A magnetar lights up the Cigar Galaxy

Using data from the European Space Agency (ESA) Integral satellite, built with the contribution of the Italian Space Agency (ASI), responsible for the main Ibis telescope, on 15 November 2023 a group of researchers led by the National Institute of astrophysics (Inaf) has identified the sudden explosion of a rare object: for just a tenth of a second, a rapid burst of gamma rays appeared from the direction of a bright galaxy near ours. What is it about? The team found out the presence of a giant flare (giant flarein English) generated by a magnetar in the Cigar Galaxy (also known by the acronyms M82 or NGC 3034), one of the most fascinating celestial objects that dot the sky. The article relating to the discovery was published today in the journal Nature.

A particular class of neutron stars (super-dense stellar remnants of supernova explosions), magnetars are the most powerful magnets in the universe, known to emit short bursts of gamma rays that typically last less than a second but are billions brighter than the Sun. Magnetars can produce giant flares, i.e. short explosions during which they can emit in less than a second the energy that the Sun radiates in a million years, but identifying them is really difficult.

The discovery was obtained thanks to the Integral Burst Alert System (Ibas), which allows the real-time localization of gamma-ray bursts and other transient phenomena in gamma rays. In particular, Integral detected a gamma-ray burst for only a tenth of a second. Ibas software, which examines data received at Integral’s science data center in Geneva, determined the precise location of this event and distributed it to astronomers around the world just thirteen seconds after Integral revealed it.

«When the automatic Ibas software alerted us to this event, we immediately realized that it was something special. It has long been suspected that some of the short-lived gamma-ray bursts (Grbs, bright gamma-ray bursts observed at a rate of one per day from unpredictable directions in the sky) could be giant flare coming from magnetars in nearby galaxies, but this had not yet been demonstrated unequivocally,” he explains Sandro Mereghetti, first author of the article and researcher at INAF in Milan. “Giant flares are the most extreme manifestation of magnetars, in terms of energy emitted and rapidity, but what produces them is not yet well known.” The one discovered by the team led by Inaf (Grb 231115A) is the first giant flare generated by a magnetar in a galaxy that does not belong to the Local Group.

«They are extremely rare events, so much so that only three have been observed in 50 years: two in our galaxy and one in the Large Magellanic Cloud. Being able to detect them even in more distant galaxies, as in the present case, allows us to study a greater number of them and in more favorable conditions”, underlines Mereghetti. «Previous cases of “candidates” giant flare outside the Local Group they had not been identified in real time and the uncertainties about their position also make associations with nearby galaxies uncertain.”

“Integral is a long-lived space telescope and 22 years after launch it continues to provide surprising contributions,” he recalls Elisabetta Cavazzuti, Asi manager of the Integral program. «The scientific team has increasingly improved the use of all the satellite’s equipment, developing software that exploits every single piece of information transmitted by the telescope in a completely new way. This way of observing and exploiting instruments from an ever-innovative perspective allows us to achieve important results, confirming that the universe is an inexhaustible source of discoveries.”

The detection of the phenomenon with Integral then started a series of rapid observations at other wavelengths (optical, X-ray, radio) which allowed us to establish its nature. In fact, in the article the researchers also present data requested from the Observatoire de Haute-Provence): If it was a gamma-ray burst caused by the collision of two neutron stars, the collision would have created gravitational waves and would have had an intense afterglow in X-rays and visible light. The Xmm-Newton observations showed only hot gas and stars in the galaxy.

The article published on Nature It therefore confirms a hypothesis that has been suspected for several years. «It is also no coincidence that this giant flare comes from one of the galaxies that is forming new high-mass stars at a high rate. In fact, in these regions we expect to find the greatest number of neutron stars and therefore magnetars”, he adds Ruben SalvaterraInaf researcher from Milan and co-author of the article.

Observable even with small telescopes, M82 is a galaxy starburst (in which the star formation process is exceptionally high) in a barred spiral which is located approximately 12 million light years from Earth, in the direction of the constellation Ursa Major. Gravitational interaction with other nearby galaxies, particularly M81, has dramatically accelerated its star formation rate which is at least ten times greater than that of the Milky Way.

«After this discovery, the M82 galaxy becomes a “special surveillance” from which we can expect other similar events in the coming years”, concludes Mereghetti.

To know more:

• Read on Nature the article “A magnetar giant flare in the nearby starburst galaxy M82”, by Sandro Mereghetti, Michela Rigoselli, Ruben Salvaterra, Dominik P. Pacholski, James C. Rodi, Diego Gotz, Edoardo Arrigoni, Paolo D’Avanzo, Christophe Adami, Angela Bazzano, Enrico Bozzo, Riccardo Brivio, Sergio Campana, Enrico Cappellaro, Jerome Chenevez, Fiore De Luise, Lorenzo Ducci, Paolo Esposito, Carlo Ferrigno, Matteo Ferro, Gian Luca Israel, Emeric Le Floc’h, Antonio Martin-Carrillo, Francesca Onori, Nanda Rea, Andrea Reguitti, Volodymyr Savchenko, Damya Souami, Leonardo Tartaglia, William Thuillot, Andrea Tiengo, Lina Tomasella, Martin Topinka, Damien Turpin and Pietro Ubertini