In the spiral galaxy NGC 4559, at a distance of about 30 million light-years from Earth, a cosmic spectacle has been playing out for decades that baffled astronomers for a long time. The massive star AT 2016blu — a hot giant with a mass of at least 33 solar masses — flared again and again so brightly that its bursts of light resembled a supernova explosion. But the star did not destroy itself. From 2012 to 2026, scientists recorded at least 27 such outbursts, recurring roughly every 113 days. It was called a “supernova impostor” — a luminous blue variable, an unstable blue variable that now blazed up, now faded, yet continued to exist.
For a long time the cause of these regular explosions remained a mystery. Optical observations showed only sharp jumps in brightness. To understand the mechanism, a different wavelength was needed — X-ray emission. And that is where the Chandra observatory entered the picture. A team led by Mojgan Aghakhanloo of the University of Virginia carefully planned the observations. The star was tracked with the KAIT telescopes, the Las Cumbres Observatory network, and even more than 30 amateur astronomers from the AAVSO. When in March 2026 AT 2016blu began to brighten again, Chandra was pointed at it in “Target of Opportunity” mode.
The data obtained on 24–27 March 2026 proved decisive. In the X-ray band the star became at least 100 times brighter than in the archival Chandra observations of 2001–2002, made before the known outbursts began. The X-ray emission was powerful enough to classify the source as a candidate ultraluminous X-ray source. Such behavior is best explained by the presence of a hidden companion — a neutron star or a stellar-mass black hole.
The two objects orbit along an elongated, elliptical path. Every 113 days they draw close. At those moments the companion begins actively capturing material ejected by the giant star. The gas falls onto the compact object, accelerates almost to the speed of light, heats up to monstrous temperatures, and flares in X-rays. That radiation, in turn, heats the surrounding material, which then shines brightly in the optical band — precisely the outbursts that were observed for years.
The results of the study were published on 9 November 2026 in The Astrophysical Journal. The work of Aghakhanloo and colleagues was the first X-ray detection of a star of this class and the first case in which a compact companion was reliably identified for a luminous blue variable with regular outbursts. The system turned out to be a massive X-ray binary — a rare and valuable object for study.
This discovery goes far beyond a single star. It helps us understand how massive stars behave before genuine supernova explosions and sheds light on the nature of ultraluminous X-ray sources. On the scale of the Universe, where millions of light-years separate us from this galaxy, two objects — an enormous unstable star and its invisible, dense partner — create a precise, almost mechanical rhythm of outbursts. Matter, temperature, gravity, and orbital motion combine into a single process that repeats again and again.
Looking at such systems, one feels especially keenly how complex and at the same time harmonious the Universe is. There are no random decorations in it: even at a distance from which light traveled to us for tens of millions of years, the same laws continue to operate, linking giant stars with the compact remnants of their brethren. AT 2016blu and its hidden companion are one more reminder of how many grand, almost invisible dramas are unfolding in space right now.
