Rare: eruption of a mega magnetic star - what astronomers saw

Published by Adrien - Thursday, May 2, 2024 - Other Languages: FR, DE, ES, PT
Source: University of Geneva

While observing the sky, ESA's INTEGRAL satellite detected a burst of gamma rays - high-energy photons - originating from the nearby galaxy M82. A few hours later, ESA's XMM-Newton space X-ray telescope searched for remnants of this explosion but found none...

An international team, including scientists from the University of Geneva (UNIGE), concluded that it was an extragalactic eruption from a magnetar, a young neutron star with an exceptionally strong magnetic field. This discovery is published in the journal Nature.

Artist's impression of a magnetar. Magnetars are cosmic objects with the strongest magnetic fields ever measured in the Universe.

On November 15, 2023, the ESA's INTEGRAL satellite detected a sudden explosion from a rare object. For only a tenth of a second, a brief burst of energetic gamma rays appeared in the sky.

"The satellite data was received by the INTEGRAL scientific data center (ISDC), located at the Ecogia site of the UNIGE Astronomy Department, from where a gamma-ray burst alert was sent to astronomers worldwide, only 13 seconds after detection," explains Carlo Ferrigno, scientific collaborator at the Astronomy Department of the Faculty of Sciences at UNIGE, head of ISDC and co-author of the publication.

The IBAS (Integral Burst Alert System) software provided an automatic localization coinciding with the galaxy M82, at 12 million light-years. This alert system was developed and is operated by scientists and engineers from UNIGE in collaboration with counterparts abroad.

A curious signal from a neighboring galaxy

"We immediately understood that this was a special alert. Gamma-ray bursts can originate from very distant regions and from any part of the sky, but this burst came from a nearby and bright galaxy," explains Sandro Mereghetti from the National Institute of Astrophysics (INAF-IASF) of Milan, Italy, principal author of the publication and contributor to the IBAS.

The team immediately requested ESA's XMM-Newton space telescope to perform a follow-up observation of the burst location as soon as possible. If it had been a short-duration gamma-ray burst, caused by the collision of two neutron stars, the collision would have created gravitational waves and a lingering glow in X-rays and visible light.

However, XMM-Newton's observations only showed the hot gas and stars of the galaxy. Using ground-based optical telescopes, including the Italian Galileo telescope and the French Haute-Provence Observatory, the team also searched for a visible light signal, just hours after the explosion, but again, nothing was detected. In the absence of X-ray and visible light signals, and without gravitational waves measured by Earth-based detectors (LIGO/VIRGO/KAGRA), the most certain explanation is that the signal came from a magnetar.

Magnetars: mega magnetic stars, recently deceased

"When stars more massive than eight times the Sun die, they explode in a supernova and leave behind either a black hole or a neutron star. Neutron stars are very compact stellar remnants, with a mass greater than that of the Sun and are concentrated into a sphere the size of the canton of Geneva. They spin rapidly and have powerful magnetic fields," explains Volodymyr Savchenko, scientific assistant at the Astronomy Department of the Faculty of Sciences at UNIGE and co-author of the publication.

Some young neutron stars possess very strong magnetic fields, over 10,000 times greater than those of typical neutron stars. These stars are called "magnetars." They emit energy in the form of eruptions, sometimes gigantic ones.

However, in the past 50 years of gamma-ray observations, only three giant eruptions have been identified as coming from magnetars in our galaxy. These eruptions are very powerful: one of them, detected in December 2004, occurred 30,000 light-years from our Earth, but was powerful enough to affect the upper layers of the Earth's atmosphere, much like solar flares that originate from much closer regions.

The detection was sent to the INTEGRAL scientific data center at UNIGE, where software determined it came from the nearby galaxy M82. The small square on the INTEGRAL map indicates the location of the explosion. The blue circle on the two cropped images pinpoints the corresponding location.
© ESA/Integral, ESA/XMM-Newton, INAF/TNG, M. Rigoselli (INAF)

The eruption detected by INTEGRAL is the first solid confirmation of a magnetar eruption outside the Milky Way. M82 is a bright galaxy rich in stellar nurseries. Within these regions, massive stars are born, live turbulent short lives, and leave behind a neutron star. "The discovery of a magnetar in this region confirms that magnetars are likely young neutron stars," adds Volodymyr Savchenko.

The search for more magnetars will continue in other extragalactic star-forming regions, to better understand these extraordinary astronomical objects. If astronomers find many more, they can begin to comprehend the frequency of these eruptions and how neutron stars lose energy through this process.

INTEGRAL, a key instrument in a race against time

Eruptions of such brief duration can only be captured by chance, when an observational instrument is already pointed in the right direction. This is why INTEGRAL, with its field of view wider than 3,000 times the area of the sky covered by the Moon, is so important for these detections.

Carlo Ferrigno explains: "Our automatic data processing system is very reliable and allows us to immediately alert the community." When unexpected observations like this are detected, INTEGRAL and XMM-Newton can be flexible in their schedules, which is essential for discoveries where time is of the essence.

In this case, if the observations had been conducted even a day later, there would not have been as solid evidence that it was indeed a magnetar and not an ordinary gamma-ray burst.
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