Weekend Feature: 40-Year-Old Mystery of Massive Gamma-Ray Burst Solved


Studies have shown that neutron stars often exist as pairs on the fringes of galaxies, and that they collide frequently, sometimes involving a black hole, also a stellar remnant but one that is much more dense. Neutron stars are often as small as the island of Manhattan with a teaspoon of its material weighing several tons.

A German supercomputer cluster at the Albert Einstein Institute in Germany models events that unfold in just 35 milliseconds — three times faster than the blink of an eye, has finally solved the Gamma-ray burst mystery by showing that colliding neutron stars can produce magnetic structures which are responsible for the cosmic event, says a NASA study.

Gamma-ray bursts are among the most energetic cosmic events known, emitting as much energy in a few seconds as our entire galaxy does in a year, most of it in the form of Gamma rays, the highest energy form of light.

"For the first time, we''ve managed to run the simulation well past the merger and the formation of the black hole," said the study''s co-author Chryssa Kouveliotou at the US space agency''s Marshall Space Flight Center in Huntsville. "This is by far the longest simulation of this process, and only on sufficiently long timescales does the magnetic field grow and reorganize itself from a chaotic structure into something resembling a jet," she added.

GRBs lasting longer than two seconds are widely thought to be triggered by the collapse of a massive star into a black hole. As matter falls toward black hole, some of it forms jets in the opposite direction that move near the speed of light and produce a blast of Gamma rays as they emerge.

"For more than two decades, the leading model of short GRBs was the merger of two neutron stars. Only now can we show the merger of neutron stars actually produces an ultrastrong magnetic field structured like jets needed for a GRB," Bruno Giacomazzo at NASA''s Goddard Space Flight Center said.

Several GRBs are recorded every day, coming from all directions. Most originate in the very distant universe. A GRB burst closer than 10 light years could trigger a mass extinction on Earth.

Gamma rays are the most intense form of radiation, more powerful than X-rays. The afterglow of a single burst, measured in X-rays, radio waves and other wavelengths, can be billions of times brighter than the entire galaxy in which it originates. Long-duration GRBs typically last about 20 seconds, released when the core of a young and very massive star collapses in a supernova event.

"Gamma-ray bursts in general are notoriously difficult to study, but the shortest ones have been next to impossible to pin down," said Neil Gehrels of NASA Goddard Space Flight Center. "All that has changed. We now have the tools in place to study these events."

Back in 2005, Astronomers observed two short-duration bursts that took place on the outskirts of a faraway galaxy, a location where old stellar remnants like neutron stars are known to reside,
in unprecedented detail and determined the likely scenario in which two dense objects collide and coalesce. The discovery involved more than 30 researchers at 16 institutions and a host of telescopes on the ground and in space.

"The observed characteristics of the short gamma-ray bursts are all consistent with models of the merger of two neutron stars, or of a neutron star with a black hole," said Piro, of the Instituto Astrofisica Spaziale in Rome.

The first burst was detected May 9, 2005 by NASA's Swift satellite. Scientists believed that morning that they were seeing, live, the merger of two neutron stars into a single black hole.

Another burst on July 9, 2005 was noted by the HETE-2 satellite, an international project run by NASA. It lasted just a tenth of a second, but was the first event for which an accurate distance is known. An afterglow of optical light from the same location was spotted by other telescopes, representing the first visible light ever associated with a short-duration burst.

At a billion light-years away, it was about 10 times closer than nearly all other recorded GRBs. Calculations also show, however, that it was about 1,000 times less energetic than the others. Too weak, in fact, to have come from a single exploding massive star, helping astronomers determine that it likely involved a merger of two old stars, as theory predicts. For every event astronomers spot, 30 other mergers go undetected.

"The mystery of short gamma-ray bursts is largely solved," said Don Lamb, a University of Chicago researcher and co-author of one of four papers describing the observations.

Albert Einstein predicted that when neutron stars collide, a shock of gravitational waves should be released. An observatory called LIGO is ramping up in an effort to detect these waves.

"It's possible now that the first gravitational wave source that LIGO observes will also be a gamma-ray burst source," said Kevin Hurley of the University of California at Berkeley. "Now that would be a spectacular discovery."

The Daily Galaxy via nature.com, space.com and news.in.msn.com

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Image credit: An artist's impression of merging neutron stars, one of the theoretical progenitors of gamma-ray bursts and the birth of a black hole. NASA, Sonoma State University, Aurore Simonnet


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