Gamma-Ray Bursts Physics

Ken-Ichi Nishikawa, Yosuke Mizuno, Bing Zhang, Phil Hardee, Mikhail Medvedev, Jacek Niemiec

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. 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 ten 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. Progress has been made in understanding how the GRB and afterglow radiation arises in terms of a relativistic fireball shock model. This is described in a non-specialist overview in Science. The observational activity is done in the GBM team. The rest of this page gives a general overview of GRB.

hypernovaUntil 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 NSSTC on the cosmological GRB distribution. 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.

swift sc1The 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 successful in explaining the various features of GRB.

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 affected 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 responsible 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).

Our recent research related to GRB physics

The studies of the effect of magnetic fields on the shock structure in jet head found a strictly high magnetization Lorentz factor boost towards the forward (bow) shock by rarefaction wave propagation into the jets. (Mizuno et al. 2009, 2010). In AGN the mechanism produces a significant acceleration into a shock driven by a highly magnetized ejection event. This provides an alternative to fast spine slow sheath scenarios reconciles the high Lorentz factor/Doppler boost requirements of blazar models with the observed more modest motion. In GRBs, highly magnetized jetted ejecta would accelerate towards the forward (bow) shock with no reverse shock present.

In the acceleration problem, studies of the effect of magnetic fields on a hydrodynamic boost mechanism found by Aloy & Rezzolla (2006) which could produce a thin high Lorentz factor sheath at the surface of an overpressured jet inside a dense external medium found the effect can be amplified by the presence of strong magnetic fields (Mizuno et al. 2008). A thin high speed sheath is likely to be stabilizing and reduce mixing of jet and external medium. In GRBs this could allow a jet to penetrate a baryon rich collapsar while maintaining the small baryon loading thought necessary to the production of ultrarelativistic GRB outflows.

Along with the gamma-ray burst radiation relativistic particle-in-cell simulations have been performed since 2002. The first simulation resultes were presented at Huntsville workshop in 2002 (Nishikawa et al. 2003, 2009). Since then several simulations have revealed that the Weibel instability created at the shock front accelerates particles perpendicular and parallel to the jet propagation direction. This instability is also responsible for generating magnetic fields in the relativistic jets. The simulations show that the growth rate of the Weibel instability depends on the Lorentz factor and composition of the jet, as well as the orientation and strength of the ambient magnetic field. The magnetic fields generated by the Weibel instability create highly nonuniform, small-scale magnetic fields, which contribute to the electron’s transverse deflection (Nishikawa et al. 2011). The radiation from electrons in these environments (jitter radiation) is different from synchrotron radiation. The details of this research is described in the section of Microscopic processes by RPIC simulations.


Magnetohydrodynamic Effects in Propagating Relativistic Jets: Reverse Shock and Magnetic Acceleration, Mizuno, Y., B. Zhang, B. Giacomazzo, K.-I. Nishikawa, P. Hardee, S. Nagataki, & H. Hartmann, ApJ, 690, L47, 2009

Magnetohydrodynamic Effects in Relativistic Jets, Mizuno, Y., B. Zhang, B. Giacomazzo, K.-I. Nishikawa, P. Hardee, S. Nagataki, & H. Hartmann, Int. J. of Mod. Phys. D, 19, 991, 2010

Magnetohydrodynamic Boost for Relativistic Jets, Mizuno, Y., P. Hardee, D. Hartmann, K.-I. Nishikawa, & B. Zhang, ApJ, 672, 72, 2008

Particle Acceleration in Relativistic Jets due to Weibel Instability, Nishikawa, K.-I., P. Hardee, G. Richardson, R. Preece, H. Sol, & G. J. Fishman, Astrophys. J., 595, 555, 2003

Weibel instability and associated strong fields in a fully 3D simulation of a relativistic shock, Nishikawa, K. -I., J. Niemiec, M. Medvedev, P. Hardee, H. Sol, Y. Mizuno, B. Zhang, M. Pohl, M. Oka, D. H. Hartmann, ApJ, 698, L10, 2009

Radiation from relativistic shocks with turbulent magnetic fields, Nishikawa, K.-I., J. Niemiec, M. Medvedev, B. Zhang, P. Hardee, A. Nordlund, J. Frederiksen, Y. Mizuno, H. Sol, M. Pohl, D. H. Hartmann, M. Oka, G. J. Fishman, Advances in Space Research, 47, pp. 1434 – 1440, 2011