Astronomers Solve the Mystery of ‘The Supernova Impostor’

For more than a decade, a massive star in a distant galaxy has been putting on a spectacular show, repeatedly erupting as if it were about to explode and die – an act that has earned it the nickname “supernova impostor.” Now, astronomers have discovered what may be driving those outbursts: a hidden companion, either a neutron star or a black hole, that’s pulling material away from the star.

The discovery, led by University of Virginia astronomer Mojgan Aghakhanloo and published in the Astrophysical Journal, provides the first X-ray detection of a system pairing a luminous blue variable — a rare, unstable type of massive star — with another, extremely dense stellar body. Aghakhanloo is a VITA-Origins Fellow at UVA, where she studies the life cycles and outbursts of the universe's most massive stars.

Catching Stellar Phenomena in the Act

The star, known as AT 2016blu, is at least 33 times more massive than the sun and lies about 29 million light-years away in the galaxy NGC 4559. Since 2012, AT 2016blu has erupted 27 times, with outbursts occurring roughly every 113 days. Earlier research by Aghakhanloo and her collaborators established the pattern and suggested that the star was part of a binary system but left them with many open questions about the cause of the phenomenon.

“We knew when the next outburst should be,” Aghakhanloo said, and that information made it possible to plan a highly coordinated observing campaign, enlisting the help of professional astronomers at observatories including the Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory in California, as well as the Las Cumbres Observatory's global network of telescopes, and more than 30 amateur astronomers around the world. The amateur observers, many using telescopes in their backyards, monitored AT 2016blu nightly and contributed their observations through the American Association of Variable Star Observers.

When the star reached the expected peak of its March 2026 outburst, the team triggered NASA’s Chandra X-ray Observatory, using a “target of opportunity” observation that allowed Chandra to interrupt its planned schedule to observe the erupting star. Chandra detected an exceptionally bright X-ray source at the location of AT 2016blu.

The X-rays provided the crucial evidence. Aghakhanloo found that the X-ray luminosity can be explained only if material is falling onto a nearby, compact object like the collapsed remnant of a star, either a neutron star or a black hole, making AT 2016blu’s violent outbursts a result of its interaction with a companion that has already died.

“It’s like having a front row seat to seeing what the star is doing before it dies,” Aghakhanloo said.

The finding could open a new window onto the lives of massive stars. Although astronomers know that massive stars play a critical role in producing the universe’s supply of elements such as iron and calcium, many questions remain about how they evolve and what happens before they die.

Taking the Next Steps

Future observations of supernova impostors could reveal whether other massive stars behave similarly, and Aghakhanloo has already secured additional Chandra time to observe another supernova impostor system, which will allow further observation of luminous blue variable related supernova impostors. Additionally, a 10-year survey by the Vera C. Rubin Observatory in Chile could help identify many more of these rare objects, providing a larger population for astronomers to study.

For Aghakhanloo, though, the goal is to understand what discoveries like this one could mean for the gaps in astronomers' understanding of massive stars. "We know a lot about them, but also there are a lot of open questions," she said. "We still don't fully understand how they evolve or how they die."

Discoveries of this kind are the result of the work of scientists who can recognize which of a nearly infinite number of objects in the night sky matter most, build a community of researchers around them, and catch them in the act of doing something extraordinary, explained Shazrene Mohamed, an associate professor of astronomy at UVA who specializes in computational stellar astrophysics.

“This kind of patient, coordinated work is exactly what we need to advance our understanding of the evolution of massive stars in binary systems,” Mohamed said.

Aghakhanloo's fellowship is supported by the Virginia Institute of Theoretical Astronomy (VITA), a center for research on theoretical problems in astronomy and astrophysics in UVA's Department of Astronomy.