Chandra Catches Hidden Black Hole Fueling 'Supernova Impostor'

The team had a hypothesis about the engine behind the flashes. "We suspected that the bright repeating flashes at optical wavelengths might actually be powered by periodic blasts of high-energy X-rays," said co-author 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."

In short: the star's visible light show may be secondhand glow, heated by invisible X-ray blasts.

What did Chandra actually see?

Chandra was essential for a simple reason. It is the only X-ray telescope with sharp enough vision to separate AT 2016blu from another bright X-ray source sitting nearby on the sky.

The team timed the observations carefully. They needed Chandra to look exactly when the star was in outburst.

Between 2012 and 2026, astronomers tracked the star's brightness using the Katzman Automatic Imaging Telescope, the Las Cumbres Observatory Global Supernova Project, and observations from the American Association of Variable Star Observers — including data from more than 30 amateur astronomers. When those monitors showed a fresh outburst in early 2026, the team triggered Chandra through its "Target of Opportunity" program, which reserves telescope time for special events. The observations came in March 2026.

The payoff: Chandra data from March 2026 showed AT 2016blu at least 100 times brighter in X-rays than in Chandra observations from 2001 to 2002, taken before the outbursts began.

What powers the blasts?

The best explanation is gravity. The star and its companion travel an oval-shaped orbit. At closest approach, material the star has ejected gets captured by the black hole or neutron star. That gas then orbits the compact object at speeds close to the speed of light, heating up so much that it blazes in X-rays. Some of that energy heats the dense surrounding gas, which glows in visible light — the bursts astronomers see from Earth.

The system is bright enough in X-rays to qualify as a candidate ultra-luminous X-ray source. These are rare, powerful sources that astronomers have not fully explained. Current thinking holds that some contain neutron stars and others black holes. Many of them also change brightness dramatically, which hints at a possible link between these sources and violent eruptions like AT 2016blu's.

The find may pay off elsewhere, too. Astronomers suspect that some of the brightness changes stars show before true supernova explosions come from massive stars orbiting compact objects or other massive stars. This system gives them a working example to study.

The takeaway: a star that faked its own death for 15 years turns out to have a dark, dense partner stealing its material — caught red-handed by an X-ray telescope.