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Office of Public Information
Eberly College of Science
Pennsylvania State University
University Park, Pennsylvania

CONTACTS: 

Lajos Balazs at the Konkoly Observatory in Budapest
phone (+36)1-375-4122, e-mail: balazs{at}konkoly.hu

Peter Meszaros at Penn State University in the United
States
phone (+1)814-865-0418, e-mail: pmeszaros{at}astro.psu.edu

Eva Engedi (PIO at the Humgarian Academy of Sciences)
phone (+36)1-411-6100, e-mail: engedi{at}office.mta.hu

Barbara K. Kennedy (PIO at Penn State)
phone (+1)814-863-4682, e-mail: science{at}psu.edu

20 February 2003

Short and Long Gamma-Ray Bursts Different to the Core
=====================================================

While the origin of gamma-ray bursts -- the most powerful explosions 
known in the universe -- remains a mystery, scientists say that the 
two major varieties, long and short bursts, arise from different types 
of events.

In an analysis of nearly 2,000 bursts, a team of researchers from 
Europe and Penn State University uncovered new discrepancies in the 
light patterns in bursts lasting less the two seconds and in bursts
lasting longer than two seconds.

"We can now say with a high degree of statistical certainty that the 
two show a different physical behavior," said Lajos Balazs of Konkoly 
Observatory in Budapest, lead author on a paper appearing in an
upcoming issue of the journal Astronomy & Astrophysics.

The analysis supports the growing consensus that long bursts originate 
from fantastic explosions of stars over 30 times more massive than our 
Sun. Short bursts have been variously hypothesized to be fiery mergers 
of neutron stars, black holes, or both, or perhaps a physically 
different type of behavior in massive collapses.

"It is suspected that, either way, with each gamma-ray burst we wind 
up with a brand new black hole," said Peter Mészáros, professor and 
head of the Penn State Department of Astronomy and Astrophysics. "The 
puzzle is in trying to identify clues that would help to elucidate 
whether these two types consist of essentially the same objects with 
different behaviors, or different objects with somewhat similar 
behavior."

Gamma-ray bursts are like a 10^45 watt bulb, over a million trillion 
times as bright as the Sun. Although common -- detectable at a rate of 
about one per day -- the bursts are fast-fading and random, never 
occurring in the same place twice. Scientists have been hard pressed 
to study the bursts in detail, for they last only a few milliseconds 
to about 100 seconds, with most around 10 seconds long. Most 
scientists agree that the majority of bursts originate in the distant 
reaches of the universe, billions of light years away.

Previous results have shown that the short bursts have "harder" 
spectra, which means that they contain relatively more higher-energy 
gamma-ray photons than the longer bursts do. Also, in short bursts, 
the photons hitting a burst detector are closely spaced, or bunched, 
compared to the longer bursts, suggesting that the source is 
physically different, as well.

This type of information is valuable because it appears to contain 
clues about the intrinsic physical mechanism by which the sources 
produce the gamma rays, but these sources have still not been 
characterized in enough detail to understand them. Balazs and his 
colleagues sought to establish what, if any, correlation exists
between different pairs of properties, when one considers separately 
the long and the short bursts.

The team examined the fluence and duration of 1,972 bursts and found a 
new relationship. The fluence is the total energy of all the photons 
emitted by the burst during its gamma-ray active stage, a measurement
incorporating both the flow and energy of individual photons.

Within both categories, long and short, there is a correlation between 
fluence and duration: the longer the burst, the greater the fluence. 
Yet the degree of this relationship is statistically different for the
two categories (at a 4.5 sigma significance level).  This difference 
places constraints on what can cause these bursts or how they can 
operate.

In long bursts, there is a direct proportionality between duration and 
fluence, suggesting that the energy conversion rate into gamma rays 
is, on average, more or less constant in time. For the short bursts, 
there is a weaker dependence, which could, for instance, be due to an 
energy conversion rate into gamma rays that drops in time, resulting 
in a less efficient gamma-ray engine.

It seems unlikely that the same engine could produce both types of 
bursts, the team said. Although not directly addressed in the paper, 
these results support the notion that if the long bursts originate 
from massive stellar explosions, then short bursts originate from 
something entirely different. In the latter scenario, this event could 
be either mergers or such a drastic Jekyll-and-Hyde-like switch in the 
stellar explosion mode that the engine appears physically quite
different. Such drastic and well-defined differences in the 
correlation between two of the major variables will need to be 
addressed quantitatively in future models of the burst physics.

The 1,972 bursts were observed by the BATSE instrument on the NASA 
Compton Gamma Ray Observatory, a mission active between 1991 and 2000. 
Coauthors also include Zsolt Bagoly, of the Laboratory for Information
Technology at Eotvos University in Budapest; Istvan Horvath, of the 
Department of Physics at Bolyai Military University in Budapest; and 
Attila Meszaros, of the Astronomical Institute at Charles University
in Prague.

This research was supported by the U. S. National Aeronautics and 
Space Administration (NASA) and the Hungarian national research 
foundation (OTKA).

For a copy of the Astronomy & Astrophysics journal article now in 
press, refer to http://lanl.arXiv.org/abs/astro-ph/0301262.

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