Harvard-Smithsonian Center for Astrophysics
Cambridge, Massachusetts
Scientific contacts:
University of Notre Dame
Peter Garnavich, 219-631-7262, pgarnavi@nd.edu
Harvard-Smithsonian Center for Astrophysics
Avi Loeb, 617-496-6808, aloeb@cfa.harvard.edu
Kris Stanek, 617-495-7042, kstanek@cfa.harvard.edu
Release No.: 00-18

For the first time, astronomers have resolved the visible blast wave produced by a gamma-ray burst. By taking advantage of a fortuitous cosmic alignment, a team of scientists were able to focus the light from a gamma-ray event some half-way across the Universe.
The object, dubbed "GRB 000301C," was discovered in March 2000. After gathering data at the Smithsonian’s Fred Lawrence Whipple Observatory and combining it with other observations of this gamma-ray burst, the research team showed that it appeared as a small, rapidly expanding ring, confirming predictions.
"This discovery really confirms what we thought a gamma-ray burst shock should look like," says Peter Garnavich of the University of Notre Dame and lead author of the paper to appear in the Astrophysical Journal Letters. "To be able to resolve an explosion so far away is really quite astounding."
Gamma-ray bursts (GRBs) are mysterious flashes of high-energy light that are detected about once a day somewhere on the sky. However, their origin remains unknown to astronomers, most of whom believe GRBs are enormous explosions that occur far across the Universe.
The researchers were able to achieve this elusive goal: to see the ring-shaped structure caused by the gamma-ray burst because of
"gravitational micolensing." Predicted by Einstein’s theory of general relativity, microlensing occurs when the light from a very distant source — in this case, a gamma-ray burst — is amplified by the gravity of an intervening object.
"Gravitational microlensing is commonly observed in our Galaxy," says Kris Stanek of the Harvard-Smithsonian Center for Astrophysics and co-author of the result. "But, this is the first time such a lensing event has been seen in a distant gamma-ray burst." The theoretical prediction about the properties of such an event was made by co-author Avi Loeb back in 1998, together with his student at the time, Rosalba Perna.
In addition to being able to show that a GRB blast wave appears as a ring, the team also discerned information about the object that magnified the burst.
"We believe that the gravity of an ordinary star, perhaps half the mass of our Sun, created the lensing phenomenon," says Avi Loeb, also of the Harvard-Smithsonian Center for Astrophysics. The authors found that the observed data perfectly fit the model for a ring structure which is expanding faster than the speed of light, a prediction first made in 1997 by Eli Waxman.
GRBs are such large explosions they drive shock waves that travel at close to the speed of light into the surrounding gas which then glows at x-ray, optical, and radio wavelengths. Because the shocks are moving at nearly the speed of light, Einstein’s theory of special relativity must be employed in calculating what an observer would see. Contrary to common sense, the relativistic shock will appear to an observer as a ring that is expanding faster than the speed of light. The ring will appear small because of the enormous distance to the GRB.
Resolving the GRB ring is equivalent to spotting a wedding ring two million miles away. Equivalently, it is like seeing an "o" on this page from the Moon.
The very small angular size of the GRB ring has prevented conventional telescopes from confirming these predictions. Earth-bound telescopes are limited to about one arcsecond resolution by turbulence in our atmosphere. Better resolution is achieved in space, but the apparent size of the GRB shock is still more than 100000 times smaller than the Hubble Space Telescope resolution of 0.1 arcsecond.
Images related to this result can be found at:
    http://cfa-www.harvard.edu/~peterg/grblens/magnification.jpg     http://cfa-www.harvard.edu/~peterg/grblens/diagram.jpg
    http://cfa-www.harvard.edu/~peterg/grblens/magnification.ps     http://cfa-www.harvard.edu/~peterg/grblens/diagram.ps