Astronomers have discovered what is triggering repeated outbursts from a giant, unstable star using NASA’s Chandra X-ray Observatory, as described in our latest press release. This volatile star, known as AT 2016blu, is located in the spiral galaxy NGC 4559 that is about 37 million light-years from Earth.
For about 15 years, astronomers have watched as AT 2016blu has repeatedly grown extremely bright in optical light only to dim back down again. AT 2016blu is a type of luminous blue variable (LBV), which are hot massive stars characterized by changes in brightness (Eta Carinae is the most famous LBV).
This change in brightness was so dramatic that it rivaled the light output of a supernova explosion, which was ruled out as an explanation since the star repeated this behavior many times. (Some have referred to this star as a “supernova imposter”.) To solve this puzzle, a team of astronomers turned to Chandra and found that during an optical outburst in March 2026, the object also glowed brightly in X-rays.
This increase in X-ray brightness Chandra provided the answer: a previously hidden companion object keeps pulling material away from the massive star every 113 days as they orbit one another. This partner is either a stellar-mass black hole or a neutron star (the dense collapsed core of a massive star). When the star gets to its closest approach to the neutron star or black hole in their oval-shaped orbit, some of its material is pulled away and gets captured by the black hole or neutron star. The material then orbits at very high speeds close to the speed of light, getting so hot that it shines brightly in X-rays.
This composite image shows AT 2016blu in X-rays from Chandra (magenta) along with its host galaxy, NGC 4559, in optical light from the Sloan Digital Sky Survey (red, green, and blue). The colors of the optical image blend to appear white in many regions. The Chandra data from March 2026 show X-ray sources in the field, some of them associated with NGC 4559, and the Sloan data shows the galaxy and foreground and background objects. The LBV AT 2016blu is in the middle of the small square and is seen in both the Chandra and Sloan data. The much brighter X-ray source within the square is unrelated to AT 2016blu.
The Chandra data from March 2026 revealed that AT 2016blu was at least 100 times brighter in X-rays than in Chandra data from 2001 to 2002, which is before the outbursts began. AT 2016blu is bright enough in X-rays to be classified as a candidate ultra-luminous X-ray source (ULX). These are rare, bright X-ray sources found with telescopes like Chandra that astronomers have yet to fully understand. The current thinking is that some of them contain neutron stars, and others have black holes.
A paper describing these results, led by Mojgan Aghakhanloo (University of Virginia), is being published in The Astrophysical Journal on November 9, 2026; it is available at https://iopscience.iop.org/article/10.3847/1538-4357/ae9a86.
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.
This release features a composite image of a spiral galaxy with a giant, unstable star at its outer edge. Using Chandra, astronomers have discovered the star's previously hidden companion, which triggers luminous outbursts every 113 days.
The spiral galaxy is NGC 4559. Shown at a sharp angle in the composite image, its flat pink and milky blue face tilts toward our upper left. Sitting near the outer edge of the dense spiral, near the center of the frame, is a small group of pink and blue dots. This group is outlined with a white square, enlarged and annotated in supporting images.
The smallest pink dot in the square is AT 2016blu, a massive star some 33 times the size of our Sun. For over 15 years astronomers have been watching this star get bright and then dim again in optical light on a regular basis. Chandra's X-ray observations suggest that a hidden companion object, either a stellar mass black hole or a neutron star, is pulling material from AT 2016blu every 113 days as they orbit one another. This captured material gets so hot that it shines brightly in X-rays.
Other pink and blue specks within the white square and enlargement are unresolved stars or star clusters, too distant to be related to the movement of material between AT 2016blu and its hidden but influential neighbor.
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