Astronomers using NASA's Chandra X-ray Observatory have finally discovered what triggers the repeated outbursts of the giant star AT 2016blu, which has spent years pretending to explode like a supernova — without ever being destroyed. The answer is a hidden companion: a black hole or a neutron star that steals material from the star every 113 days.
A star that “dies” several times a year
The star, called AT 2016blu, lies in the spiral galaxy NGC 4559, about 30 million light-years from Earth, and has an estimated mass of at least 33 times that of the Sun. It is a luminous blue variable, a type of hot, massive star known for large changes in brightness. In the case of AT 2016blu, the brightness spikes are so extreme that they resemble supernova explosions — with one crucial difference: unlike a supernova, the star is not destroyed. That is why astronomers call it a “supernova impostor”.
Between 2012 and 2026, at least 27 bursts of optical light were detected from the star, roughly every 113 days. “For 15 years, astronomers have been watching this star get really bright and then dim again on a regular basis,” said Mojgan Aghakhanloo of the University of Virginia, who led the study. “Until we were able to get Chandra data at the right time, we couldn't figure out what was making this happen.”
The X-ray trap
To understand the cause of the eruptions, the team needed X-ray data timed to observe the star during an outburst. Chandra was chosen because it is the only X-ray telescope with vision sharp enough to distinguish AT 2016blu from another nearby bright X-ray source.
When optical observations pointed to an outburst in early 2026, Chandra was triggered in March 2026 under its Target of Opportunity program. The data showed that AT 2016blu was at least 100 times brighter in X-rays than in Chandra archival data from 2001 and 2002, before the outbursts began. It is the first X-ray detection of this class of erupting stars — and the first such object found with a black hole or neutron star companion.
A hidden companion
The best explanation, according to the study published on Monday (9) in The Astrophysical Journal, is that AT 2016blu orbits a compact object in an oval-shaped orbit. At each closest approach, every 113 days, the companion captures material ejected by the star. That material orbits at speeds close to the speed of light, gets so hot that it shines brightly in X-rays, and heats the dense gas around it, which then glows in optical light — producing the observed outburst.
Researchers still cannot say whether the companion is a stellar-mass black hole or a neutron star: the observations are consistent with both hypotheses, and that identity remains open. “This star's behavior was confusing at first because it kept acting like a supernova, but clearly it's not because it keeps surviving these outbursts about three times a year,” said co-author Núria Torres-Albà, also of the University of Virginia.
Why it matters
AT 2016blu is bright enough in X-rays to be classified as a candidate ultraluminous X-ray source, a rare class of sources that is still poorly understood. The new observations suggest there could be a link between these sources and the violent stellar eruptions seen in AT 2016blu, especially because many ultraluminous X-ray sources undergo large changes in X-ray brightness.
Geekiki has already covered another extreme neutron star: PSR J2053+6905, which spins almost 600 times per second.
The discovery may also help studies of stars that change in brightness before they explode as supernovae: astronomers suspect that much of this behavior is caused by systems containing a massive star orbiting a compact object or another massive star.
To collect the optical data, the team used the Katzman Automatic Imaging Telescope, the Las Cumbres Observatory Global Supernova Project, and observations from the American Association of Variable Star Observers, including more than 30 amateur astronomers who monitored the star's brightness.
Image credit: X-ray: NASA/CXC/Univ. of Virginia/M. Aghakhanloo et al.; Optical: SDSS; Image Processing: NASA/CXC/SAO/P. Edmonds, via Wikimedia Commons. The image shows star AT 2016blu in Chandra X-rays together with its host galaxy, NGC 4559, in SDSS optical light.
Sources
- NASA — NASA's Chandra Finds Source of Star's Mysterious Outbursts
- Phys.org — The supernova impostor: Chandra finds likely source of giant star's repeated outbursts
- Scientific paper — Aghakhanloo et al., The Astrophysical Journal (2026)
- Chandra Blog — The Supernova Impostor With a Hidden Partner
Transparency: this article was produced with the help of artificial intelligence, by cross-checking public posts on X and sources on the internet, and reviewed against the original sources cited below. We recommend consulting the original sources for more details.
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