Scientists working at the University of Warwick, a public research university in Coventry, UK, have identified an undocumented type of gamma-ray burst, the product of massive stars destroyed in a way previously unknown to astronomers.
These events are extremely rare, and the only ones observed to date have occurred outside our galaxy. And thank goodness for that; a gamma-ray burst pointed directly at Earth would very likely cause a mass extinction event by ionizing our atmosphere. And in fact, it’s quite possible that the Ordovician extinction, which happened about 450 million years ago, was caused by a gamma-ray burst.
The Long and the Short of It
Until this new discovery, gamma-ray bursts were placed into one of two categories.
Short duration bursts last for a few seconds and occur when two neutron stars collide, producing a black hole.
Long duration gamma-ray bursts last for about a minute and happen when a Wolf-Rayet star explodes as a supernova, producing a black hole at its core. These massive stars are about 20 times the mass of our own sun and with a radius 1,000 times larger (that’s a mind-boggling 3.2 billion kilometres wide).
NORM ‘n’ AL Note: A Wolf-Rayet star is one of the final stages in the life of a very massive star, with more than 20 times the mass of the Sun. These stars lose mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s.
But on Christmas Day of 2010, scientists using NASA’s Swift satellite noticed a very strange celestial event, one that was subsequently analyzed by Andrew Levan and colleagues at the University of Warwick.
“A new population of ultra-long duration gamma-ray bursts”
Levan’s re-interpretation of the data revealed a supergiant that had gone supernova in a galaxy about 7 billion years away (twice the distance of the previous estimate). And strangely, it exhibited an unusually long duration gamma-ray burst — one that poured out of the supernova for hours instead of seconds.
Since this initial observation, astronomers have discovered two more similar examples.
To explain this unprecedented phenomenon, Levan and his team proposed a new theory suggesting that these ultra-long bursts happen because it takes much longer for the supernova explosion to propagate through the massive star, which in turn causes the gamma-ray burst to last for a much longer time.
Typically, most stars that generate gamma-ray bursts are relatively small and dense, resulting in the minute-long events. In these explosions, matter is pulled into the black hole, but some of the energy escapes quickly and violently at nearly the speed of light.
But in these ultra-long duration events, the astronomers speculate that a newly formed black hole in the core of the dying star is powering the explosion. The gamma-ray bursts in these massive stars need to power and plough through the copious amounts of solar material, which results in these extra-long bursts.
Alternatively, Levan’s team considered the possibility that they arise from the tidal disruption of stars by supermassive black holes.
This research was presented at the GRB 2013 Symposium in Nashville, Tennessee on Tuesday, April 16th. Levan’s paper can be read here: “A new population of ultra-long duration gamma-ray bursts.”
(Images from University of Warwick and NASA)
[by George Dvorsky, writing for IO9.com.]
As always, posted for your edification and enlightenment by
NORM ‘n’ AL, Minneapolis