Chandra Finds Hidden Partner Behind a Distant Star's Outbursts Every 113 Days

NASA's Chandra X-ray Observatory has found a hidden black hole or neutron star pulling material from the giant star AT 2016blu in the galaxy NGC 4559, according to a study published Monday, November 9, 2026, in The Astrophysical Journal, NASA said.

What NASA says

The star has been flaring on a regular schedule for years. A companion object that astronomers could not see before now is the likely cause. NASA's release says the companion keeps pulling material off the massive star every 113 days as the two orbit each other. The partner is either a stellar-mass black hole or a neutron star, which NASA describes as the dense collapsed core of a massive star.

NASA says this is the first X-ray detection of one of this class of erupting stars. It is also the first of these stars found with a black hole or neutron star companion.

The star sits in NGC 4559, a spiral galaxy about 30 million light-years from Earth, per NASA. Astronomers estimate its mass at no less than 33 times that of the Sun, NASA says.

"For 15 years, astronomers have been watching this star get really bright and then dim again on a regular basis," said Mojgan Aghakhanloo of the University of Virginia in Charlottesville, who led the study. "Until we were able to get Chandra data at the right time, we couldn't figure out what was making this happen."

How they caught it

AT 2016blu is a luminous blue variable, NASA explains: a hot, massive star whose brightness changes a lot. Eta Carinae is the well-known example. This one flares so hard that it looks like a supernova, which is the death of a massive star. But the star survives. That is why NASA calls it a "supernova impostor."

Between 2012 and 2026, astronomers recorded at least 27 bursts of visible light from the star, roughly every 113 days, NASA says. Co-author Núria Torres-Albà of the University of Virginia put it this way: the star keeps surviving these outbursts "about three times a year," so it cannot be a supernova.

The timing was the hard part. Co-author Nathan Smith of the University of Arizona in Tucson said the team suspected the visible flashes were powered by blasts of X-rays. Gas around the system is heated by the X-rays and then glows in visible light. So the Chandra observations had to be planned to land during an outburst.

The team watched the star's brightness with the Katzman Automatic Imaging Telescope, the Las Cumbres Observatory Global Supernova Project, and observations from the American Association of Variable Star Observers, including more than 30 amateur astronomers, NASA says. When the star went into outburst in early 2026, that triggered Chandra observations, taken in March 2026 under its "Target of Opportunity" program, which is built for special events.

NASA says the team used Chandra because it is the only X-ray telescope sharp enough to tell AT 2016blu apart from another bright X-ray source nearby.

What Chandra saw

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

NASA calls the best explanation a close pass in an oval-shaped orbit. When the two objects come near each other, material thrown off by the star is captured by the black hole or neutron star. That material orbits at very high speeds, close to the speed of light, and gets hot enough to shine in X-rays.

The system is bright enough in X-rays to count as a candidate ultra-luminous X-ray source, NASA says. These are rare, bright sources that astronomers have yet to fully understand. Current thinking, per NASA, is that some hold neutron stars and others hold black holes. NASA says the new results suggest a possible link between those sources and violent eruptions like this one, since many of them change a lot in X-ray brightness.

What is still open

NASA's release does not say which one the companion is. It names a black hole or a neutron star, and nothing in the release settles it.

Context

NASA says the find may also help studies of stars that change in brightness before they explode as supernovas. Astronomers suspect much of that behavior comes from a massive star orbiting a compact object or another massive star.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center runs science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts, NASA says.

This is one of several recent Chandra results from NASA. A separate release describes a possible destroyed planet at the center of the Helix Nebula, where NASA says a white dwarf may have torn apart a planet, which would explain an X-ray signal seen since 1980. The status there is stated as "may," and NASA says the case is unconfirmed.

What is next

NASA's release names no follow-up observations or dates. The detail worth noting is the timing: the whole result rests on pointing Chandra at the star while it was mid-outburst. Fifteen years of patient watching, including by amateurs, set that up.

Stay curious.

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