Mega burst of radiation from a magnetar

Magnetars have the strongest known magnetic fields in the universe. Artist’s impression © ESA

What flooded the nearby galaxy M82 with intense gamma radiation last November? Research by a team of astronomers has now led to an interesting answer: the strong burst of radiation can be traced back to a mega flare from an extremely magnetic neutron star – a magnetar. An investigation of the source area initiated immediately after the gamma ray flash made this conclusion possible, the researchers report.

For some astronomical research questions, a broader rather than a focused look into space is necessary: ​​the European Space Agency (ESA)’s INTEGRAL satellite is dedicated to the broad search for gamma ray sources in the universe. The space telescope’s spectrometer detects the high-energy photon radiation and then enables its cosmic origin to be localized. As the international research team led by Sandro Mereghetti from the National Institute of Astrophysics in Milan reports, INTEGRAL received a particularly exciting signal online on November 15, 2023: a strong gamma ray burst that only lasted a fraction of a second. The system then automatically informed the astronomers involved in INTEGRAL monitoring.

Thanks to automatic data processing, the origin of the signal could be quickly located: It therefore came from the galaxy M82, which is approximately twelve million light-years away from us. “It was immediately clear to us that this was a special case,” says Mereghetti. So he and his colleagues took additional measures to clarify the origin of the gamma ray burst. At the team’s request, additional astronomical tools were deployed to conduct follow-up observations of the event region as quickly as possible. One possibility was that the radiation was due to the collision of two neutron stars. In this case, gravitational waves would have been detectable and an afterglow in X-ray and visible light should have been visible in the area of ​​origin, the researchers explain.

Missing observations point to a special origin

But the follow-up observations showed nothing of the sort: the gravitational wave detectors LIGO, VIRGO and KAGRA did not record any signals and observations by ESA’s XMM-Newton X-ray space telescope also remained inconclusive. There was also nothing to record in the visible wavelength range: the ground-based optical telescopes that began searching for the signal just a few hours were unable to identify any afterglow in the M82 galaxy. But as the researchers explain, the lack of observations became an indication: a special cause of the gamma ray burst could be deduced. The most plausible explanation is that the signal does not come from a 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 strong gamma ray sources.

Co-author Volodymyr Savchenko from the University of Geneva explains: “Neutron stars can form after the supernova explosion of stars with more than eight times the mass of the sun. “They are very compact remnants of star material that rotate rapidly and develop very strong magnetic fields.” However, in some – probably young neutron stars – they are more than 10,000 times stronger than in typical neutron stars. These specimens are therefore called magnetars. They are also known to be particularly prone to eruptions: in so-called flares, they release enormous amounts of energy in the form of radiation.

Mega flare from an extragalactic magnetar

Three exceptionally strong magnetar outbursts have already been recorded in our galaxy. A gamma ray flash reached us in December 2004 from a distance of 30,000 light years. The photon radiation was so intense that it could affect the upper layers of the Earth’s atmosphere – similar to the radiation from relatively nearby solar flares. The gamma ray burst discovered by INTEGRAL now represents the first reliable confirmation of a magnetar flare outside the Milky Way, say the researchers.

What is also interesting is that M82 is a particularly bright galaxy in which intensive stellar development processes occur: massive stars are born there, which lead a short, turbulent life and can then leave neutron stars behind. “The track of the magnetar in this region now supports the assumption that these versions are probably young neutron stars,” says Savchenko.

Finally, the researchers once again emphasize the importance of the rapid transmission of information in their discovery: If the observations had taken place just one day later, the evidence of the particular 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 further evidence of magnetars in extragalactic star-forming regions with the help of INTEGRAL. This could show, for example, how frequently mega flares occur and to what extent the special neutron stars lose energy in the process.

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

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