If the relationship is confirmed by future observations, this potentially new breed of burst, called an X-ray flash, will provide key information to solve the decades-old puzzle of how these most powerful explosions in the universe are produced.

R. Marc Kippen, a staff member at the U.S. Department of Energy's Los Alamos National Laboratory, presented the report at the American Astronomical Society High Energy Astrophysics Division meeting in Albuquerque. Collaborators in the work are researchers John Heise and Jean J. M. in 't Zand of The Netherlands' National Institute for Space Research (SRON) and Peter M. Woods, Michael S. Briggs, and Robert D. Preece of the National Space Science & Technology Center in Huntsville, Ala.

Since the 1960s, scientists have used orbiting platforms to measure high-energy radiation (X-rays and gamma rays), finding a range of perplexing cosmic burps, buzzes, and pops. These are now explained by a variety of extreme mechanisms involving strange objects such as neutron stars, black holes, and quasars.

The most energetic and powerful of these phenomena are gamma-ray bursts, which typically last less than a minute, and emit a large majority of their energy in the form of high-energy photons called gamma rays.

After recent discoveries of lingering X-ray, optical and radio afterglow emissions from the sites of these bursts, scientists now generally agree that they occur in some of the most distant galaxies known, but how they are produced remains a mystery.

Similarly mysterious are lower- energy X-ray transients that also typically last less than a minute, and have been observed for decades by several different instruments. It has long been hypothesized that these X-ray events and gamma-ray bursts are related to the same phenomenon, but only now does evidence support the idea.

The evidence presented by Kippen linking X-ray transients to gamma-ray bursts is based on a set of events dubbed "X-ray flashes," detected at a rate of approximately four per year. These flashes were discovered starting in 1997 by a team led by Heise using their Wide Field Cameras (WFC) on the then newly launched Italian-Dutch X-ray astronomy satellite BeppoSAX.

Only the lack of detectable gamma-ray emission distinguished these X-ray flashes from the ordinary gamma-ray bursts observed with BeppoSAX. However, without measuring gamma-ray emission, there was nothing to conclusively link the flashes to gamma-ray bursts.

Fortunately, in the right place at the right time to simultaneously observe ten of the WFC X-ray flashes was the Burst and Transient Source Experiment (BATSE) aboard NASA's Compton Observatory.

Kippen, Heise, and their colleagues strongly suspected that because BATSE was a much more sensitive gamma-ray instrument than those on BeppoSAX, there was a chance of measuring the gamma-ray signals, and thus obtaining data that could link the two phenomena. Indeed, using BATSE, weak gamma-ray emission was detected from nine of the ten observed flashes.

"Experimental astronomers love to tout their favorite instruments, but in this case the great sensitivity of BATSE really was crucial for observing the weak gamma-ray emission from these events," said Kippen.

Just because the flashes showed weak gamma-ray emission, however, did not prove that they are related to gamma-ray bursts-several types of mainly X-ray emitting objects extend (weakly) into the gamma-ray regime.

The astronomers therefore compared the newly obtained gamma-ray properties of the flashes to those of the thousands of gamma-ray bursts observed with BATSE from 1991-2000. According to Kippen, "The flashes are remarkably similar to gamma-ray bursts in nearly all respects, except that they emit most of their energy in X-rays."

The team then attempted to quantify the spectrum of energies in X-rays and gamma rays for the flashes. By jointly examining data from both the WFC and BATSE, the researchers identified a familiar pattern: the detailed spectra of the flashes appear similar to those of typical gamma-ray bursts, only shifted to lower (X-ray) energies.

Furthermore, the characteristic spectral energies of the flashes are consistent with a trend observed for gamma-ray bursts-namely that weaker bursts have spectra with lower characteristic energies than more energetic bursts.

These findings lead the researchers to tentatively conclude that X-ray flashes are the low-energy relatives of gamma-ray bursts, created by similar mechanisms. However, because of the small number of flashes, and the weakness of their detected emissions, the preliminary findings need to be confirmed by more observations, such as those being made with BeppoSAX and NASA's High Energy Transient Explorer (HETE). Los Alamos is responsible for the wide-field X-ray monitor aboard HETE.

Another key to understanding the burst/flash relationship is the identification of flash afterglow counterparts, which remain elusive.

The best hope in this arena, the scientists say, is NASA's Swift Gamma Ray Burst Explorer satellite mission, scheduled for launch next year, which includes telescopes to measure gamma-ray, X-ray, and optical emissions nearly simultaneously.

If the results of this study are confirmed, these new additions to the family of gamma-ray bursts could provide crucial information needed to understand how the tremendous bursts are generated.

"It's like finding a distant cousin living in the next hollow that can tell you more about your family tree than your own mother or father," said Kippen.

Indeed, theorists are already speculating about possible scenarios that explain the entire clan of bursts using the differences and similarities of the new family members. This work was supported by the National Aeronautics and Space Administration under the Burst and Transient Source Experiment program.

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