NASA’s Chandra finds star’s X-ray brightness rose at least 100-fold
Researchers say material falling onto a black hole or neutron star could explain AT 2016blu’s recurring outbursts during an orbit of roughly 113 days.
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Researchers say material falling onto a black hole or neutron star could explain AT 2016blu’s recurring outbursts during an orbit of roughly 113 days.
NASA’s Chandra X-ray Observatory found the star AT 2016blu at least 100 times brighter in X-rays in March 2026 than in observations from 2001 to 2002, before its first observed outburst in 2012. Researchers say the increase supports an explanation for the star’s repeated eruptions: a companion captures material from it as the two approach each other in an elongated orbit.
The companion is either a stellar-mass black hole or a neutron star, the dense collapsed core of a massive star. The observations do not determine which. NASA said on 9 November that this was the first X-ray detection of this class of erupting star and the first time one had been found with such a companion.
AT 2016blu is a luminous blue variable: a hot, massive star whose brightness changes. Its eruptions resemble supernova explosions, but the star survives them, earning it the name “supernova impostor.” Astronomers estimate its mass at least 33 times that of the Sun. It lies in the spiral galaxy NGC 4559, about 30 million light-years from Earth.
Astronomers detected at least 27 outbursts in visible light between 2012 and 2026, roughly every 113 days. Earlier research had established the pattern and suggested that the star had a companion, but had not resolved what caused the eruptions.
In the team’s proposed explanation, the companion captures material ejected by the star near their closest approach. The captured material heats up and emits X-rays, which heat surrounding gas until it glows in visible light. This sequence is the researchers' interpretation, not a directly observed transfer of material.
The team timed the Chandra observations using visible-light monitoring by the Katzman Automatic Imaging Telescope, the Las Cumbres Observatory Global Supernova Project and contributors to the American Association of Variable Star Observers, including more than 30 amateur astronomers. When monitoring showed an outburst in early 2026, the team triggered the Chandra observations. NASA said Chandra was the only X-ray telescope able to distinguish the star from a nearby bright X-ray source.
Mojgan Aghakhanloo of the University of Virginia led the study, published in The Astrophysical Journal. Its co-authors include Núria Torres-Albà, also of the University of Virginia, and Nathan Smith of the University of Arizona.
Analysis
NASA said AT 2016blu is a candidate ultraluminous X-ray source, a type of unusually bright source that astronomers do not yet fully understand. The observations suggest a possible link between such sources and stellar eruptions. NASA also said the system may help studies of stars that vary in brightness before a supernova.
Aghakhanloo has secured additional Chandra observing time for another supernova impostor system, allowing researchers to investigate whether other massive stars behave similarly.
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