Dying supergiant stars behind gamma-ray bursts
Washington: Two international teams of astronomers studying long-lasting gamma-ray bursts (GRBs) have concluded that they likely arose from the catastrophic death of supergiant stars hundreds of times larger than the Sun.
GRBs are the most luminous and mysterious explosions in the universe. The blasts emit surges of gamma rays-the most powerful form of light-as well as X-rays, and they produce afterglows that can be observed at optical and radio energies.
Bruce Gendre, a researcher who led this study while at the Italian Space Agency`s Science Data Center in Frascati, Italy and his colleagues made a detailed study of GRB 111209A, which erupted on Dec. 9, 2011, using gamma-ray data from the Konus instrument on NASA`s Wind spacecraft, X-ray observations from Swift and the European Space Agency`s XMM-Newton satellite, and optical data from the TAROT robotic observatory in La Silla, Chile.
The burst continued to produce high-energy emission for an astonishing seven hours, making it by far the longest-duration GRB ever recorded.
Another event, GRB 101225A, exploded on Christmas Day in 2010 and produced high-energy emission for at least two hours. Subsequently nicknamed the "Christmas burst," the event`s distance was unknown, which led two teams to arrive at radically different physical interpretations.
One group concluded the blast was caused by an asteroid or comet falling onto a neutron star within our own galaxy. Another team determined that the burst was the outcome of a merger event in an exotic binary system located some 3.5 billion light-years away.
"We now know that the Christmas burst occurred much farther off, more than halfway across the observable universe, and was consequently far more powerful than these researchers imagined," said Andrew Levan, an astronomer at the University of Warwick in Coventry, England.
Using the Gemini North Telescope in Hawaii, Levan and his team obtained a spectrum of the faint galaxy that hosted the Christmas burst. This enabled the scientists to identify emission lines of oxygen and hydrogen and determine how much these lines were displaced to lower energies compared to their appearance in a laboratory. This difference, known to astronomers as a redshift, places the burst some 7 billion light-years away.
As a part of this study, Levan`s team also examined 111209A and the more recent burst 121027A, which exploded on Oct. 27, 2012. All show similar X-ray, ultraviolet and optical emission and all arose from the central regions of compact galaxies that were actively forming stars.
The astronomers concluded that all three GRBs constitute a hitherto unrecognized group of "ultralong" bursts.
And both groups suggested that the likely candidate is a supergiant, a star with about 20 times the Sun`s mass that still retains its deep hydrogen atmosphere, making it hundreds of times the Sun`s diameter.
Gendre`s team goes further, suggesting that GRB 111209A marked the death of a blue supergiant containing relatively modest amounts of elements heavier than helium, which astronomers call metals.
The astronomers found that 75 percent of long GRBs occurred among the 10 percent of star formation with the lowest metal content. While the study found a few long GRBs in environments with high-metal content, like our own galaxy, these occur at only about 4 percent the rate seen in low-metal environments per unit of underlying star formation.
"Most stars form in metal-rich environments, and this has a side effect of decreasing the prevalence of long GRBs as the universe grows older. And while a nearby long GRB would be catastrophic to life on Earth, our study shows that galaxies like our own are much less likely to produce them," Graham explained.
The findings appeared in The Astrophysical Journal.
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