A giant star in the spiral galaxy NGC 4559 has faked a supernova 27 times since 2012 by feeding a hidden binary partner.
Astronomers spent 15 years watching AT 2016blu undergo repeated and rapid increases in brightness. The star sits 30 million light-years away from Earth, though other estimates place the spiral galaxy in the constellation Coma Berenices at 25 million light-years away.
While the optical spikes resembled a massive star collapsing onto itself, the star repeatedly survived the eruptions. Researchers classified it as a supernova impostor. The findings appeared on 9 November in The Astrophysical Journal.
Researchers turned to Space to uncover the driving force behind the events. Europe’s XMM-Newton telescope blurs the target and a brighter X-ray source, NGC 4559 X7, which sits ten arcseconds away. Astronomers needed Chandra’s sharp vision to distinguish the target.
Four observations of AT 2016blu were also carried out by XMM-Newton, an ESA x-ray observatory, during its monitoring runs directed at NGC 4559 X7.
“Until we were able to get Chandra data at the right time, we couldn’t figure out what was making this happen,” said Mojgan Aghakhanloo, lead author and a fellow at the University of Virginia in Charlottesville.
Observations obtained in March 2026 revealed that the system was 100 times brighter in X-rays than archival baseline data captured between 2001 and 2002. The data indicate that the system is an eccentric binary containing a luminous blue variable with 33 times the mass of the Sun and a compact companion.
The companion is a stellar-mass black hole or a neutron star. The two objects complete an oval-shaped orbit every 113 days.
The high-gravity partner pulls material away from the unstable giant star during close approaches. Telescopes at Lick Observatory and the Las Cumbres Observatory network checked the star almost nightly.
An alert from the American Association of Variable Star Observers brought in more than thirty observers. The researchers relied on optical data gathered by the Katzman Automatic Imaging Telescope, the Las Cumbres Observatory Global Supernova Project, and records from the American Association of Variable Star Observers encompassing inputs from upwards of 30 amateur astronomers to track the star’s luminosity changes.
“The dense gas surrounding the system is heated by the X-rays and then glows in the optical,” said co-author Nathan Smith of the University of Arizona in Tucson. “We suspected that the bright repeating flashes at optical wavelengths might actually be powered by periodic blasts of high-energy X-rays.”
The stolen material forms an accretion disk, moving at speeds close to the speed of light and generating intense X-rays. This energy heats the surrounding cloud of gas previously ejected by the giant star, creating the visible light flashes that mimic a supernova explosion.
Co-author Núria Torres-Albà, also of the University of Virginia, said that the star’s behavior was confusing at first because it kept acting like a supernova while surviving these outbursts about three times a year, adding that to understand the cause of these eruptions, the team realized they needed X-ray data timed so that they would observe the star when it was in an outburst.
The system shines brightly enough in X-rays to qualify as a candidate ultra-luminous X-ray source. The authors write that “AT 2016blu represents the first observationally known LBV system with a compact companion.”
“We present the first X-ray detection of the supernova (SN) impostor AT 2016blu in NGC 4559,” the authors wrote in The Astrophysical Journal.
Earlier coverage: A Supernova Impostor: X-Ray Outbursts Every 113 Days Reveal Massive Star with a Compact Companion
