Decades-Old Mystery of Gamma-Ray Bursts Solved by International Team of Astronomers

PASADENA, Calif.—Astronomers have solved the mystery of the elusive gamma-ray bursts with very short duration that go off once a day.

Using a variety of space- and ground-based instruments, the international team has found good support for the idea that short gamma-ray bursts are caused by violent collisions in star-forming galaxies between a black hole and a neutron star, as well as by collisions of two neutron stars. These bursts typically shine in gamma radiation for only a few thousandths of a second, but can be brighter than a billion suns during the short interval they are at peak power.

Reporting in the October 6 issue of the journal Nature, Penn State University astronomer Derek Fox, his former mentor Shri Kulkarni of the California Institute of Technology, and 31 colleagues from 16 other institutions announce that the detailed observations of short gamma-ray bursts that occurred on May 9 and July 9 of this year led to the discovery.

The short burst of May 9 was originally detected by NASA's Swift Satellite, and the July 9 burst by NASA's High-Energy Transient Explorer (HETE). In particular, the July 9 event was not caused by the explosion of a single star, as is known to be the case in gamma-ray bursts of longer duration. Theorists had already suspected that the short bursts were caused by the extremely heavy black holes and/or neutron stars slamming together, and the new observations provide the empirical evidence to back up the theory.

"Our observations do not prove the coalescence model, but we surely have found a lady with a smoking gun next to a dead body," said Kulkarni, who is the MacArthur Professor of Astronomy and Planetary Science.

The coalescence of neutron stars and black holes has been the primary driver for research at gravitational-wave observatories such as LIGO, and will continue to be important to future space missions such as LISA.

"I am thrilled that work may have bearing on the exciting field of gravitational wave astronomy, surely a great frontier in physics," said Fox, who is the lead author on one of the four Nature papers.

Before assuming his faculty position at Penn State this year, Fox was part of the Caltech team that has been at the forefront of investigating gamma-ray bursts. Kulkarni and his colleagues, in fact, first demonstrated in 1997 that gamma-ray bursts were extragalactic in origin.

The May 9 event was the first short burst for which an afterglow was later detected (in other words, the flash of visible light from the explosion was seen after the gamma rays quickly subsided).

The July 9 explosion, however, allowed scientists to follow up with observations sufficient to eliminate many other possible explanations for the burst. Using the Chandra X-Ray Observatory, the Danish 1.5-meter telescope, the 8.2-meter Subaru telescope, the Very Large Array radio telescope, the 40-inch Swope and 100-inch Du Pont telescopes, the Gemini North, and the Hubble Space Telescope, the team concluded that the burst originated in a star-forming galaxy about 2 billion light-years from Earth.

Further evidence gathered with the Hubble Space Telescope led the researchers to believe that the explosion was too faint to be a supernova-in fact, fainter than any supernova that has ever been observed. Further, the energies observed in both the July 9 and May 9 bursts are consistent with the results seen in computer simulations for collisions of "compact object binaries"-that is, neutron stars and black holes.

Also, the location of the May 9 burst on the outskirts of a star-forming galaxy is at a place where one would expect to see old stars that have collapsed into the extremely massive neutron stars and even more massive black holes. The July 9 burst appears to have come from deeper within the galaxy, however.

"I am amazed that we have been able to make such great strides in the space of a few months." Kulkarni added. "Now it is time to start addressing what beast lies at the heart of these explosions."

In addition to Kulkarni and Fox, who was a Caltech postdoctoral scholar at the time of the discovery, the other Caltech authors are Fiona Harrison, a professor of physics and astronomy at Caltech; Tsvi Piran, a professor at Hebrew University who recently spent part of his sabbatical at Caltech as a Moore Distinguished Scholar; Avishay Gal-Yam and Dae-Sik Moon, all postdoctoral scholars; Ehud Nakar, a senior research fellow; and Mansi Kasliwal, Alicia Soderberg, Stephen Bradley Cenko, and Patrick Brian Cameron, all graduate students.

Research on gamma-ray bursts at Caltech is supported in part by funding from NASA and the National Science Foundation.