Solving the Mystery of Gamma-Ray Bursts

Until 1997, the big problem was that, with the exception of soft gamma-ray repeaters (which may be a distinct class of objects), no gamma-ray burst had been seen in wavelengths longer than X-rays and no quiescent counterpart had been seen. This greatly hampered the study of gamma-ray bursts, since we didn't know what objects to concentrate on theoretically.

The Breakthrough Observations

In 1997, Astronomers were able to take the first steps to solve one of the longest standing puzzles in science. Two optical counterparts to gamma-ray bursts (GRBs) were discovered by the most powerful telescopes on Earth and in space. The results of these observations showed the bursts originate billions of light-years from Earth.

The discoveries were a result of new instruments and a new cooperation between gamma-ray, X-ray and optical astronomy. While aloft, the Gamma-Ray Observatory (GRO) detected on average one burst per day. Unfortunately GRO could not pinpoint the position of the object or objects causing the burst to a small region of the sky. This made searches at other wavelengths difficult. The launch of the BeppoSAX satellite in 1996 changed that. Now more precise positions of GRBs could be determined at X-ray wavelengths by BeppoSAX and relayed to astronomers using powerful optical telescopes on the ground and the Hubble Space Telescope (HST). Data from these telescopes showed the GRBs to originate far outside our Milky Way galaxy.

The first break in the case of the mysterious bursts came on February 28, 1997. Within hours of the detection of a burst designated GRB970228 by the BeppoSAX satellite, astronomers began searching the sky near the source of the X-rays with optical telescopes. They were rewarded with the first optical images of a GRB object.

Analysis of the HST images of the February 28 burst revealed the burst object is associated with a faint, fuzzy patch of light dwarfed by the brighter emission of the GRB source. This faint extended emission is presumed by many scientists to be a 'host' galaxy from within which some cataclysmic event led to the GRB. The unmatched resolution of the Hubble telescope allowed astronomers to determine that the source of the burst does not lie at the center of the faint galaxy, but is offset, most likely in the disk population of normal stars. This would seem to rule out the possibility that the bursts are powered by massive black holes at the center of galaxies and suggests the products of typical stellar evolution like colliding neutron stars as GRB candidates. A galaxy like our own Milky Way could produce a bursting object every few million years, an explosion that for a few seconds out shines the entire galaxy.

Within a week of the February outburst the optical component had faded nearly out of sight, leaving only the faint smudge of its host galaxy. Out of more than 2,000 bursts detected by satellites from 1990-1997, only one burst location ever produced a second GRB. In all likelihood the source of the February 28 burst will not shine again.

Subsequent Observations and Confirmation

Further insight to the source of GRBs came after another BeppoSAX observation, this time of a burst on May 8, 1997. Again within a few hours of the burst detection telescopes all over the world were pointed towards the elusive source. Scientists at the Palomar Observatory identified an optical counterpart to the GRB that exhibited unusual variability in its brightness.

The astronomers then turned to the worlds most powerful ground based telescopes, the 10 meter Keck pair in Hawaii. By examining the wavelengths of specific spectra features present in the light form the variable source, the scientists were able to determine the distance to the source object. The GRB host was found to lie at a redshift of at least 0.8, or several billion light years from earth, over one half way across the observable universe. This is definitive proof that at least some if not all the observed gamma-ray bursts are due to objects far beyond our own galaxy. Mark Metzger, a Caltech astronomy professor, said he was thrilled by the result. "When I finished analyzing the spectrum and saw features, I knew we had finally caught it. It was a stunning moment of revelation. Such events happen only a few times in the life of a scientist." For a few seconds the burst was over a million times brighter than an entire galaxy.

GRO recorded a bright GRB event, GRB 970616, in June 97. The position of the burst accurate to about 2 degrees was derived and promptly disseminated. The region of sky was scanned by the Rossi X-Ray Timing Explorer (RXTE) within four hours of the GRO discovery. RXTE discovered an X-ray source several times brighter than those detected previously by SAX. Meanwhile, a greatly improved position was derived by combining the GRO measurement with a separate detection of GRB 970616 by the Ulysses spacecraft. The RXTE X-ray source lies within this region of the sky. Follow up observations with RXTE one day later revealed that the source had faded below the detection limit. Thus, the source detected by RXTE was, in all likelihood, the X-ray counterpart to GRB 970616.

Through the late 1990's, these three NASA satellites, working in tandem, continued to greatly expanding the number of gamma-ray burst counterpart detections. No other events produce so much energy in so short a time. The study of gamma-ray bursts has continued in an attempt to discover just what can cause such violent and spectacular displays.