Chandra Finds Hidden Partner Behind Star’s 113-Day Outbursts
NASA's Chandra X-ray Observatory found a hidden black hole or neutron star behind the 113-day outbursts of the giant star AT 2016blu in NGC 4559.
A giant star about 30 million light-years away has a hidden companion that is either a black hole or a neutron star, according to NASA. The companion pulls material from the star every 113 days, and astronomers using the Chandra X-ray Observatory say this explains the star’s repeated flares. The study was published Monday in The Astrophysical Journal.
NASA said it is the first X-ray detection of this class of erupting star, and the first one found with a black hole or neutron star partner.
A star that flares like a supernova but survives
The star, called AT 2016blu, sits in the spiral galaxy NGC 4559. Astronomers estimate its mass at no less than 33 times that of the Sun. It belongs to a group called luminous blue variables, hot, massive stars whose brightness changes. Eta Carinae is a well-known example.
AT 2016blu brightens so sharply that the spikes resemble supernova explosions. Unlike a real supernova, 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.
“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 of the University of Virginia.
How the team caught it
The team, led by Mojgan Aghakhanloo of the University of Virginia, followed the star’s brightness with the Katzman Automatic Imaging Telescope, the Las Cumbres Observatory Global Supernova Project, and data from the American Association of Variable Star Observers, including more than 30 amateur astronomers. When those observations showed an outburst in early 2026, Chandra was triggered through its Target of Opportunity program for special events, and it observed the star in March 2026.
Chandra was needed because it is the only X-ray telescope sharp enough to separate AT 2016blu from another bright X-ray source nearby. Co-author Nathan Smith of the University of Arizona said the team suspected the optical flashes were powered by periodic bursts of high-energy X-rays. Dense gas around the system is heated by the X-rays and then glows in visible light, so the X-ray observation had to coincide with an optical outburst.
The March 2026 data showed the star at least 100 times brighter in X-rays than in Chandra data from 2001 to 2002, before the outbursts began. The team’s best explanation is that the two objects follow an oval orbit. At closest approach, material thrown off by the star is captured by the black hole or neutron star. NASA said that material orbits at very high speeds close to the speed of light and grows hot enough to shine in X-rays.
What is still unclear
The study does not say whether the companion is a black hole or a neutron star. NASA describes it as one or the other.
Why it matters for understanding bright X-ray sources
AT 2016blu is bright enough in X-rays to count as a candidate ultra-luminous X-ray source. These are rare, very bright sources that astronomers have yet to fully understand. Current thinking is that some contain neutron stars and others black holes. Many of them change a great deal in X-ray brightness, so the new observations suggest a possible link between these sources and violent stellar eruptions like those of AT 2016blu.
The finding may also help studies of stars that vary in brightness before they explode as supernovae. Astronomers suspect much of that behavior comes 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. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center runs science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.
Source: NASA
