Hidden Black Hole or Neutron Star Blamed for Star's Eruptions Every 113 Days

Astronomers have found what is behind a giant star's repeated, supernova-like flare-ups: a hidden companion that is either a black hole or a neutron star. According to NASA, the companion pulls material off the massive star every 113 days as the two orbit each other.

The finding, made with NASA's Chandra X-ray Observatory, was published Monday in The Astrophysical Journal. NASA says it is the first X-ray detection of this class of erupting star, and the first of these objects found with a black hole or neutron star partner.

A star that keeps surviving

The star, called AT 2016blu, sits in the spiral galaxy NGC 4559, about 30 million light-years from Earth. Astronomers estimate its mass at no less than 33 times that of the Sun. It belongs to a group known as luminous blue variables, which are hot, massive stars whose brightness changes. Eta Carinae is a well-known member of the group.

AT 2016blu is an especially restless example. Its brightness spikes are so extreme that they resemble supernova explosions, yet the star is not destroyed, which is why it is sometimes called a supernova impostor. Between 2012 and 2026, astronomers recorded at least 27 bursts of visible light from it, roughly every 113 days.

Mojgan Aghakhanloo of the University of Virginia, who led the study, said astronomers had watched the star brighten and fade on a regular schedule for 15 years without knowing the cause. Her colleague Núria Torres-Albà noted that the star kept acting like a supernova but could not be one, since it survives about three outbursts a year.

How Chandra solved the puzzle

The team suspected that the visible flashes were powered by bursts of high-energy X-rays. Nathan Smith of the University of Arizona explained that dense gas around the system is heated by the X-rays and then glows in visible light. To test the idea, the observations had to be timed to catch an outburst in progress.

Chandra was the right tool because, according to NASA, it is the only X-ray telescope sharp enough to separate AT 2016blu from another bright X-ray source nearby. Ground-based telescopes and the American Association of Variable Star Observers, including more than 30 amateur astronomers, tracked the star's brightness. When it flared in early 2026, a special Chandra program for unexpected events was triggered, and the observation took place in March 2026.

The data showed AT 2016blu was at least 100 times brighter in X-rays than in Chandra observations from 2001 to 2002, before the outbursts began. NASA says the best explanation is that the two objects follow an oval orbit. At their closest approach, material thrown off by the star is captured by the black hole or neutron star. That material circles at very high speeds, close to the speed of light, and gets hot enough to shine brightly in X-rays.

Why it matters for astronomers

The result may matter well beyond one star. AT 2016blu is bright enough in X-rays to count as a candidate ultra-luminous X-ray source, a rare class of objects that astronomers still do not fully understand. Current thinking is that some contain neutron stars and others contain black holes. Because many of these sources change sharply in X-ray brightness, the new observations suggest a possible link between them and violent stellar eruptions like those of AT 2016blu.

The discovery could also help researchers who study stars that change in brightness before they explode as supernovae. Astronomers suspect that much of this behavior may come from systems in which a massive star orbits a compact object or another massive star.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program, and the Smithsonian Astrophysical Observatory's Chandra X-ray Center runs its science operations from Cambridge, Massachusetts.

Read next
Webb Telescope Finds Dense Star Clusters That May Have Birthed Black Hole Seeds
Hunting Lone Black Holes Within 50 Light-Years of Earth, Hidden in Gas Clouds
Black Hole Jets Light Up Gas Far Beyond a Galaxy’s Stars, Study Finds

Around Prague, based on reporting by NASA