24/04/2024
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While ESA’s Integral was observing the sky, it suddenly spotted a burst of gamma-rays coming from the nearby galaxy M82. Only a few hours later, ESA’s XMM-Newton searched for an afterglow from the explosion but found none. Astronomers realised that the burst must have been an extra-galactic flare from a magnetar, a young neutron star with an exceptionally strong magnetic field.
A curious signal from a nearby galaxy
In November 2023 ESA’s Integral spotted a sudden explosion from a rare object. For only a tenth of a second, a short burst of energetic gamma-rays appeared from the direction of the bright galaxy M82.
The satellite data were received in the Integral Science Data Centre in Geneva, from where a gamma-ray burst alert was sent out to astronomers worldwide, only 13 seconds after its detection. The IBAS (Integral Burst Alert System) software gave an automatic localisation coinciding with the nearby galaxy M82.
Now it was up to astronomers to figure out what had happened; was this one of the more common gamma-ray bursts or a rare occasion of a giant flare from a magnetar?
“We immediately realised 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,” explains Sandro Mereghetti of the National Institute for Astrophysics (INAF–IASF) in Milan, Italy, and lead author of a paper on this discovery.
The team requested ESA’s XMM-Newton space telescope to perform a follow-up observation of the burst’s location as soon as possible. If this had been a short gamma-ray burst, caused by two colliding neutron stars, the collision would have created gravitational waves and have an afterglow in X-rays and visible light.
“XMM-Newton’s observations only showed the hot gas and stars in the galaxy. If this explosion had been a short gamma-ray burst, we would have seen a fading source of X-rays coming from its location, but this afterglow was not present,” adds co-author Michela Rigoselli from INAF.
“Using ground-based optical telescopes, including the Italian Telescopio Nazionale Galileo and the French Observatoire de Haute-Provence, we looked for a signal in visible light, starting only a few hours after the explosion, but again we did not find anything. With no signal in X-rays and visible light, and no gravitational waves measured by detectors on Earth (LIGO/VIRGO/KAGRA), we are certain the signal came from a magnetar,” concludes Sandro.
Jan-Uwe Ness, ESA’s Integral Project Scientist explains: “When unexpected observations like this are picked up, Integral and XMM-Newton can be flexible in their schedules, which is essential in time-crucial discoveries. In this case, if the observations had been performed even just a day later, we would not have such strong proof that this was indeed a magnetar and not a gamma-ray burst.”
Magnetars: mega-magnetic, recently dead
When stars more massive than eight times the Sun die, they explode in a supernova that leaves a black hole or neutron star behind. Neutron stars are very compact stellar remnants with more than the mass of the Sun packed into a sphere with the size of a city. They rotate quickly and have strong magnetic fields.
“Some young neutron stars have extra strong magnetic fields, more than 10 000 times that of typical neutron stars. These are called magnetars. They emit energy away in flares, and occasionally these flares are gigantic,” clarifies Ashley Chrimes, Research Fellow at ESA.
However, in the past 50 years of gamma-ray observations, only three giant flares have been seen from magnetars in our galaxy. These outbursts are very strong: one that was detected in December 2004, came from 30 000 light-years from us but was still powerful enough to affect the upper layers of Earth’s atmosphere. Similar to how Solar flares, coming from much closer to us, influence it.
“The flare detected by Integral is the first confirmation of a magnetar outside of the Milky Way. We suspect that some of the other ‘short gamma-ray bursts’ Integral and other satellites have revealed are also giant flares from magnetars,” continues Sandro.
“This discovery opens our search for other extra-galactic magnetars. If we can find many more, we can start to understand how often these flares happen and how these stars lose energy in the process,” adds Ashley.
“However, outbursts of such short duration can only be captured serendipitously when an observatory is already pointing in the right direction. This makes Integral with its large field of view, more than 3000 times greater than the sky area covered by the Moon, so important for these detections,” emphasises Jan-Uwe.
M82 is a bright galaxy where star-formation takes place. In these regions massive stars are born, live short turbulent lives and leave behind a neutron star. The discovery of a magnetar in this region confirms that magnetars are likely young neutron stars. The search for more magnetars will continue in other star-forming regions, to understand these extraordinary astronomical objects.