- Scientists found a massive outburst of gamma rays from the galaxy M82, 12 million light-years away.
- The observation didn’t fit with typical gamma ray bursts, and astronomers found no afterglow in visible light to X-rays. Plus, they didn’t detect any gravity waves either.
- Now researchers have concluded the event originated from a magnetar in the galaxy M82. This is the first magnetar eruption we’ve ever seen in another galaxy! Magnetars are a type of neutron star, but with vastly more powerful magnetic fields.
Massive outburst of gamma rays in nearby galaxy
On November 15, 2023, the European INTEGRAL space observatory detected a massive burst of gamma rays coming from the nearby galaxy M82, which is 12 million light-years from Earth. The eruption lasted for only 1/10 of a second. What was it? A team of European astronomers said on April 24, 2024, that it originated from a giant magnetar eruption in that galaxy.
Magnetars are an exotic type of young neutron star with extremely powerful magnetic fields. The magnetic field is about 10,000 times stronger than a normal neutron star and about a trillion times stronger than Earth’s! Only three such giant flares have been identified in our own galaxy in the past 50 years. And now we have the first one confirmed from outside our galaxy.
The peer-reviewed discovery was published in the journal Nature on April 24, 2024. The paper is also available free as a preprint on arXiv.
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Giant gamma ray eruption brief but powerful
The huge outburst of gamma rays was intense but brief, lasting only 1/10 of a second. Lead author Sandro Mereghetti at the National Institute for Astrophysics (INAF–IASF) in Milan, Italy, said:
We immediately realized that this was a special alert. Gamma-ray bursts come from far away and anywhere in the sky, but this burst came from a bright nearby galaxy.
Incredibly, the Integral Burst Alert System (IBAS) automatically sent out the gamma ray burst notification to other astronomers only 13 seconds after INTEGRAL initially detected the eruption. Carlo Ferrigno, senior research associate at the University of Geneva (UNIGE) Faculty of Science, and co-author of the paper, said:
The satellite data were received in the INTEGRAL Science Data Centre (ISDC), based on the Ecogia site of the UNIGE Astronomy Department, from where a gamma-ray burst alert was sent out to astronomers worldwide, only 13 seconds after its detection.
Lack of afterglow and gravity waves
This allowed other astronomers to quickly follow up on the discovery. This included using the European Space Agency’s XMM-Newton space telescope. The telescope searched for an afterglow – in both visible light and X-rays – from the eruption but didn’t find one. This narrowed down the possible explanations. Sometimes neutron stars collide, creating a short gamma ray burst. But that would have created gravitational waves and left an afterglow.
The astronomers then also used the Telescopio Nazionale Galileo (TNG) in Italy, the Haute-Provence Observatory in France and other ground-based telescopes to search for an afterglow in visible light. Again, nothing. Instead, the telescopes only saw the usual hot gas and stars found in galaxies.
No gravitational waves were detected, either. If there had been any, the LIGO (U.S.), VIRGO (Italy) and KAGRA (Japan) gravity wave detectors should have picked them up.
Huge magnetar eruption the best explanation
There was another possibility that could explain the lack of an afterglow and gravity waves: magnetars. They are a type of neutron star, a very small dense stellar remnant of a massive star that exploded in a supernova. Neutron stars rotate quickly and have powerful magnetic fields. But the magnetic fields of magnetars are much more powerful than those of typical neutron stars, up to 10,000 times stronger. In fact, they are the strongest known magnetic fields of any objects in the universe. As the paper stated:
Its spectral properties, along with the length of the burst, the limits on its X-ray and optical counterparts obtained within a few hours, and the lack of a gravitational wave signal, unambiguously qualify this burst as a giant flare from a magnetar in M82.
FIrst magnetar eruption seen outside the Milky Way
Magnetars release energy in explosive flares. And on occasion those flares can be truly gigantic. They are not common, and astronomers have only seen three such eruptions during the past 50 years of observing gamma-ray bursts. Notably, all three came from magnetars within our own galaxy. One of those bursts, detected in 2004, originated on a magnetar 30,000 light-years from Earth. It was so powerful that it even had an effect on Earth’s upper atmosphere.
This new flare from galaxy M82 is similar to those previous detections. The big difference, however, is this magnetar is in another galaxy, 12 million light-years away. This also supports the theory that magnetars are young neutron stars, as Volodymyr Savchenko at the University of Geneva noted:
The discovery of a magnetar in this region confirms that magnetars are likely young neutron stars.
Astronomers will now continue to search for evidence of more magnetars outside our galaxy.
A serendipitous discovery made easier with INTEGRAL
It’s also noteworthy that this would have been a more serendipitous discovery if another gamma ray observatory had made the detection. INTEGRAL covers a lot of the night sky, an area about 3,000 times larger than the moon. But most observatories need to be pointed in the right direction at the right time to catch such short gamma ray bursts. INTEGRAL and XMM-Newton are also more flexible in their schedules. This also helps them to be able to detect gamma ray bursts or other anomalies and alert astronomers much faster.
Bottom line: The European INTEGRAL satellite detected a giant magnetar eruption of gamma rays. The rare, powerful event of this size is the first we’ve ever seen in another galaxy.
Source: A magnetar giant flare in the nearby starburst galaxy M82
Source (preprint): A magnetar giant flare in the nearby starburst galaxy M82
Via University of Geneva
Read more: Magnetars are the most powerful magnets in the universe
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