💥 When a superpowerful supernova reveals a magnetar

An international team analyzed data from NASA's Fermi space telescope and detected gamma rays from a rare and exceptionally bright supernova. According to scientists, the luminosity of this explosion was amplified by the birth of a super-magnetized neutron star, called a magnetar, resulting from the collapse of the star that caused the supernova.

The results of this research were published on May 20 in the journal Astronomy & Astrophysics.

Extraordinary stellar explosions

Core-collapse supernovas occur when a star, much more massive than our Sun, exhausts its fuel and collapses in on itself before exploding. This collapse can give birth to a city-sized neutron star, or even a smaller black hole. The shockwave then propels the remainder of the star into space, forming a hot, dense, and ionized gas cloud that expands rapidly.

Over the past two decades, nearly 400 exceptional supernovas of this type, known as superluminous supernovas, have been identified. Each of them produced at least ten times more visible light than a classic supernova.

In 2024, a study led by Li Shang from Anhui University in Hefei, China, revealed that the Fermi telescope had detected gamma rays emitted by a superluminous supernova that occurred years earlier. Dubbed SN 2017egm, this superpowerful explosion took place in the galaxy NGC 3191, located about 440 million light-years from Earth, in the constellation Ursa Major. Despite this distance, it remains one of the closest of its kind ever observed.

The scientists searched for gamma rays emitted by the six closest superluminous supernovas detected during the first 16 years of the Fermi mission. Only SN 2017egm shows traces of gamma rays, confirming that some supernovas can be as luminous in gamma rays as in visible light. This opens a new avenue for studying these phenomena.

The role of magnetars

Scientists have long debated the energy sources that can make these explosions so exceptionally powerful. Among the hypotheses, the formation of a magnetar — a neutron star with the most intense magnetic fields known, up to 1,000 times stronger than those of ordinary neutron stars — tops the list. To give an idea, that is 10,000 billion times the strength of a refrigerator magnet!

The team analyzed the optical and gamma-ray characteristics of the supernova in depth to compare different theoretical models. A model developed by co-authors Indrek Vurm (University of Tartu, Estonia) and Brian Metzger (Columbia University, New York) simulated how light and particles produced by a newly formed magnetar interact with the expanding debris of the supernova.

A complex mechanism

A freshly formed magnetar spins on its axis more than 100 times per second. This rapid rotation generates an intense stream of electrons and positrons (their antimatter counterparts), forming a vast cloud of energetic particles. In this cloud, called a pulsar wind nebula (or magnetar wind nebula in this case), various interactions drive the production and absorption of gamma rays, the most energetic form of light. Thus, gamma rays interact with the supernova debris. Unable to escape directly, they are converted into lower-energy visible light, which boosts the supernova's luminosity.

About three months after the collapse, as the supernova debris expands and cools, gamma rays begin to escape. The magnetar model best reproduces the supernova's luminosity and the timing of its gamma-ray arrival during the first few months, but improvements are needed to explain later phases, when visible light fades irregularly.

Additional processes

The scientists suggest that other mechanisms likely played a role in the prolonged decline of SN 2017egm, such as debris falling onto the magnetar or interactions between the shockwave and matter ejected by the star centuries before its death.

The team also assessed the capability of a new ground-based gamma-ray observatory, the Cherenkov Telescope Array Observatory (CTAO), to detect events similar to SN 2017egm. With about 50 hours of observation, such a phenomenon could be spotted up to 500 million light-years away and would open new perspectives for studying the role of magnetars in energetic events in the universe.

The CTAO telescope network is under construction with two sites: one on the island of La Palma in the Canary Islands and one in Chile's Atacama Desert. The CNRS is a major player in this consortium.