On 2 July 1967, American Vela satellites, launched to watch for secret nuclear weapons tests, picked up a flash of gamma rays. It did not come from Earth. More followed, and in 1973 the team published 16 of them: brief, violent bursts arriving from random directions in space. For more than two decades nobody knew what they were, or even whether they came from inside our galaxy.
Two kinds
Today satellites detect about one a day. They fall into two groups. About 70 percent last longer than two seconds, sometimes for minutes. These long bursts come from the collapse of massive stars into black holes, the same events that can produce a hypernova. The other 30 percent are over in under two seconds, often in a third of a second. Most of these come from two neutron stars colliding.
The breakthrough came in February 1997, when the Italian-Dutch BeppoSAX satellite caught the fading afterglow of a burst in X-rays, and telescopes found it in visible light about 20 hours later. That made it possible to measure distances, and the answer was astonishing: billions of light-years.
The energy
To be seen from so far away, a gamma-ray burst must release about 3 × 10⁴⁴ joules in a few seconds. That is as much as the Sun will give off in its entire life of about ten billion years. The energy is not sprayed in every direction but focused into two narrow jets, between about 2 and 20 degrees wide, so we only see the bursts whose beams happen to point at us.
On 19 March 2008 the afterglow of GRB 080319B reached magnitude 5.8, just visible to the naked eye under a dark sky. It had travelled 7.5 billion years to get here.
The BOAT
On 9 October 2022 the Swift and Fermi satellites recorded GRB 221009A, quickly nicknamed the BOAT: the Brightest Of All Time. It was 50 to 70 times brighter than the previous record holder, from about 2.4 billion light-years away in the direction of Sagitta. The LHAASO observatory in China caught more than 5 000 very-high-energy photons from it, some carrying a record 18 trillion electronvolts. Even at that distance it disturbed Earth’s upper atmosphere for several hours. A burst this bright and this close is estimated to happen about once every 10 000 years.
Could one hit Earth?
A burst inside the Milky Way, within about 5 000 to 8 000 light-years and aimed at us, could do real harm by stripping away the ozone layer. Some researchers have suggested that one may have contributed to the Late Ordovician mass extinction about 450 million years ago. It is a hypothesis, not an established cause.