Oct 15, 2000

Backyard Gamma-ray Bursts




With the successful launch of NASA's HETE-2 satellite, amateur astronomers will be able to spot the most powerful explosions in the Universe.


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  October 16, 2000 -- Every day our planet is bathed in high-energy radiation from distant and mysterious cosmic explosions that astronomers call "gamma-ray bursts" (GRBs). If humans could see gamma-radiation, stargazers would notice bright popping flashes all over the sky, rarely from the same direction twice and totally unpredictable. Scientists aren't sure what causes the outbursts, but they do know that GRBs come from the distant reaches of the observable universe and that they explode with mind-boggling power.

"Gamma-ray bursts are about a million times more luminous than supernovae," says Jerry Fishman of the NASA Marshall Space Flight Center. "There's considerable debate about what causes them. They might signal the collapse of super-massive stars or they could be the aftermath of a collision between two compact objects, like a neutron star and a black hole. We just don't know."

Above: An artist's concept of a gamma-ray explosion in progress. [



On October 9th, NASA launched the first observatory entirely devoted to the study of enigmatic GRBs -- the High Energy Transient Explorer-2, or HETE-2 for short. The spacecraft flew into orbit on board an expendable Pegasus rocket that was deployed by an L-1011 aircraft over the Pacific Ocean.

"The successful launch of HETE-2 means that for the first time we can locate hundreds of these bursts with pinpoint accuracy," says MIT's George Ricker, the principal investigator for the HETE-2 mission. "HETE-2's ability to relay the accurate location of each burst in real-time to ... optical and radio observatories will surely revolutionize [the study of gamma-ray bursts]."



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In the past, space-based gamma-ray observatories usually couldn't pinpoint the sky coordinates of a GRB until hours or even days after the blast wave had passed our planet. As a result, astronomers had trouble following up GRB alerts with observations at wavelengths outside the gamma-ray band. They simply didn't know where to point their telescopes while the rapidly-fading afterglows were still visible.

HETE-2 will change that. Instruments on board the spacecraft can quickly localize bright gamma-ray bursts with a precision of 10 arcminutes. Bursts that happen to shine brightly at lower energy "soft" X-ray wavelengths will be pinpointed with an unheard-of precision of 10 arcseconds, more than an order of magnitude finer than any previous GRB instrument.

By transmitting the coordinates to a global network of ground stations, HETE-2 will provide nearly real-time alerts that astronomers can use to study gamma-ray explosions at optical, radio and other wavelengths while the event is still in progress.

"The GRB detectors on HETE-2 will cover a 30-degree-square field of view centered on the anti-solar direction," explains Fishman. "This is the best part of the sky for follow-up optical observations because it's far from the Sun and dark."


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In 1999, a robotic optical telescope in New Mexico caught a gamma-ray burst in action. What it saw stunned astronomers. The expanding fireball of GRB990123 reached 9th magnitude -- bright enough to spot with binoculars or a small telescope if only one knew where to look! Since then several groups of amateur astronomers have captured images of fading afterglows using modest telescopes equipped with CCD cameras. HETE-2's fast, high-accuracy alerts will allow anyone with a good telescope and an internet connection (to receive GRB notices) to view the explosions from their own back yard.

Right: The negative image sequence shows the visible light from a gamma-ray blast (peak magnitude 9) as recorded by the Robotic Optical Transient Source Experiment (ROTSE) in January 1999. Credit: Carl Akerlof/University of Michigan/Los Alamos National Laboratory/Lawrence Livermore National Laboratory. [more information]

"Amateur astronomers can make a real contribution to GRB research by observing such fireballs and measuring their light curves," says Fishman. Observing time on professional telescopes is in high demand, and powerful instruments like the Keck telescope and the Hubble Space Telescope frequently can't be redirected to view an ongoing gamma-explosion, especially on short notice. With little competition for telescope time, amateur astronomers are more flexible. Eventually a global network of amateurs could monitor GRB afterglows nearly around the clock -- a feat no single professional observatory can match.


Timing is Everything
This table, courtesy of Scott Barthelmy (NASA-GSFC) and Jerry Fishman (NASA-MSFC), shows how the brightness of a typical faint GRB afterglow might be expected to diminish with time. Fading fireballs can be dimmer than 20th magnitude just a few hours after the onset of the explosion.
 Time After Burst Maximum Visual Magnitude Minimum Visual Magnitude


 10 min






 30 min






 1 hour






 2 hours






 4 hours






 6 hours






 24 hours





Fishman and Janet Mattei, director of the American Association of Variable Star Observers (AAVSO), are working to make amateur involvement a reality. They've established a program to notify amateurs of GRB coordinates soon after they are received by Goddard's GRB Coordinates Network (GCN). Along with HETE-2, several other spacecraft contribute data to the GCN. These include Ulysses, BeppoSAX and the NEAR spacecraft, in orbit around asteroid 433 Eros. Combinations of these spacecraft also form an Inter-Planetary Network (IPN), which uses the precise arrival times of bursts at each spacecraft to provide an accurate GRB position. The IPN is headed by Kevin Hurley of UC Berkeley, who is also a co-investigator on the HETE-2 mission.

The AAVSO's amateur observing network is in its early stages, but already one member -- Joe Dellinger of the Fort Bend Texas Astronomy Club -- has bagged a gamma-ray burst. Dellinger spotted a fading 19th magnitude glow from GRB000926 on September 28, 2000, using his astronomy club's reflecting telescope and a CCD camera on loan from Rice University.


"This is just the beginning," noted Fishman. "There are hundreds of amateurs around the world with CCD cameras. With rapid notification of bursts in their nighttime sky, most are capable of seeing the afterglows."


Above: This Hubble telescope image of GRB 990123 was captured two weeks after the gamma-ray blast wave passed Earth on January 23, 1999. The image covers a square region about the gamma-ray burst, 3.2 arcseconds on each side. The fading fireball, a point source at the center of the image, is near an irregular galaxy that could be home of whatever exploded. [more information]

The future of gamma-ray burst research looks promising, he continued. In 2003 NASA plans to launch the SWIFT spacecraft, which will improve on HETE-2's already impressive ability to detect and localize gamma-ray bursts. SWIFT will carry aloft its own set of x-ray, ultraviolet, and optical telescopes, which can zero in on new GRB detections within minutes. With ever-improving coordinates and rapid-fire alert times, astrophysicists hope to solve the puzzle of the biggest explosions in the cosmos.

And what of amateur astronomers? For observers with 8 inch or larger telescopes, there's never been a better time to see cosmic explosions from the dawn of the Universe -- from the comfort of one's own backyard!

For information about joining the AAVSO GRB Network, please visit this URL:

HETE-2 is a collaboration between NASA; MIT; Los Alamos National Laboratory, New Mexico; France's Centre National d'Etudes Spatiales (CNES), Centre d'Etude Spatiale des Rayonnements (CESR), and Ecole Nationale Superieure de l'Aeronautique et de l'Espace (Sup'Aero); and Japan's Institute of Physical and Chemical Research (RIKEN). The science team includes members from the University of California (Berkeley and Santa Cruz) and the University of Chicago.


Web Links


HETE-2 Home Page -- from MIT

HETE-2 Launch Press Release -- from NASA-GSFC

A collection of gamma-ray burst images -- from NASA-GSFC

More Science@NASA stories about gamma-ray bursts


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