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The gamma-ray burst indicated an extragalactic outburst from a magnetar, a young neutron star with an exceptionally strong magnetic field.
In November 2023, the Integral space observatory of the European Space Agency accidentally noticed an unusual phenomenon. For just a tenth of a second, the nearby Cigar galaxy (M82) was illuminated by a short burst of gamma radiation. The analysis showed that for the first time scientists have observed a rare case of a giant magnetar flare outside the Milky Way.
Just a few seconds after the explosion was detected, the data was transmitted to Earth, and the programs automatically determined the location of the explosion. Astronomers aimed space and ground-based telescopes at the Cigar galaxy to determine the source of the radiation.
The X-ray telescope XMM-Newton, connected to the observation, showed only gas and stars. This ruled out the version about a possible collision of neutron stars: such an event could create a burst of gamma radiation, but should be accompanied by an afterglow in the X-ray spectrum and gravitational waves.
A bright burst of gamma radiation and no afterglow in the X-ray and optical spectrum. Image: ESA/Integral, ESA/XMM-Newton, INAF/TNG, M. Rigoselli (INAF), license details
Ground-based telescopes that studied the galaxy a few hours after the explosion also showed no anomalies. "Given the absence of a signal in X-rays and visible light, as well as the absence of gravitational waves measured by detectors on Earth, we are confident that the signal came from the magnetar," says Sandro Mereghetti, co-author of the study from the National Institute of Astrophysics in Milan.
When stars with a mass at least eight times that of the Sun die, they go supernova and leave behind a black hole or neutron star. Dense remnants resembling giant atomic nuclei rotate rapidly and generate powerful magnetic fields.
Magnetars are dead stars with extremely strong magnetic fields - 10 or more times greater than those of ordinary neutron stars. They are distinguished by sharp bursts of radiation, which in rare cases can reach gigantic sizes. For example, in December 2004, such a release of energy from a dead star located 30 light-years from Earth affected the upper layers of the Earth's atmosphere, similar to flares on the Sun.
The flare detected by "Integral" is the first confirmation of the existence of a magnetar outside the Milky Way, scientists note. And this discovery is a rare stroke of luck: the split-second event can only be caught if the observatory is looking in the right direction at the right time.
