Astronomers announced today the
discovery of what may be the lower-energy “poor relations” of cosmic
gamma-ray bursts, the fantastically powerful explosions occurring
daily in distant galaxies throughout the universe. 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

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.

Los Alamos National Laboratory is operated by the University of
California for the National Nuclear Security Administration (NNSA) of
the U.S. Department of Energy and works in partnership with NNSA’s
Sandia and Lawrence Livermore national laboratories to support NNSA
in its mission.

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