Black Hole Caught Faking 27 Supernovas in a Nearby Galaxy

What did Chandra actually see?

The breakthrough came from timing. "To understand the cause of these eruptions, we realized that we needed X-ray data that was timed so that we'd observe the star when it was in an outburst," said team member Núria Torres-Albà, also of the University of Virginia.

The team got their chance in March 2026, directing Chandra at AT 2016blu during an outburst. Chandra had coincidentally observed the same galaxy in 2001 and 2002 and seen nothing unusual. This time it caught a powerful blast of X-rays arriving exactly alongside the fake supernova. "Until we were able to get Chandra data at the right time, we couldn't figure out what was making this happen," Aghakhanloo said.

The X-rays point to an unseen companion: a neutron star or stellar-mass black hole circling the LBV every 113 days. Each close pass lets the compact object's extreme gravity tear material from the unstable star. That stolen gas forms a whirling accretion disk — a disk of infalling matter — that heats until it radiates X-rays.

Those X-rays then slam into a dense cloud of gas the LBV had ejected years earlier. The cloud heats up and blazes in visible light. Voila: a supernova that isn't one. "We suspected that the bright repeating flashes at optical wavelengths might actually be powered by periodic blasts of high-energy X-rays," said team member Nathan Smith of the University of Arizona in Tucson. "The dense gas surrounding the system is heated by the X-rays and then glows in the optical."

How much material is being stolen?

The numbers are staggering, then immediately humbling:

• At minimum, 440 billion trillion pounds of material are stolen per year — possibly 100 times more.
• Even so, that's only 800 millionths of the sun's mass annually.
• The LBV itself tips the scales at no less than 33 solar masses.

That feeding rate far exceeds the Eddington limit — the theoretical maximum rate at which a neutron star can swallow material safely — which helps explain the extreme energies involved. Whether the companion is truly a black hole or a neutron star remains undetermined; the data can't yet distinguish the two.

What does this change for astronomy?

Possibly quite a bit. AT 2016blu is bright enough to count as an ultra-luminous X-ray source, or ULX — a rare class of object whose engine is still debated, though compact objects like black holes and neutron stars are leading candidates. Some researchers wonder whether AT 2016blu could be a Rosetta stone linking supernova impostors and ULXs.

It's too early to say. Only one other impostor, SN 2010da, shows similar X-ray emission from stolen material, and there the companion is a red supergiant — a puffy massive star, not a compact object. One object doesn't make a class. Caveat noted. But after 15 years of crying wolf, this impostor finally has a mechanism, and it's a hungry one.