WORLDTECHChandra Traces Repeated Outbursts of Star AT 2016blu to Hidden Companion
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Astronomers using NASA's Chandra X-ray Observatory say they have identified the force behind repeated outbursts from a giant, unstable star. The system is the first of its kind found to include a black hole or neutron star as a companion.
A hidden partner draws material away
A study published Monday in the latest issue of The Astrophysical Journal describes a previously concealed companion object that keeps pulling material from the massive star. The two bodies circle each other every 113 days.
The companion is either a stellar-mass black hole or a neutron star, the dense remnant left when a massive star's core collapses. The team reports that this is the first X-ray detection of a star of this erupting class.
A star that mimics a supernova
The star, designated AT 2016blu, belongs to the luminous blue variables, hot massive stars whose brightness shifts sharply. Its brightness spikes are so extreme that they resemble supernova blasts, yet the star survives them, which is why it is termed a supernova impostor. It sits in the spiral galaxy NGC 4559, roughly 30 million light-years from Earth.
Astronomers put the star's mass at no less than 33 times that of the Sun. Mojgan Aghakhanloo of the University of Virginia in Charlottesville, who led the study, said astronomers had watched the star brighten and fade regularly for 15 years. Co-author Núria Torres-Albà, also of the University of Virginia, noted that the star keeps enduring these outbursts about three times a year, which is why the supernova label does not fit.
Chandra's X-ray vision made the difference
The team relied on Chandra because it is the only X-ray telescope able to resolve AT 2016blu apart from another bright X-ray source close by. Chandra data from March 2026 showed the star was at least 100 times brighter in X-rays than in Chandra observations from 2001 to 2002, before the outbursts began.
When the two bodies approach one another, the compact companion captures material the star has ejected. That material swirls at speeds approaching that of light and heats up enough to glow strongly in X-rays. The system is bright enough in X-rays to be classed as a candidate ultra-luminous X-ray source, a rare category astronomers have yet to fully understand.
Possible link to ultra-luminous X-ray sources
Scientists currently believe some ultra-luminous X-ray sources harbour neutron stars and others black holes. The new observations hint at a possible connection between these sources and violent stellar eruptions such as those of AT 2016blu, particularly since many ultra-luminous X-ray sources show large swings in X-ray brightness. Astronomers suspect much of this behaviour may arise from systems in which a massive star orbits a compact object or another massive star.
Optical data came from the Katzman Automatic Imaging Telescope, the Las Cumbres Observatory Global Supernova Project and the American Association of Variable Star Observers, whose monitoring included more than 30 amateur astronomers tracking the star's brightness.
Open questions
- Is the companion a black hole or a neutron star?