NASA’s Fermi Telescope Captures First Clear Signal from the Engine of a Record-Bright Star Explosion

Photo credit: NASA’s Goddard Space Flight Center/Chris Smith (USRA/GESTAR)
Astronomers have tracked thousands of exploding stars over the years yet a handful stand out for their extreme brightness. Data from NASA’s Fermi Gamma-ray Space Telescope has now delivered a direct answer for one of those rare cases and changed how scientists view these events.

A supernova that shook the galaxy NGC 3191 unexpectedly in 2017, SN 2017egm, has now revealed some of its mysteries. This giant, located 440 million light years away in the direction of Ursa Major, exploded so dramatically that by July 1 of that year, it was brighter than all of the stars in its home galaxy combined. Initially, astronomers could only see the visible light it emitted, but it appears that there was much more going on in that explosion that was unseen to the naked eye.

Fermi’s Large Area Telescope detected gamma rays from the same patch of sky 43 to 155 days after the initial discovery, between July 5 and October 25 of that year. Prior to this, no one has discovered any evidence of gamma rays from a supernova of this magnitude. Researchers scoured Fermi’s data for further candidates, but all they could uncover were small traces of gamma radiation from other huge stars. Finally, it has been confirmed, and solidly so.

The investigation was led by Fabio Acero of the French National Centre for Scientific Research and his team. They’ve pieced together the facts and are confident that the explosion caused the development of a magnetar, a neutron star so young that it spins hundreds of times per second and has a magnetic field a thousand times stronger than any other neutron star. That’s what drives the steady stream of electrons and positrons that collide inside a massive cloud encircling the core, resulting in those gamma rays.

Initially, the thick shell of exploding material efficiently trapped all gamma rays and converted their energy to visible light. This is what gave SN 2017egm its remarkable brilliance. However, after roughly three months, the debris begins to cool and spread out, enabling some of the stored gamma rays to pass through. That’s exactly what the Fermi team detected as the first “leakage” of high-energy particles from the explosion.

To verify this hypothesis Indrek Vurm and Brian Metzger constructed computer models which accurately reproduced the timing and overall brightness of the explosion as picked up by optical equipment. Meanwhile, researchers at Louisiana State University contributed significantly. Michela Negro conducted the Fermi analysis, while Manos Chatzopoulos contributed crucial supernova experience. So, together, they were able to explain how valuable it is to be able to see straight into the heart of a supernova, rather than relying solely on the light reflected off it, as in a second-hand narrative.

Superluminous supernovae, which emit so much energy that they outshine a whole galaxy, are already uncommon, but those that allow us to see what’s causing the explosion are extremely rarer. In the past two decades, we’ve cataloged almost 400 of them. However, despite their energy production, the subject of what powers them has been debated. On the one hand, there was the magnetar theory, and on the other, the idea that the supernova interacts with gas emitted by the star years previously. The detection of gamma rays in this situation provides us with a much clearer picture of what is going on, helping to rule out some of the competing explanations.