Astronomers link a giant star's repeating outbursts to a hidden black hole or neutron star

For more than a decade, a massive star in the spiral galaxy NGC 4559, about 30 million light-years from Earth, has flared and faded on a regular beat. A study that NASA says was published on Monday, November 9, 2026, in The Astrophysical Journal names a likely cause: an unseen companion, either a black hole or a neutron star, that strips material from the star each time the pair swing close together, roughly every 113 days. The key evidence is X-ray light from NASA's Chandra X-ray Observatory. NASA announced the result the same day; our summary of the paper follows the authors' accepted manuscript, posted on the arXiv preprint server in August.

AT 2016blu is a luminous blue variable, one of a class of hot, massive and unstable stars whose brightness changes. The team estimates it has at least 33 times the mass of the Sun. Its outbursts are bright enough to look like supernova explosions, but the star survives them, which is why astronomers call it a supernova impostor. The authors describe it as the first known impostor of this kind whose outbursts are driven by material falling, on and off, onto a compact object. NASA says it is also the first X-ray detection of any star in this erupting class.

Timing the X-ray look

The team tracked the star's visible light around a predicted outburst, then triggered Chandra's Target of Opportunity program, which NASA says is designed for special events. Chandra observed from March 24 to 27, 2026, and the paper reports detections over five closely spaced epochs. NASA says Chandra was needed because no other X-ray telescope can separate AT 2016blu from a nearby bright X-ray source. The optical monitoring drew on the Katzman Automatic Imaging Telescope, the Las Cumbres Observatory Global Supernova Project and the American Association of Variable Star Observers, including more than 30 amateur astronomers.

Chandra had seen the same patch of sky by chance in 2001 and 2002, years before the first recorded outburst, and the stacked archival data show no X-ray source there. In March 2026 the system was at least 100 times brighter in X-rays, according to NASA. The paper estimates that material is falling onto the companion at a rate of at least about 8 hundred-millionths of the Sun's mass each year, and the authors call their X-ray measurements consistent with a compact companion. In NASA's description, the material grabbed from the star orbits the companion at close to the speed of light and grows hot enough to glow in X-rays.

“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. The study was led by Mojgan Aghakhanloo of the University of Virginia.

How the picture built up

The new result caps a run of papers led by Aghakhanloo, each first posted on arXiv. The star was discovered by the Lick Observatory Supernova Search in 2012.

In 2022 the authors already proposed that the outbursts were set off by close passes in an eccentric binary, comparing the system with SN 2000ch in the galaxy NGC 3432. The 2024 spectra hinted at a companion but did not say what it was. The X-ray data point to a compact object.

What it may explain, and what stays open

NASA says AT 2016blu is bright enough in X-rays to count as a candidate ultra-luminous X-ray source, a rare class that astronomers have yet to fully understand, and that the finding hints at a link between such sources and violent stellar eruptions. The agency adds that the system may help studies of stars that change in brightness before they explode as supernovae. The paper classes AT 2016blu as a high-mass X-ray binary, similar to SN 2010da, where the donor star is a red supergiant rather than a luminous blue variable.

The nature of the companion remains open: NASA describes it only as either a stellar-mass black hole or a neutron star, and the paper's summary as a compact object. Its long-term fate is open too. The 2022 paper noted the star's resemblance to stars seen erupting before a supernova and said it may be headed for a catastrophe, a possibility the authors raised rather than a forecast.