How do gamma-ray bursts work?
That’s one question that astronomers are asking after our current understanding of these bright explosions is challenged by new observations.
These maps show the sky at energies roughly 500,000 times that of visible light as seen by Fermi’s LAT instrument. Left: The sky during a 3-hour interval before GRB 130427A. Right: A 3-hour map ending 30 minutes after the burst. GRB 130427A was located in the constellation Leo, near its border with Ursa Major.
It was the tell-tale blast of light of a star in its death throes in a far away galaxy – a gamma-ray burst designated GRB 130427A – that not only captured the attention of astronomers around the world due to its sheer brightness, one of the most luminous yet most energetic ever seen, but also because it has challenged our understanding of how these explosions really work.
Found in distant galaxies, gamma-ray bursts are one of the brightest events in our Universe, throwing out an intense beam of radiation from a rapidly rotating, high-mass star that has collapsed to form a hefty neutron star, quark star or black hole. This is followed by a gentler afterglow in X-ray, ultraviolet, optical, infrared, microwave and radio wavebands. They are thought to be the end point of a massive star running out of fuel, before collapsing under its own weight and creating an exotic black hole. Gamma rays are then driven, drilling through the devastated star before breaking free to race through the cosmos at almost the speed of light.
Packed full of energy, gamma rays can be found travelling all over the Universe; from the solar flares that are angrily thrown out by our Sun, to the hot matter that swirls around a newly born black hole. In these high gravity objects gamma rays are thought to exist, packing a punch some 500,000 times the energy of visible light thanks to internal shock waves resonating due to collisions in the jet. More energetic emissions of gamma rays are thought to happen when a jet slams into its surroundings, creating an external shock wave.
In the most common type of gamma-ray burst, illustrated here, a dying massive star forms a black hole (left), which drives a particle jet into space. Light across the spectrum arises from hot gas near the black hole, collisions within the jet, and from the jet’s interaction with its surroundings.
“We expect to see an event like this only once or twice a century, so we’re fortunate it happened when we had the appropriate collection of sensitive space telescopes with complementary capabilities available to see it,” says Paul Hertz of NASA’s Astrophysics Division in Washington, D.C.
It was the combined efforts of a trio of NASA satellites – the Fermi Gamma ray Space Telescope, the Swift Gamma-ray Burst Mission as well as the agency’s newest X-ray member, the Nuclear Spectroscopic Telescope Array (NuSTAR) – alongside telescopes operated by Los Alamos National Laboratory in New Mexico as part of the Rapid Telescopes for Optical Response (RAPTOR) Project, that caught the initial waves of gamma rays from what has been hailed as the second brightest flash ever detected from a gamma-ray burst, shortly after 3:47 am EDT on 27th April of this year. However, while this gamma-ray burst isn’t the brightest we have ever detected, it is the most energetic. And, this is where our current theories come unstuck.
“We thought the visible light for these flashes came from internal shocks, but this burst shows that it must come from the external shock, which produces the most energetic gamma rays,” says Fermi team member Sylvia Zhu at the University of Maryland.
Streaming radiation for a good 20 hours – far longer than any previous burst that we have ever witnessed – GRB 130427A is relatively nearby with its light travelling some 3.8 billion years before arriving at Earth. This is about one-third the travel time for light from your usual gamma-ray burst.
“Detailed observations by Swift and ground-based telescopes clearly show that GRB 130427A has properties more similar to typical distant bursts than to nearby ones,” Brera Observatory’s Gianpiero Tagliaferri, a Swift team member, says.
Images courtesy of NASA/DOE/Fermi LAT Collaboration (top) and NASA’s Goddard Space Flight Center (bottom)