst seen by the Fermi GBM satellite detector, GRB170817A, as well as by the INTEGRAL satellite (see figure on the right; top two panels GBM, third panel INTEGRAL, bottom panel LIGO, showing theincreasing chirp frequency). This long-awaited proof-positive that binary neutron star mergers produce both gravitaional waves and gamma-tay bursts can be taken (apart from the neutrinos and the light from the supernova SN1987a) as marking the beginning of the multi-messenger astrophysics era, where at least two completely different but complementary types of messengers (in this case gravitons and photons) are used to study the same object.

GW170817/GRB170817A was also detected, soon after the initial GRB and GW trigger, with a large number of other electromagnetic detectors, including X-ray, optical, IR and radio signals. The gamma rays appear to have been observed some 20 to 30 degrees off the jet axis (see Figure on the left), e.g. Ioka and Nakamura, 2017; Kasliwal, et al, 2017, while a broader outflow at large angles shows, after a day, a chracteristic macronova type of optical emission, believed to be powered by the decay of radioactive elements made by the r-process of rapid neutron capture, which also makes all the other stable A>56 elements, including gold, platinum, uranium, etc.

A recent emphasis on GRB research at Penn State and elsewhere has been the investigation of ultra-high energy neutrino production in GRB, AGN and other sources, which could be 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 10^{20} eV, which are the target for large cosmic ray detectors such as the Pierre Auger Observatory . 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 are being measured with the Fermi satellite. The other active interest is that GRBs can be sources of gravitational waves (GW). This is especially the case for short bursts, which have been thought to arise from binary neutron star (or neutron star-black hole) mergers, which have a large quadrupole mass moment ideal for generating GWs, a paradigm now confirmed (see above) by the LIGO Laser Interferometric Gravitational Observatory.

Research sponsors: NASA, NSF