Dead star lights up a neighboring galaxy

A curious signal from a nearby galaxy

In November 2023, ESA’s Comprehensive Space Observatory detected a sudden explosion, just a tenth of a second old. The data was received at the Integral Science Data Center in Geneva, which in turn sent a gamma-ray burst alert to astronomers around the world just 13 seconds after detection. The IBSA (Integral Burst Alert System) software provided a location that coincided with the nearby galaxy M82.

It was now up to astronomers to discover what had happened: Could it have been a common gamma ray burst or a rare giant magnetar explosion?

“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 nearby, bright galaxy,” explains Sandro Mereghetti of the National Institute of Astrophysics (INAF-IASF) in Milan, Italy, and lead author of a study on this discovery published in the journal Nature.

The team asked ESA’s XMM-Newton space telescope to carry out a follow-up observation of the explosion’s location as quickly as possible. If this had been a short gamma-ray burst caused by the collision of two neutron starsthe collision would have created gravitational waves and would have an afterglow in X-rays and visible light.

“The XMM-Newton observations showed only the hot gas and stars in the galaxy. If this explosion had been a short burst of gamma rays, we would have seen a weakened source of X-rays coming from the site, but this afterglow was not present,” adds study co-author Michela Rigoselli of INAF-IASF.

Astronomers then used ground-based optical telescopes, including the Italian Telescopio Nazionale Galileo and the French Observatoire de Haute-Provence to look for a signal in visible light a few hours after the explosion.

“But once again we found nothing. With no signal from X-rays and visible light and no gravitational waves measured by Earth’s detectors (LIGO/VIRGO/KAGRA), we are sure that the signal came from a magnetar”, concludes Sandro Mereghetti.

Magnetars: megamagnetic stars, recently dead

When stars with a mass greater than eight times that of the Sun die, they explode in a supernova which gives rise to a black hole or a neutron star.

To the neutron stars they are very compact stellar remnants, with more mass than the Sun, compacted into a sphere 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 stronger than those of typical neutron stars – they are called magnetars. They emit energy in explosions and, occasionally, these explosions are gigantic”, explains Ashley Chrimes, ESA researcher.

However, in the last 50 years of gamma-ray observations, only three giant magnetar explosions in our galaxy. What was it detected in December 2004, coming from 30,000 light years awaywas so powerful that it affected the upper layers of Earth’s atmosphere in the same way that they affect solar flares, which come from much closer.

“The first confirmation of a magnetar outside the Milky Way”

This recent explosion detected by ESA’s Integral “It is the first confirmation of a magnetar outside the Milky Way. We suspect that some of the other ‘short gamma-ray bursts’ that Integral and other satellites have revealed are also giant magnetar bursts.”says Sandro Mereghetti.

“This discovery expands our search for other extragalactic magnetars. If we can find more, we can begin to understand how often these explosions happen and how these stars lose energy in the process,” adds Ashley Chrimes.

Jan-Uwe Ness, ESA Integral Project scientist, explains that “explosions of such short duration can only be captured accidentally when an observatory is already pointing in the right direction. This is why Integraç is so important, as it has a large field of view, more than 3,000 times larger than the area of ​​the sky covered by the Moon.”

M82 is a bright galaxy where massive stars are born, live short, turbulent lives and, when they die, give rise to neutron stars. The discovery of a magnetar in this region confirms that the magnetars are likely young neutron stars.