AT 2016blu: NASA’s Chandra finds what causes a giant star’s outbursts every 113 days

A giant star in a galaxy 30 million light-years away is no longer a mystery. Astronomers used NASA’s Chandra X-ray space telescope and identified what causes the repeating outbursts of the star AT 2016blu in the spiral galaxy NGC 4559: a so-far hidden object that strips matter from it every 113 days, as the two bodies orbit one another. NASA announced it on Monday, November 9, 2026, in its release on the discovery.

The star’s companion is either a stellar-mass black hole or a neutron star, the dense shrunken core of a massive star that collapsed. NASA notes this is the first X-ray detection of a star of this class and the first such object found with a black hole or neutron star companion. The study was published in The Astrophysical Journal and is available on the journal’s website.

AT 2016blu is a luminous blue variable, a hot, massive star with strong changes in brightness. The best-known example of the class is Eta Carinae. This particular star stands out for the rapid, repeated surges in its luminosity, so extreme that they resemble a supernova explosion. In such an explosion a massive star collapses on itself and its matter is thrown into space. Here, however, the star is not destroyed, which is why AT 2016blu is also called a “supernova impostor.” Its mass is estimated at no less than 33 times the mass of the Sun.

The key was timing

From 2012 to 2026 astronomers recorded at least 27 outbursts in visible light, about every 113 days. “For 15 years astronomers watched this star brighten intensely and then fade again, at regular intervals,” said Mojgan Aghakhanloo of the University of Virginia, who led the study. “Until we managed to get Chandra data at the right time, we couldn’t understand what was causing it.” Co-author Núria Torres-Albà, also of the University of Virginia, noted that the star’s behaviour initially puzzled researchers because it resembled a supernova, but it clearly is not, since the star survives the explosions about three times a year.

Nathan Smith of the University of Arizona explained that the team suspected the repeating flashes in visible light were actually powered by periodic bursts of high-energy X-rays: the dense gas around the system is heated by the X-rays and then glows in visible light. That is why Chandra’s observation had to be made at the same time as an outburst. The team monitored the star’s brightness with the Katzman Automatic Imaging Telescope (KAIT), the Las Cumbres Observatory Global Supernova Project and observations from the American Association of Variable Star Observers (AAVSO), to which more than 30 amateur astronomers contributed. When the star flared in early 2026, Chandra’s Target of Opportunity programme was triggered and the measurements were made in March 2026.

Chandra was essential because it is the only X-ray telescope with sharp enough “vision” to separate AT 2016blu from another, very close bright X-ray source. The March 2026 data showed that AT 2016blu was at least 100 times brighter in X-rays than in Chandra’s observations in 2001 and 2002, before the outbursts began. The most likely explanation is that, when the two bodies come closest in their elliptical orbit, the matter ejected by the star is captured by the black hole or neutron star. That matter circles at speeds close to the speed of light and heats up so much that it shines intensely in X-rays.

Link to ultra-luminous X-ray sources

The system is bright enough in X-rays to qualify as a candidate ultra-luminous X-ray source (ULX). These sources are rare and very bright, and astronomers do not yet fully understand them: the prevailing view is that some contain neutron stars and others black holes. The new observations suggest a possible link between ULXs and violent stellar eruptions like those of AT 2016blu, especially since many ULXs show large variations in their X-ray brightness. The discovery may also help in studying stars that change brightness before exploding as supernovae, since astronomers suspect much of this behaviour is due to systems where a massive star orbits a compact object or another massive star.

The Chandra programme is managed by NASA’s Marshall Space Flight Center in Huntsville, Alabama. The Chandra X-ray Center of the Smithsonian Astrophysical Observatory controls science operations from Cambridge and flight operations from Burlington, Massachusetts. For another NASA space telescope, see the Swift telescope, which ended its mission after more than 2,000 gamma-ray bursts.