On April 27, a powerful flash of radiation erupted from deep space. The flash, a gamma-ray burst (GRB), was the brightest on record, challenging some of the leading theories on how the most powerful explosions in the known Universe occur.
Triggered by the sudden collapse of a dying massive star, GRBs are thought to be energized by the resulting black hole that forms in its wake. The black hole birthing drives relativistic particles through the collapsing star material, generating a shock wave, producing a highly collimated beam of gamma-ray radiation. GRBs are considered to be the more energetic cousins of supernovae, but for the first time, this particular GRB — called GRB 130427A — was seen to occur alongside a supernova; an unprecedented observation.
“We normally detect GRBs at great distance, meaning they usually appear quite faint. In this case the burst happened only a quarter of the way across the Universe meaning it was very bright. On this occasion, a powerful supernova was also produced, something we have not recorded before alongside a powerful GRB and we will now be seeking to understand this occurrence,” said Paul O’Brien, of the University of Leicester, who collaborated on one of the five papers devoted to GRB 130427A published in the journals Science and Astrophysical Journal Letters on Thursday (Nov. 21).
Another noteworthy factor of this event was the high number of observatories in space and on the ground that were able to slew in the direction of GRB 130427A just after it occurred. The initial discovery was made almost simultaneously by NASA’s orbiting Swift Gamma-Ray Burst Mission and Fermi Gamma-ray Space Telescope, then an alert was sent out to observatories on the ground such as the Rapid Telescopes for Optical Response (RAPTOR) project to watch the gamma-ray glow brighten.
Even NASA’s newest X-ray space observatory, the Nuclear Spectroscopic Telescope Array (NuSTAR), was able to get in on the act, recording hard X-ray data in the GRB’s afterglow.
“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,” said Paul Hertz, director of NASA’s Astrophysics Division in Washington.
Due to GRB 130427A’s relatively close proximity and the vast quantities of data collected from the multi-instrument campaign, it has become something of a GRB “Petri dish.”
“The rapid reaction of Swift has enabled us to discover many new and unexpected aspects of GRBs, the strong confirmation of the basic theory by this new very bright burst reassures us that we are on the right track in understanding these extraordinary explosions,” said Julian Osborne, Swift team leader at the University of Leicester.
Although astrophysicists will be picking through the data for some time to come, this event has already poked a couple of holes in our understanding of how GRBs work. For example, as highlighted in Fermi data, just as the optical light from the GRB peaked, there was an anomalous spike in highly energetic gamma-rays. The energies associated with this gamma-ray peak topped out at 95 GeV, the most powerful radiation ever seen from a GRB event.
“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,” said Sylvia Zhu, a Fermi team member at the University of Maryland in College Park.
To probe the very limits of astrophysics, sometimes you need violent events like GRB 130427A to let us know if we really are on the right track.
Image (top): These maps show the sky at energies above 100 MeV 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. Credit: NASA/DOE/Fermi LAT Collaboration