May 19, 2024

Huge radiation explosion from a magnetar –

What bathed nearby galaxy M82 in intense gamma radiation last November? Research by a team of astronomers has now led to an interesting answer: the powerful burst of radiation can be traced back to the massive flare of a highly magnetic neutron star – a magnetar. Investigation of the source area that began immediately after the gamma-ray flash made this conclusion possible, the researchers reported.

For some astronomical research questions, it is necessary to take a broader view of space rather than a focused view: the European Space Agency's (ESA) INTEGRAL satellite is dedicated to the broad search for gamma-ray sources in the Universe. The space telescope's spectrograph detects high-energy photon radiation and thus allows its cosmic source to be determined. According to the international research team led by Sandro Mereghetti of the National Institute of Astrophysics in Milan, INTEGRAL received a particularly exciting signal online on November 15, 2023: a powerful gamma-ray burst that lasted only a fraction of a second. The system then automatically informs astronomers participating in INTEGRAL observations.

Thanks to automatic data processing, the location of the signal source could be quickly determined: it therefore came from the galaxy M82, which is about twelve million light-years away from us. “It was immediately clear to us that this was a special case,” Merghetti says. So he and his colleagues took additional measures to clarify the origin of the gamma-ray burst. At the team's request, additional astronomical instruments were deployed to conduct observations of the event area as quickly as possible. One possibility is that the radiation was due to the collision of two neutron stars. In this case, gravitational waves would have been detectable, and afterglows in X-rays and visible light should have been visible in the origin region, the researchers explained.

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The missing notes indicate a special origin

But subsequent observations showed nothing of the sort: the LIGO, VIRGO and KAGRA gravitational wave detectors recorded no signals, and observations by the European Space Agency's XMM-Newton X-ray space telescope remained inconclusive. There was also nothing to record in the visible wavelength range: ground-based optical telescopes that began searching for the signal after only a few hours were unable to identify any aurora in the galaxy M82. But as the researchers explained, the lack of observations became a clue: a special cause could be inferred for the gamma-ray burst. The most plausible explanation is that the signal comes not from the collision, but from the activity of a so-called magnetar. These celestial bodies are neutron stars with extremely strong magnetic fields, which in rare cases have already made themselves felt in the Milky Way as extremely powerful sources of gamma rays.

“Neutron stars can form after the supernova explosion of stars with a mass more than eight times the mass of the Sun,” explains co-author Volodymyr Savchenko from the University of Geneva. “It is a very compact remnant of rapidly rotating stellar material that forms very strong magnetic fields.” However, in some probably young neutron stars, they are more than 10,000 times more powerful than typical neutron stars. Therefore, these samples are called magnetars. They are also known to be particularly vulnerable to eruptions: in so-called volcanic flares, they release huge amounts of energy in the form of radiation.

A huge glow from an extragalactic magnetar

Three exceptionally powerful magnetic explosions have already been recorded in our galaxy. A gamma-ray flash reached us in December 2004 from a distance of 30 thousand light-years. The photon radiation was so intense that it could affect the upper layers of the Earth's atmosphere, similar to radiation from relatively nearby solar flares. The gamma-ray burst detected by INTEGRAL now represents the first reliable confirmation of the glow of a magnetar outside the Milky Way, researchers say.

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What's also interesting is that M82 is a particularly bright galaxy in which intense stellar evolution processes take place: massive stars are born there, which live short, turbulent lives and can then leave neutron stars behind. “The track of the magnetar in this region now supports the assumption that these emission are probably young neutron stars,” says Savchenko.

Finally, the researchers once again emphasize the importance of rapid information transmission in their discovery: if the observations had been made only one day later, the evidence of the specific source of the radiation would not have been clear. “Our automatic data processing system allows us to immediately alert the research community,” says co-author Carlo Ferrigno from the University of Geneva. The research team now hopes to find more evidence of the existence of magnetars in star-forming regions outside the galaxy with the help of INTEGRAL. This could show, for example, how frequently massive flares occur and to what extent special neutron stars lose energy in the process.

Source: University of Geneva, specialized article: Nature, 10.1038/s41586-024-07285-4