A massive star has produced at least 27 dramatic outbursts since 2012, brightening and fading instead of blowing itself apart. An X-ray observation timed to catch it in the act now points to a hidden partner: a black hole or neutron star taking material from it.
The star, AT 2016blu, sits in the galaxy NGC 4559. Its flare-ups arrive roughly every 113 days, giving astronomers something unusually useful when investigating a cosmic mystery: an approximate appointment.
A supernova impostor with a schedule
AT 2016blu belongs to a group of enormous, unstable stars called luminous blue variables. Researchers estimate it has at least 33 times the Sun’s mass. Its repeated eruptions earned it the description “supernova impostor,” a term for an outburst that resembles a stellar death but leaves the star alive.
The timing suggested a companion following an elongated orbit. Each close approach could disturb the star or capture some of its escaping material. The unresolved question was what kind of object was coming around.
Chandra caught the crucial flare
Optical telescopes tracked the system, with more than 30 amateur astronomers joining the monitoring effort. When it brightened in March 2026, the team triggered observations with NASA’s Chandra X-ray Observatory.
Chandra found the source at least 100 times brighter in X-rays than in observations from 2001 and 2002, before the recurring eruptions began. Its sharp vision also separated AT 2016blu from an unrelated bright X-ray source nearby.
In her account of the research, study leader Mojgan Aghakhanloo explains that the X-ray output is too strong to plausibly come from two ordinary stellar winds colliding. The compelling explanation is material falling toward a compact remnant, heating intensely as it gathers around a black hole or neutron star.
The visible fireworks may be secondhand
In the team’s proposed picture, those X-rays heat dense gas around the system. That gas then shines in visible light, producing the flare that optical telescopes notice.
The observations do not yet settle whether the companion is a black hole or a neutron star. They do turn a repeating spectacle into a testable physical picture: the surviving giant supplies the material, while its unseen neighbor helps light up the surroundings.
The findings were announced by NASA’s Chandra team on November 9, 2026, alongside a paper in The Astrophysical Journal.
Featured image: NGC 4559 in Chandra X-rays and Sloan Digital Sky Survey optical light. X-ray: NASA/CXC/University of Virginia/M. Aghakhanloo et al.; Optical: SDSS; Image processing: NASA/CXC/SAO/P. Edmonds. Official Chandra image, uploaded unchanged. Chandra material is available under its public-use policy; SDSS imagery is CC BY 4.0 under its image policy.
