Rice Scientists Discover New Property of Gamma Ray Bursts

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RICE SCIENTISTS DISCOVER NEW PROPERTY OF GAMMA RAY BURSTS

By studying the evolution of radiation
emissions from gamma ray bursts, Rice University scientists
discovered an energy pattern that will provide new insight into
physical properties of the source of these cosmic enigmas.

Rice astrophysicist Edison Liang and former graduate student
Vincent Kargatis reported the findings in the May 2 issue of the
science journal Nature. Their discovery may eventually help unravel
the underlying physical mechanism, and the origin, of gamma ray
bursts.

Gamma ray bursts are mysterious random flashes of
electromagnetic radiation, or photons, 100,000 times more energetic
than visible light photons in the sky. These high-energy bursts
occur about once a day, and last from 100th of a second to 100s of
seconds.

Discovered accidentally in the late 1960s by United States
nuclear test surveillance satellites, the origin of the bursts has
puzzled astronomers for almost three decades.
Currently astronomers are deeply divided over whether the bursts
originate from an extended sphere around our own galaxy, at a
distance of hundreds of thousands of light years away, or if they
originate in galaxies and quasars billions of light years away.

To shed light on these opposing models, scientists have
investigated every detail of the time-based and spectrum-based
behaviors of these bursts. But until now, very few clear trends or
patterns have emerged. The findings by Liang and Kargatis, obtained with support from
NASA scientists on the Burst and Transient Source Experiment (BATSE)
team at Marshall Space Flight Center in Huntsville, Ala., show that
for many gamma ray bursts whose spectrum, or relative distribution
of photons at different energies, can be analyzed in short time
intervals, a pattern is found. A gamma ray burst is typically made
up of many individual pulses.

The Rice scientists found that during
the decay or declining evolution of many separate pulses, the
characteristic energy of the gamma rays generally decreases
exponentially, not over time, but with the total amount of photons
emitted by the source. Physically, this implies that the energy of
the gamma rays decreases at a rate proportional to the power emitted
from the source. In other words, the average photon energy decreases
exponentially as more photons leave the source.

“This is the first time that we have had an inkling that such a
physical law is operating at the source,” Liang says. “This will
provide information about the physical mechanism of the source, and
later, combined with other information, we are hopeful that this
will help us to pin down the distance of the source.”

In the Nature article, the Rice scientists venture this
interpretation of their findings: They propose that if particles
energized at the source cool down only by radiating gamma rays with
an energy proportional to the particle energy, then the gamma ray
energy decay pattern they found can be explained by energy
conservation — no additional energy is supplied to the particles
during the pulse decay. In this case, the exponential constant of
the gamma ray energy decay is proportional to the total number of
radiating particles. However, they emphasize that this
interpretation is not unique and other viable alternatives are being
explored.

Liang and Kargatis further observed that the energy decay
constants of the individual pulses within a single burst are often
the same, even when the pulses have very different qualities. If the energy decay constant represents a measure of the total number of
radiating particles, then the number of particles must remain the
same from pulse to pulse. This would hint that the radiating
particles originate from the same burst site and that the pulses
come from a regenerative source, Liang says. The implications of
these results for the debate about the distance models are
potentially far reaching and remain to be investigated.

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