Yet when the light of the 'supernova' faded, the star was still there, only to continue bursting into light every 113 days.
Thanks to Chandra ’s X-ray vision, Aghakhanloo has led a team of astronomers who think they have now uncovered an explanation.
The supernova imposter is named AT 2016blu, and is a type of star that we call a 'luminous blue variable,' or LBV.
An illustration shows a black hole feeding on a star (Image credit: NASA/CXC/A.
It’s little wonder that the accretion onto the compact object unleashes such extreme energies.
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A composite optical and X-ray image of NGC 4559. The purple dots are X-ray sources; AT 2016blu is highlighted in the square box
A cosmic imposter that has faked being a supernova 27 times and counting is actually a hot, luminous, and decidedly unstable star that is being fed on by a vampiric black hole or neutron star, NASA’s Chandra X-ray Observatory has discovered.
When a massive star reaches the end of the road and runs out of nuclear fuel to power fusion reactions in its core, the star is destroyed in a supernova explosion. That’s what astronomers thought they had found on 11 January 2012, when they observed what appeared to be a supernova on the outskirts of the spiral galaxy NGC 4559, which is about 25 million light-years away in the constellation Coma Berenices.
Yet when the light of the 'supernova' faded, the star was still there, only to continue bursting into light every 113 days.
"For 15 years, astronomers have been watching this star get really bright and then dim again on a regular basis," Mojgan Aghakhanloo of the University of Virginia in Charlottesville said in a statement .
Several other supernova imposters have been observed over the past decade or two. Until now, no one knew what causes these stars to repeatedly cry wolf. Thanks to Chandra ’s X-ray vision, Aghakhanloo has led a team of astronomers who think they have now uncovered an explanation.
How did a cosmic vampire hide as a supernova?
The supernova imposter is named AT 2016blu, and is a type of star that we call a 'luminous blue variable,' or LBV. These are the most massive and luminous stars in the universe and their extreme characteristics result in them being highly unstable, undergoing intermittent bouts of mass loss as they eject material from their outer layers into the surrounding space.
However, faking a supernova is not part of their typical behavior, so a new way of looking at AT 2016blu was needed.
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"To understand the cause of these eruptions, we realized that we needed X-ray data that was timed so that we’d observe the star when it was in an outburst," team member Núria Torres-Albà, also from the University of Virginia in Charlottesville, said.
Aghakhanloo’s team were granted the opportunity to commandeer Chandra to target an outburst of AT 2016blu at the earliest opportunity. That opportunity arose in March 2026.
A comparison between Chandra images from 2001–02 and 2026 at top, and optical light images at bottom, showing the supernova imposter outbursts and associated X-rays. (Image credit: X-ray: NASA/CXC/University of Virginia/M. Aghakhanloo et al; Optical: NSF/KAIT.)
Chandra had previously and coincidentally looked in the direction of NGC 4559 back in 2001 and 2002 and did not notice anything unusual back then. This time around, however, it captured a powerful blast of X-rays coinciding with yet another false supernova outburst.
The X-ray detection allowed Aghakhanloo’s team to assemble a picture of what is happening.
"Until we were able to get Chandra data at the right time, we couldn’t figure out what was making this happen," he said.
Chandra’s X-ray observations indicate that AT 2016blu has an unseen companion, a compact, high-gravity object that is either a neutron star or a stellar-mass black hole on an elliptical 113-day orbit around the luminous blue variable.
Neutron stars and stellar-mass black holes are the core remnants of massive stars that have gone supernova for real, meaning that at one time AT 2016blu was a binary star system .
Blue star falls victim to a cosmic vampire
Each time the compact object reaches the point in its orbit that is closest to the luminous blue variable, its powerful gravity is able to pull material from the unstable star. This sets in motion a chain of events that results in the brilliant outbursts.
As the stolen starstuff is dragged away from the wounded star and towards the compact object, it forms an accretion disk swirling around the neutron star or black hole so fast and growing so hot that it emits X-rays. These are the X-rays that Chandra saw.
The X-rays are immense, energizing the surrounding cloud of material that had been ejected by the luminous blue variable years before. The cloud is heated and glows extremely brightly in visible light, creating the impression of a supernova.
"We suspected that the bright repeating flashes at optical wavelengths might actually be powered by periodic blasts of high-energy X-rays," said team member Nathan Smith of the University of Arizona in Tucson. "The dense gas surrounding the system is heated by the X-rays and then glows in the optical."
An illustration shows a black hole feeding on a star (Image credit: NASA/CXC/A. Hobart)
Calculations suggest that each year a 440 billion trillion pounds of material at minimum is stolen by the compact object, and possibly two orders of magnitude more than that. This is a huge amount, yet it is only 800 millionths of the mass of the sun , and AT 2016blu’s luminous blue variable is at least 33 times the mass of the sun.
Nevertheless, it is far greater than the maximum rate of material that a neutron star can safely accrete, referred to as the Eddington limit . It’s little wonder that the accretion onto the compact object unleashes such extreme energies.
AT 2016blu is also connected with another mysterious and rare astrophysical phenomenon, that of ultra-luminous X-ray (ULX) sources. The identity of whatever it is that is powering ULXs remains unclear, but the involvement of compact objects such as black holes and neutron stars is high on the list.
AT 2016blu is bright enough to be considered a ULX, so could it act as a Rosetta stone connecting supernova imposters with ULXs? It’s too early to say that; only one other supernova impostor has so far been found to have X-ray emission typical of material being stolen from one star and accreting onto a compact body, and that is SN 2010da, but in that case it is known that the companion is another massive star, a red supergiant, rather than a compact object.
