Gamma bursts: Researchers reveal a little of their secret

Two neutron stars with gamma ray burst

Two merging neutron stars emit a gamma-ray burst

Source: picture alliance / Photoshot

Until now, astronomers have assumed that there are two types of cosmic gamma-ray bursts – those that result from neutron star mergers or supernova explosions. However, not all gamma-ray flashes can be explained with this.

AOn December 11, 2021, the “Swift” satellite registered the NASA an intense shower of gamma rays from space. Such gamma flashes or “gamma ray bursts” are not uncommon. Satellite observatories register one of these events almost every day.

But the flash called “GRB 211211A” did not fit into the astronomers’ usual scheme: The duration of this gamma shower pointed to the explosion of a star as the cause, but other properties of the radiation rather to the merger of two neutron stars, as several teams of scientists now in the The journals “Nature” and “Nature Astronomy” report.

Astronomers distinguish between two types of gamma-ray bursts: Short ones last a maximum of two seconds and are hypothesized to occur when two neutron stars collide and merge.

Long gamma-ray bursts lasting more than two seconds and up to several minutes, on the other hand, are the result of supernovae, i.e. massive stars that collapse into a neutron star or black hole at the end of their lives, expelling their outer layers explosively into space.

This gamma burst was particularly bright

But sometimes cosmic gamma-ray bursts don’t fit into this simple breakdown—they’re long, but they don’t show any sign of a stellar explosion. So far, the scientists have not been able to make any real sense of it.

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“GRB 211211A” now provides astronomers with the most detailed view of such a mysterious event to date. Because the cause of this gamma-ray burst is about a billion light-years closer to Earth than most such events – which is why this flash also lit up extraordinarily brightly in the sky.

Several research teams were able to observe “GRB 211211A” with numerous other detectors and telescopes immediately after the discovery with Swift – and not only in the gamma radiation range, but also in the optical, ultraviolet and X-ray range.

White dwarf meets neutron star

In contrast to typical long gamma-ray bursts, which slowly fade out after bright lighting, “GRB 211211A” showed an extremely complex behavior. The actual flash lasted 13 seconds, followed by another, albeit weaker, flash lasting 55 seconds. High-energy gamma rays arrived again sixteen minutes later and lasted for more than five hours.

This variety of different components of the gamma-ray burst allowed a team led by Jun Yang from the University of Nanjing in China to develop a model for the cause of the event: Neither a supernova nor a collision of neutron stars, but the merger of a white dwarf star and a neutron star can explain all observations consistently, the scientists say.

In this scenario, the merger initially forms a rapidly rotating neutron star with a strong magnetic field. With such a so-called magnetar, the various components of the unusual gamma-ray burst could be explained, according to Yang and his research colleagues.

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However, not all researchers are convinced of this – in their opinion, a black hole is necessary as a collision partner for the white dwarf star, especially for the long afterglow. On the other hand, such a merger would probably only produce a short gamma burst.

The scientists have now taken a step further in explaining the strange gamma-ray bursts, but further observations of as many similar events as possible are necessary for a satisfactory explanation.

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