Gamma Ray Bursts Point to New Cosmic Flashes in the Sky

Researchers from the Naval Research Laboratory (NRL) in Washington, DC, and Rice University in Houston, Texas, announced today at a meeting of the American Astronomical Society the strong likelihood of new, undiscovered classes of transient astrophysical sources. The existence of these sources is implied by the blast-wave model, the most widely accepted model of gamma-ray bursts.

This conclusion was reached by Drs. Charles Dermer and James Chiang of NRL's Space Science Division and Dr. Markus Boettcher of Rice University, who used the blast-wave model to interpret and model prompt and delayed afterglow data from gamma-ray bursts observed at many different wavelengths across the electromagnetic spectrum.

Diagram of flares from fireballs with various amounts of contaminating matter. A clean fireball (left) produces a short, bright flare at high-energy gamma rays, whereas a dirty fireball (right) produces an extended, weak flare at X-ray and optical energies. The central contour illustrates a flare from a typical gamma-ray burst.

The blast-wave model has been developed by dozens of theoretical astrophysicists and has gained widespread acceptance through its compelling explanation of the bizarre gamma-ray emissions and afterglow behaviors observed from gamma-ray burst sources.

In the blast-wave model, an enormous amount of energy is deposited into a small volume which contains a tiny amount of proton and neutron matter. The resulting fireball expands at nearly the speed of light, producing a blast wave which sweeps up gas in its path. The swept-up gas causes the blast wave to radiate its energy furiously and to slow down. By varying the initial amount of matter initially in the fireball, the researchers found that new cosmic flashes with specific flaring behaviors should exist.

The researchers note that earlier serendipitous detection of X-ray flashes may represent members of the highly contaminated "dirty fireball" class, but that design features of previous high-energy gamma-ray telescopes may have prevented discovery of the weakly contaminated "clean fireball" class.

"A whole new area of discovery could open up," remarks Dr. Dermer, "namely the discovery of new types of 'cosmic flashes in the sky.' So far we have been looking where we were lucky enough to find gamma-ray bursts. But telescopes with the right designs should also find the sister classes of gamma-ray bursts predicted by the blast-wave model, not to mention new types of unimagined cosmic activity."

Background

The gamma-ray burst phenomenon represents one of the great astronomical mysteries of our time. Gamma-ray bursts (GRBs) are intense flashes of gamma rays which last from a fraction of a second to hundreds of seconds. (Gamma-ray photons have nearly a million times more energy than the visual photons we see.) A GRB occurs about once per day and typically outshines all the other gamma-ray sources in the sky during its brief appearance. There is no conclusive evidence that GRB sources repeat.

The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) found that although GRBs flare up at random directions in the sky, there is a notable absence of weak GRBs. The most natural explanation is that GRBs are produced by cosmologically distant sources, and that the lack of weak sources is due to the expansion of the universe and to a declining GRB event rate when the universe was young.

The Italian/Dutch Beppo-SAX satellite (Beppo refers to the Italian scientist Giuseppe "Beppo" Occhialini; SAX stands for Satellite for X-ray Astronomy) revolutionized GRB studies by pinpointing variable X-ray sources hours after the initial gamma-ray burst events. Follow-up observations using optical and radio telescopes led to the discovery of GRB counterparts and their host galaxies. Optical spectroscopy of the fading counterpart of GRB 970508 and the host galaxy of GRB 971214 implied that the burst sources were located at such great distances that cosmic expansion had stretched the wavelengths of the emitted photons by at least a factor of 1.83 for GRB 970508 and by a factor of 4.4 for GRB 971214. At these distances, incredible amounts of energy are implied. In the case of GRB 971214, this represents more than a hundred times the total energy that the Sun radiates in its lifetime!

To explain these astonishing observations, many theorists have seized upon the fireball/blast wave model pioneered by Profs. R. Blandford (CalTech) and C. McKee (UC Berkeley) and first applied to GRBs by Profs. P. Meszaros (Penn State University) and M. Rees (Institute of Astronomy, Cambridge, England). Irrespective of the original source of the GRB energy, which might be due to the coalescence of a neutron star and a compact object, the extraction of the spin energy of a black hole through a catastrophic accretion event, or the collapse of a massive star, the injection of so much energy into a confined volume will cause a fireball to form.

In a paper submitted to The Astrophysical Journal, Drs. Dermer, Chiang, and Boettcher show that the crucial quantity regulating the appearance of a fireball as it expands and decelerates is the amount of proton and neutron (baryon) matter which is originally mixed in the fireball. The amount of mixed-in contaminating matter, or "baryon loading," determines the maximum speed reached by the blast wave before it interacts with the surrounding medium. For a typical GRB, this speed is so close to the speed of light that the ratio of the kinetic energy of the particles in the blast wave compared to the particles' rest mass energy reaches values of 30 - 300. This ratio is called the blast-wave Lorentz factor.

When the blast-wave Lorentz factor is less than about 30, then the fireball is laden with contaminating matter and termed "dirty." Such dirty fireballs are found to produce most of their energy at X-ray and optical wavelengths, though at a lower power level and over a much longer period of time than for a standard GRB. A "clean" fireball has very little contaminating matter so that the blast-wave Lorentz factor can reach values much greater than 300. A clean fireball produces very luminous subsecond bursts of very high energy radiation, with a typical photon containing one hundred million to a billion times more energy than carried by an optical photon.

The bright X-ray sky glow has made it difficult to detect X-ray flashes from dirty fireballs except for chance observations. Nevertheless, X-ray telescopes such as the Einstein Observatory and ROSAT (Roengten Satellite) have reported transients which may be members of the dirty fireball class. On the other hand, the discovery of clean fireballs will require more sensitive gamma-ray telescopes which are not overwhelmed by a flurry of high-energy gamma-ray photons arriving within a fraction of a second. This effect may have prevented the discovery of clean fireballs with previous generations of gamma-ray telescopes, such as the Energetic Gamma Ray Experiment Telescope (EGRET) on CGRO. Discovery of these gamma-ray burst relatives would confirm the fireball/blast wave model, but may require new telescopes which are designed to detect cosmic flashes with the predicted behaviors.

Copies of the press release and related images may be obtained via the World Wide Web at: http://osse-www.nrl.navy.mil/dap-aps.

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