Chandra finds a hidden companion behind a giant star's repeating outbursts

A massive star in a galaxy 30 million light-years away has been flaring up like clockwork for more than a decade, and NASA's Chandra X-ray Observatory has now found out why. The star has an unseen partner, either a black hole or a neutron star, that pulls material off it every 113 days. NASA announced the result on November 9, 2026; the study appears in The Astrophysical Journal.

A "supernova impostor" on a schedule

The star, called AT 2016blu, lies in the spiral galaxy NGC 4559 and has at least 33 times the mass of the Sun. It belongs to a rare class called luminous blue variables. These unstable giants can throw off huge outbursts that look a lot like supernovae, which is why some are nicknamed supernova impostors, but the star survives.

Between 2012 and 2026, observers recorded at least 27 optical outbursts from AT 2016blu, spaced roughly 113 days apart. That regular rhythm hinted at an orbiting companion, but nobody could see one.

Timing the X-ray look

The team, led by Mojgan Aghakhanloo of the University of Virginia, needed Chandra to look during an outburst. When the star flared in early 2026, Chandra observed it from March 24 to 27. It found the star at least 100 times brighter in X-rays than in Chandra images from 2001 and 2002, before the outbursts began. Chandra was the only X-ray telescope sharp enough to separate the star from a bright X-ray source next to it.

The explanation: each time the companion swings closest, it strips gas from the giant star. That gas heats up as it falls in and glows in X-rays. "Until we were able to get Chandra data at the right time, we couldn't figure out what was making this happen," Aghakhanloo said.

It is the first time a star of this type has been detected in X-rays, and the first found with a black hole or neutron star companion. AT 2016blu is bright enough in X-rays to count as a candidate ultraluminous X-ray source, suggesting a possible link between those mysterious sources and erupting giant stars.

Amateurs in the loop

The outburst record came partly from professional robotic telescopes and partly from the American Association of Variable Star Observers (AAVSO), including data from more than 30 amateur astronomers. It is a good reminder that careful backyard measurements of faraway stars can feed directly into front-line research. If you want to try it, our guide to choosing a telescope is a good place to start.

What comes next: with the 113-day cycle known, astronomers can plan future observations around the next outbursts to study how the companion feeds.