NASA finds a hidden companion behind a star’s repeated outbursts

NASA said astronomers using NASA’s Chandra X-ray Observatory discovered that a hidden companion is triggering repeated outbursts from the giant, unstable star AT 2016blu. The companion is either a stellar-mass black hole or a neutron star. It pulls material from the star every 113 days as the two orbit one another.

The study, published Monday in the latest issue of The Astrophysical Journal, places AT 2016blu in the spiral galaxy NGC 4559, about 30 million light-years from Earth. Astronomers estimate the star’s mass is at least 33 times that of the Sun. It is a luminous blue variable that undergoes rapid and repeated increases in brightness.

Between 2012 and 2026, astronomers detected at least 27 bursts of optical light from AT 2016blu roughly every 113 days. The spikes are so extreme that they resemble supernova explosions, but the star is not destroyed. It is also referred to as a supernova impostor.

Chandra data from March 2026 showed that AT 2016blu was at least 100 times brighter in X-rays than in data from 2001 to 2002, before the outbursts began. The best explanation is that material from the star falls onto a neutron star or black hole during the closest approach in their oval-shaped orbit. The material heats up and shines in X-rays.

According to Mojgan Aghakhanloo of the University of Virginia, who led the study, astronomers had watched the star brighten and dim regularly for 15 years. They could not determine the cause until Chandra observed it at the right time. Co-author Nathan Smith of the University of Arizona said the bright optical flashes may be powered by periodic blasts of high-energy X-rays that heat surrounding gas.

AT 2016blu is bright enough in X-rays to be classified as a candidate of an ultra-luminous X-ray source. The observations suggest a possible link between these rare sources and violent stellar eruptions. The team monitored the star with the Katzman Automatic Imaging Telescope, the Las Cumbres Observatory Global Supernova Project and the American Association of Variable Star Observers, including more than 30 amateur astronomers. Chandra observations were triggered in March 2026.