Long but short burst of gamma rays

It took no more than fifty seconds to turn everything upside down. comet? GRB 211211A, a high-energy burst of light that occurred roughly a billion light-years away, is relatively close for such events. It was discovered on December 11, 2021 by the space telescope SwiftBy virtue of its duration, this electromagnetic burst falls into the category of long gamma-ray bursts – as opposed to short bursts lasting less than two seconds. After its discovery, Eleonora Troja, of the University of Rome Tor Vergata, and her colleagues, as well as other teams, performed in-depth analyzes of the gamma-ray burst. And its result is unexpected, to say the least: GRB 211211A may be long, but it has all the characteristics of a short burst!

When a black hole forms, very narrow and very fast jets of matter are emitted along its axis of rotation. “Once these jets are at a good distance from the black hole, their energy is transformed into short, very intense radiation. This is an actual gamma-ray burst,” explains Frédéric Daigne, a researcher at the Astrophysical Institute in Paris. What makes a gamma-ray burst longer or shorter depends On the event that led to the formation of the black hole. If the black hole was caused by the explosion of a huge supernova star, then the gamma ray burst is long. On the other hand, if it was formed after the merger of two neutron stars, then the eruption is short. » Since GRB 211211A is a long burst, it is expected to come from the collapse of a massive star. But everything indicates that, nevertheless, as a result of the merger of two neutron stars …

The first evidence of this comes from the site of the explosion, which was detected on the edge of a galaxy. However, massive stars only live a few million years, which is a very short period on a galactic scale. Therefore, we expect to find such objects in star-forming regions within galaxies. Conversely, pairs of neutron stars can live for hundreds of millions of years before merging, thus having time to migrate out of their host galaxy.

The spectrum of GRB 211211A’s gamma-ray burst most clearly bears witness to its origin. There is an increase in luminosity in the infrared typical of a so-called “kilonova”. At the time of the merger of neutron stars, in addition to those that will form the black hole and the accretion disk, some of the matter is instantly ejected. “There are a lot of heavy nuclei and very hot neutrons that are ejected at a very high speed,” explains Frédéric Diagne. These conditions lead to a process of rapid capture of neutrons by the nuclei, and the formation of very heavy elements. » Radiation, associated with the radioactivity of elements that have just been synthesized, is most intense in the infrared: it’s a kilonova. “So this is a very clear signature of neutron star mergers.”

But then, how do we explain the explosion of GRB 211211A? Jun Yang of Nanjing University, China, and colleagues hypothesized that before a black hole was formed, neutron star mergers gave rise to a magnetar (a neutron star with a very intense magnetic field), allowing for a longer jet emission, and thus a longer gamma-ray burst. “But it is too early to conclude, and other hypotheses will be proposed in the coming months,” stresses Frédéric Diagne. Its development is sure to lead to a leap in our understanding of binary systems.