Dying supergiant stars implicated in hours-long gamma-ray bursts

GRB 111209A exploded on Dec. 9, 2011. 

The blast produced high-energy emission for an astonishing seven hours, earning a record as the longest-duration GRB ever observed. 

This false-colour image shows the event as captured by the X-ray Telescope aboard NASA's Swift satellite. 

Credit: NASA/Swift/B. Gendre (ASDC/INAF-OAR/ARTEMIS) 

Three unusually long-lasting stellar explosions discovered by NASA's Swift satellite represent a previously unrecognised class of gamma-ray bursts (GRBs).

Two international teams of astronomers studying these events conclude that they likely arose from the catastrophic death of supergiant stars hundreds of times larger than the sun.

Swift Satellite

The astronomers discussed their findings Tuesday at the 2013 Huntsville Gamma-ray Burst Symposium in Nashville, Tenn., a meeting sponsored in part by the University of Alabama at Huntsville and NASA's Swift and Fermi Gamma-ray Space Telescope missions.

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.

Swift, Fermi and other spacecraft detect an average of about one GRB each day.

"We have seen thousands of gamma-ray bursts over the past four decades, but only now are we seeing a clear picture of just how extreme these extraordinary events can be," said Bruce Gendre, an ASI researcher.

Three recent GRBs (blue dots) emitted high-energy gamma-ray and X-ray light over time spans up to 100 times greater than typical long bursts and constitute a new ultra-long class.

This plot compares the energy received and the event duration among different classes of transient high-energy events: long GRBs (green); the disruption of an asteroid or comet by a neutron star or stellar-mass black hole in our own galaxy, or the break-out of a supernova shock wave in another galaxy (orange); and the tidal disruption of a star by a supermassive black hole in another galaxy (purple). 

Credit: NASA's Goddard Space Flight Center, after B. Gendre (ASDC/INAF-OAR/ARTEMIS)

Traditionally, astronomers have recognized two GRB types, short and long, based on the duration of the gamma-ray signal. Short bursts last two seconds or less and are thought to represent a merger of compact objects in a binary system, with the most likely suspects being neutron stars and black holes.

Long GRBs may last anywhere from several seconds to several minutes, with typical durations falling between 20 and 50 seconds.

These events are thought to be associated with the collapse of a star many times the sun's mass and the resulting birth of a new black hole.

Both scenarios give rise to powerful jets that propel matter at nearly the speed of light in opposite directions.

As they interact with matter in and around the star, the jets produce a spike of high-energy light.

Gendre 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.

The team's findings appear in the March 20 edition of The Astrophysical Journal.