SANTA FE, NM–Astronomers led by an MIT team have solved the mystery
of why nearly two-thirds of all gamma-ray bursts, the most powerful
explosions in the Universe, seem to leave no trace or afterglow: In
some cases, they just weren’t looking fast enough.

New analysis from the speedy High Energy Transient Explorer (HETE),
which locates bursts and directs other satellites and telescopes to
the explosion within minutes (and sometimes seconds), reveals that
most gamma-ray bursts likely have some afterglow after all.

Scientists will announce these results today at a press conference at
the 2003 Gamma Ray Burst Conference in Santa Fe, N.M., a culmination
of a year’s worth of HETE data.

“For years, we thought of dark gamma-ray bursts as being more
unsociable than the Cheshire Cat, not having the courtesy to leave a
visible smile behind when they faded away,” said HETE Principal
Investigator George Ricker, a senior research scientist at the
Massachusetts Institute of Technology’s Center for Space Research.

“Now we are finally seeing that smile. Bit by bit, burst by burst,
the gamma-ray mystery is unfolding. This new HETE result implies
that we now have a way to study most gamma-ray bursts, not just a
meager one third.”

Gamma-ray bursts, likely announcing the birth of a black hole, last
only for a few milliseconds to upwards of a minute and then fade
forever. Scientists say that many bursts seem to emanate from the
implosion of massive stars, over 30 times the mass of the Sun. They
are random and can occur in any part of the sky at a rate of about
one per day. The afterglow, lingering in lower-energy X-ray and
optical light for hours or days, offers the primary means to study
the explosion.

The lack of an afterglow in a whopping two thirds of all bursts had
prompted scientists to speculate that the particular gamma-ray burst
might be too far away (so the optical light is “redshifted” to
wavelengths not detectable with optical telescopes) or the burst
occurred in dusty star-forming regions (where the dust hides the

More reasonably, Ricker said, most of the dark bursts are actually
forming afterglows, but the afterglows may initially fade very
quickly. An afterglow is produced when debris from the initial
explosion rams into existing gas in the interstellar regions,
creating shock waves and heating the gas until it shines. If the
afterglow initially fades too quickly because the shock waves are too
weak, or the gas is too tenuous, the optical signal may drop
precipitously below the level at which astronomers can pick it up and
track it. Later, the afterglow may slow down its rate of decline–but
too late for optical astronomers to recover the signal.

HETE, an international mission assembled at and operated by MIT for
NASA, determines a quick and accurate location for about two bursts
per month. Over the past year, HETE’s tiny but powerful Soft X-ray
Camera (SXC), one of three main instruments, accurately determined
positions for 15 gamma-ray bursts. Surprisingly, only one out of the
SXC’s fifteen bursts has proven to be dark, whereas ten would have
been expected based on results from previous satellite.

An MIT-led team has concluded that the reason that afterglows are
finally being found are twofold: The accurate, prompt SXC burst
locations are being searched quickly and more thoroughly by optical
astronomers; and the SXC bursts are somewhat brighter in X rays than
the more run-of-the-mill gamma-ray bursts studied by most previous
satellites, and thus the associated optical light is also brighter.

Thus, HETE seems to have accounted for all but about 15 percent of
gamma-ray bursts, greatly reducing the severity of the “missing
afterglow” problem. Studies planned by teams of optical astronomers
over the next year should further reduce, and possibly even
eliminate, the remaining discrepancy.

Gamma-ray hunters are challenged. Because of the nature of
gamma-rays and X-rays, which cannot be focused like optical light,
HETE locates bursts within only a few arcminutes by measuring the
shadows cast by incident X-rays passing through an accurately
calibrated mask within the SXC. (An arcminute is about the size of
an eye of a needle held at arm’s length.) Most gamma-ray bursts are
exceedingly far, so myriad stars and galaxies fill that tiny circle.
Without prompt localization of a bright and fading afterglow,
scientists have great difficulty locating the gamma-ray burst
counterpart days or weeks later. HETE must continue to localize
gamma-ray bursts to settle the discrepancy of the remaining dark

The HETE spacecraft, on an extended mission into 2004, is part of
NASA’s Explorer Program. HETE is a collaboration among MIT; NASA;
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 del’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, as well as from Brazil, India and Italy.

At MIT, the HETE team includes Ricker, Geoffrey Crew, John Doty,
Roland Vanderspek, Joel Villasenor, Nat Butler, Allyn Dullighan,
Gregory Prigozhin, Steve Kissel, Alan Levine, Francois Martel, and
Fred Miller; at Los Alamos National Laboratory, team members are
Edward E. Fenimore and Mark Galassi; at the University of California
at Berkeley, Kevin Hurley and J. Garrett Jernigan; at the University
of California at Santa Cruz, Stanford E. Woosley; at the University
of Chicago, Don Lamb, Carlo Graziani, and Tim Donaghy; and at NASA’s
Goddard Space Flight Center, Thomas L. Cline. In Japan, HETE
scientists include Masaru Matsuoka at NASDA, Nobuyuki Kawai at Tokyo
Institute of Technology, and Atsumasa Yoshida at Aoyama Gakuen
University; in France, Jean-Luc Atteia at Observatoire Midi-Pyrenees,
and Michel Boer and Gilbert Vedrenne at CESR.

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