A 33-Solar-Mass Star 29 Million Light-Years Away Has 'Erupted' 27 Times Since 2012, Like a Supernova About to Blow. Chandra Just Showed It Is Feeding a Dead Companion Instead.

For more than a decade, a star in the galaxy NGC 4559 has been behaving like a supernova that never quite happens. AT 2016blu, a luminous blue variable about 33 times the mass of the sun and roughly 29 million light-years away, has produced 27 outbursts since its discovery in 2012, each bright enough to trip the alarms that astronomers set for stars about to explode. New X-ray observations now show why it keeps crying wolf: the star is not tearing itself apart. It is being drained, on a schedule, by a dead companion.

The finding, led by Mojgan Aghakhanloo of the University of Virginia and accepted by The Astrophysical Journal, rests on timing. The outbursts recur roughly every 113 days, a pattern that earlier papers by the same group in 2023 and 2025 had suggested was the signature of an eccentric binary orbit, with the eruptions triggered each time the two objects swing closest to each other. That hypothesis made a prediction: the next flare would come in March 2026.

"We knew when the next outburst should be," Aghakhanloo said. Her team lined up ground-based telescopes and more than 30 amateur astronomers to watch, and filed a Target of Opportunity request with NASA's Chandra X-ray Observatory. When the outburst arrived on cue, Chandra broke from its schedule and pointed at the star. In five separate exposures during the March peak, AT 2016blu was shining in X-rays. Archival Chandra observations of the same patch of sky from 2001 and 2002, taken before the star began erupting, show nothing there at all.

X-rays are the calling card of accretion. When gas falls onto a compact object such as a neutron star or a black hole, it heats up violently and radiates at high energies. From the X-ray luminosity Chandra recorded, the team calculated the rate at which matter would have to be falling onto a companion to produce it, and found the numbers "consistent with the presence of a compact companion," as the paper puts it. "We therefore conclude that AT 2016blu is the first known case of an LBV SN impostor whose outbursts are driven by intermittent accretion onto a compact object," the authors write.

That makes AT 2016blu a high-mass X-ray binary, a class of systems in which a massive living star feeds a stellar corpse, and the first such system ever identified with a luminous blue variable as the donor. Each time the orbit brings the pair together, the companion skims material off the giant star's outer layers, the gas lights up, and observers on Earth see what looks like a pre-supernova eruption. "It's like having a front-row seat to seeing what the star is doing before it dies," Aghakhanloo said.

Luminous blue variables are among the most massive and unstable stars known, the kind that forge heavy elements and scatter them across their galaxies when they finally do explode. Their outbursts have long been assumed to come from within, driven by instabilities in the star itself. AT 2016blu suggests that at least some of the eruptions catalogued as supernova impostors are instead the work of an unseen partner, and the paper ties the system to a growing set of models in which massive-star-plus-remnant binaries explain a range of pre-supernova transients.

Aghakhanloo has been awarded more Chandra time to check other impostors for the same X-ray signature, and the Vera C. Rubin Observatory's 10-year survey is expected to turn up many more candidates. "We know a lot about them, but also there are a lot of open questions," she said. The star, meanwhile, will keep erupting on its 113-day clock, and this time the telescopes will be waiting.