Over many decades of observing gamma-ray bursts the universe has a coherent theory. All events were strictly divided into two categories: short bursts (no longer than two seconds) occurred as a result of neutron star mergers, and long bursts (from two seconds to six hours) were observed during the collapse of supernovae. Burst sources never crossed before, but a year ago it is happened: a long gamma-ray burst originated from a source clearly out of its category.
Image Source: Aaron M. Geller/Northwestern/CIERA and IT Research Computing Services
In December 2021, a team of astronomers led by scientists from Northwestern University (Chicago) discovered a long gamma-ray burst with a duration of 50 s. The event was numbered GRB211211A and was seen by the Fermi Space X-ray Telescope and the Neil Gerels Swift Earth Observatory Multispectral Telescope. Since long gamma-ray bursts were always recorded after a supernova explosion, an afterglow in all electromagnetic ranges remained at the site of the collapse of massive stars for a long time.
The prolonged afterglow, up to a week or more, made it possible to point optical, infrared, and even radio telescopes at the site of the explosion and collect as much data as possible about the event. But not at this time. When they looked there through the “optics”, they did not see anything at the site of the explosion. There were no supernova remnants. This meant that the long gamma-ray burst produced something different than the theory suggested.
Scientists were a little surprised and began to connect everything that was possible to the observation, since the object exploded relatively close – at a distance of 1.1 billion light years from the Earth and it was easy to observe it (apart from interference in the form of thick clouds, which greatly interfered with this particular work).
It should be noted that short gamma-ray bursts with a duration of less than two seconds have always been recorded in binary star systems in the case of a merger of two neutron stars, a neutron star with a black hole, or, which is still a pure theory, in the case of a merger of two black holes. The masses of objects (with the possible exception of the last case) were clearly not enough for a long-term release of energy in the gamma range. The merger and subsequent explosion of such objects since 2010 is commonly called kilonovasince up to 1000 times more energy is released than in a supernova explosion.
A detailed study of the GRB211211A event in other ranges showed that there is still an afterglow, but it is rather dim and bears signs of a kilonova explosion. In other words, a long gamma-ray burst was born under conditions that were not conducive to this and had never been observed before. This opening, at least, will force scientists to change their understanding of the nature of gamma-ray bursts in the Universe. New instruments like the upcoming X-ray telescope to launch next year can help. “Einstein probe”and “James Webb”, whose infrared sensors are excellent for finding kilonovae.
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