Astronomers using NASA’s Chandra X-ray Observatory found a hidden companion powering repeated eruptions from the giant star AT 2016blu.
Located 30 million light-years away in the spiral galaxy NGC 4559, AT 2016blu has a mass 33 times that of the Sun. For 15 years, researchers tracked the massive star as it grew bright and then dimmed on a predictable schedule, mimicking the death throes of a true supernova while repeatedly surviving the ordeal. Between 2012 and 2026, observers recorded at least 27 optical outbursts occurring every 113 days.
“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, who led the study published in The Astrophysical Journal. “Until we were able to get Chandra data at the right time, we couldn’t figure out what was making this happen.”
AT 2016blu generated confusion because its outbursts resembled supernovae without destroying the underlying star. To pinpoint the mechanism, a research team synchronized ground-based optical monitoring with targeted space telescope observations.
Investigators used the Katzman Automatic Imaging Telescope, the Las Cumbres Observatory Global Supernova Project, and observations coordinated by the American Association of Variable Star Observers, which included contributions from more than 30 amateur astronomers. When these optical networks spotted an outburst in early 2026, researchers activated a Chandra Target of Opportunity program to capture the system in high-energy X-rays in March 2026.
Chandra’s resolution isolated AT 2016blu from ULX NGC 4559 X7. Data from March 2026 showed AT 2016blu was 100 times brighter in X-rays than it was in Chandra data from 2001 to 2002, before optical outbursts began.
Observations suggest an eccentric binary system where the primary star orbits a compact companion—either a stellar-mass black hole or a neutron star. As the two bodies complete their oval path every 113 days, the compact object strips material from the massive star during their closest approach. This captured gas accelerates to high speeds and gets hot enough to shine in X-rays, heating surrounding gas and driving periodic optical flares.
“This star’s behavior was confusing at first because it kept acting like a supernova, but clearly it’s not because it keeps surviving these outbursts about three times a year,” said co-author Núria Torres-Albà of the University of Virginia. Researchers said the discovery provides a link between supernova impostors, luminous blue variables, and interacting supernovae.