Mysterious Extra-Long Gamma Ray Bursts Baffle Astronomers

NASA/Swift/A. Simonnet, Sonoma State Univ. Artists's conception of the X-Ray producing blast of gas exiting GRB 130925A as it turns into a black hole

An extraordinarily long Gamma Ray Burst is creating excitement among astronomers as a possible portal into the way very large, very early stars died.

As the name suggests, Gamma Ray Bursts (GRBs) are short blasts of very high energy radiation. Most last two seconds or less often leaving astronomers with a precious afterglow to study

However, a class of GRBs known as long Gamma Ray Bursts last from 20 to 50 seconds. The most popular theory is that short GRBs are the product of two objects merging, be they black holes or neutron stars, while long bursts come from stars collapsing to form black holes.

What then to make of a burst measured not in seconds, but hours? GRB 130925A lasted 1.9 hours. Intense and variable X-Rays were seen by the Swift Telescope for six hours before a steady fadeout began.

"GRB 130925A is a member of a rare and newly recognized class we call ultra-long bursts," said Eleonora Troja of NASA's Goddard Space Flight Center. "But what really sets it apart is its unusual X-ray afterglow, which provides the strongest case yet that ultra-long GRBs come from stars called blue supergiants." 

GRB 130925A is not the longest lasting GRB. GRB 111209A lasted an astonishing seven hours, and NASA established a symposium for rival teams to debate the most likely cause. 

Naturally an event as extraordinary as GRB 130925A has drawn plenty of interest, with attempts to explain it coming thick and fast.

Troja is one of the authors of a paper in The Astrophyiscal Journal suggesting GRB 111209A must have come from a blue supergiant with low proportions of elements heavier than helium, which astronomers refer to as metals. 

This is an intriguing possibility, because the first stars were very low in metals. Each generation of stars form metals as they expire, creating an increased concentration in younger stars.GRB 130925A is not far enough away to have occurred when the first generation of stars was common through most of the universe, but some galaxies have lagged behind and GRB 130925A may have come from one of these, providing insight into the earliest stars..

Troja's theory holds that when the core of the star turned into a supernova its outer atmosphere was so large it took two hours to fall into the hole, generating gamma rays throughout. After the initial burst GRB 130925A was unusually consistent in its afterglow, which indicates a fairly empty area of space around it, rather than one filled with thrown-off gasses, also consistent with a blue giant, but not most other very large stars.

The most unusual feature of GRB 130925A, the repeated X-Ray flaring, was also the hardest to explain. The paper proposes a high-energy jet boring through the collapsing star, heating the cooler stellar gas as the collide. The warmed gas flowed down the sides of the jet and created an X-Ray-emitting sheath. The cocoon appears to have remained intact as the jet left the star, which the authors attribute to magnetic fields.

"This is the first time we have detected this thermal cocoon component, likely because all other known ultra-long bursts occurred at greater distances," said lead author Luigi Piro of the Institute for Space Astrophysics and Planetology.

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