Chandra found a hidden companion behind a giant star’s mysterious 113-day outbursts

A star roughly 30 million light-years away has been flaring on a schedule, and astronomers now think something dark is pulling the strings. The star, AT 2016blu in the galaxy NGC 4559, has erupted at least 27 times between 2012 and 2026, roughly once every 113 days. A new study proposes that an unseen companion, either a stellar-mass black hole or a neutron star, drags material off the star each time the two swing close.

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The evidence for the companion is X-rays. NASA’s Chandra X-ray Observatory caught the star shining far brighter in that light than it did before the outbursts began.

A supernova impostor on a 113-day clock

AT 2016blu is classed as a luminous blue variable and, in the jargon, a supernova impostor, because its eruptions mimic an exploding star without destroying it. Astronomers estimate its mass at no less than 33 times that of the Sun, according to the NASA release on the Chandra finding. Luminous blue variables are massive, unstable stars that shed material in outbursts, and the label impostor exists because the eruptions resemble supernovae while leaving the star intact.

The cycle was spotted earlier. A 2023 paper led by Mojgan Aghakhanloo identified at least 19 outbursts between 2012 and 2022, each brightening the star by one to two magnitudes and lasting one to four weeks, and found the bursts clustering on a period of about 113 days. Archival telescope data from 1999 to 2009 showed no outbursts, so the behavior appears to have started around 2012. The 2023 authors used the period to predict outbursts in early 2023 and in 2024, and the later spectroscopy paper reports a 21st outburst around May or June 2023 that fit the forecast.

A follow-up spectroscopy study tracked the star for more than a decade and linked the pulses to close approaches in an eccentric binary, while noting that some absorption features move faster than the star’s own emission, a hint that a companion may be involved. Some narrow emission lines in those spectra appear to come from a nearby ionized-gas region in NGC 4559 rather than the star itself, a reminder of how crowded the field is.

What Chandra saw in March 2026

Optical data alone could not show what the companion was. Aghakhanloo, of the University of Virginia, said the puzzle held until Chandra data arrived at the right time. The team scheduled observations through the observatory’s Target of Opportunity program, timed to catch an optical outburst, and Chandra looked at the star from March 24 to 27, 2026. Optical images from the Katzman Automatic Imaging Telescope came on March 14, when the star was quiet, and March 25, during the outburst.

Chandra was the only X-ray telescope sharp enough to separate AT 2016blu from a nearby bright X-ray source. The March data showed the star at least 100 times brighter in X-rays than in observations from 2001 and 2002, before the outbursts started. The likely explanation, per the NASA account, is that gas pulled from the star heats up and glows in X-rays as it falls onto the companion near the closest approach. Light curves from the Las Cumbres Observatory Global Supernova Project and the American Association of Variable Star Observers, including contributions from more than 30 amateur astronomers, supplied the optical record.

The 2001 and 2002 observations matter because they predate the outbursts, which began around 2012, so they work as a baseline for the quiet star. A brightening of at least a hundredfold between that baseline and March 2026 is what moved the discussion from a bright star with a periodic habit toward a system with a compact object in it.

Proposed, not yet pinned down

The study, published in The Astrophysical Journal, proposes the companion rather than imaging it. The object is described as either a black hole or a neutron star, and the X-ray data alone do not settle which. The earlier papers likewise treated the binary reading as favored but unconfirmed, with single-star instability still possible. Aghakhanloo’s name appears on all three papers, alongside collaborators including Nathan Smith and Alexei Filippenko in the earlier work.

NASA calls the result the first X-ray detection of this class of erupting star, and the first such star found with a black hole or neutron star partner. The star is also bright enough in X-rays to be a candidate ultraluminous X-ray source, a rare class that scientists do not fully understand, so the finding may connect those sources to violent stellar eruptions.

Hubble images taken before the outbursts, a Hubble Space Telescope archive staple, supplied the mass limit used in the earlier work. The next step is more X-ray coverage across a full 113-day cycle, to see whether the glow rises and falls with the orbit the way a companion would require. The study, published in The Astrophysical Journal, leaves the companion’s exact identity as the main open item.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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