The map on the left shows the sky before the gamma-ray burst; the one on the right shows it 30 minutes after. Image Credit: NASA/DOE/Fermi LAT Collaboration
We know a fair bit about black holes, those dense remnants of dead stars that lie in the centre of galaxies whose gravitational pull is so strong that even light can’t escape them. But for all we know, astronomers have never recorded one’s creation; scientists typically study the after effects of this cosmic process. Until now. It turns out that a bright flash observed on April 27 of this year was actually a black hole forming. Called GRB 130427A, it’s already being touted as a watershed moment for astronomy.
It might sound counterintuitive, but a star’s death is actually a dramatic and energetic event. As a star ages, the core that has provided its life-force for millions or billions of years runs out of fuel. This causes the star to collapse, and the act of imploding triggers a shockwave that blows up the star's outer layer. This is the bright supernova astronomers on Earth have seen from time to time for centuries. At the same time, the dying star shoots out a powerful blast of gamma-ray radiation.
While the star shoots out light and gamma-rays, its core shrinks and becomes steadily more dense. The increase in density has the effect of increasing the star’s core’s gravity to the point that nothing, not even light, can escape it. The border around that new black hole where the escape velocity is faster than the speed of light is called the event horizon. Whatever happens beyond that border we’ll never know.
The recent observation is the first time a star's death and a black hole's birth has ever been recorded. Never before have a gamma-ray burst and its associated supernova explosion been measured simultaneously. This confluence of observations is unprecedented, and it comes thanks to a barrage of orbiting satellites working in tandem with ground based telescopes.
This illustration of a gamma-ray burst shows a dying star forming a black hole, which drives a particle jet into space. Image Credit: NASA's Goddard Space Flight Center
NASA’s Fermi Gamma-ray Space Telescope and Swift Gamma-ray Burst Mission were the first to detect the gamma-ray burst. They sent out an alert to observatories on the ground, including where to look (the constellation Leo). The Rapid Telescopes for Optical Response (RAPTOR) Project telescopes in Los Alamos, New Mexico, were quick to respond. These telescopes, which scan the sky for optical events like flashes from distant stars, are also designed to react quickly to capture astronomical phenomena as they happen. They quickly honed in on the blast and imaged the optical light, an event so bright (a magnitude 7 event on the astronomical brightness scale) that it could have been seen through simple binoculars. This optical measurement was complemented by the gamma-ray information gathered by Fermi and Swift, as well as x-ray data gathered by NASA’s NuSTAR mission.
The blast was more powerful than anything astronomers have ever seen. Right as the optical blast peaked, Fermi’s Large Area Telescope detected the largest spike in GeV gamma-rays ever recorded in association with an astronomical event like this—so big, astronomers thought it was theoretically impossible. As the gamma-ray burst subsided, so too did the optical light, leaving behind an afterglow akin to the embers of a dying fire. It’s another feature of these cosmic events astronomers have never before witnessed.
It’s the array of data types—optical, x-ray, and gamma-ray information—that has scientists so excited. Though astronomers have yet to really dig into the data, preliminary results are already challenging assumptions about the universe. Like the sheer brightness of the blast, for example. That GRB 130427A was so much brighter and more powerful than what astronomers thought was possible is forcing scientists to rethink existing theories of radiation to allow for these kinds of events. It’s also challenging the long-held theory that the light from supernovas come exclusively from an internal shockwave. GRB 130427A suggests that there might be an external shockwave at play, too.
It’s too soon to say just how much this new data is going to change how we look at and understand the universe, but this is likely not the last we’ll hear about GRB 130427A.