Scientists working at the University of Warwick have identified an undocumented type of gamma-ray burst, the product of massive stars destroyed in a way previously unknown to astronomers.
Gamma-ray bursts are among the most powerful forces in nature. As a supergiant star goes supernova, it shoots out directed beams of concentrated gamma-rays across vast distances. A typical burst can release as much energy in a few seconds as the sun does its entire 10-billion-year lifespan.
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.
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).
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.
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: University of Warwick; NASA.