AT 2016blu: the fake supernova that flares every 113 days

What is AT 2016blu?

It is a luminous blue variable, one of the most massive and brightest kinds of star known. They are unstable and regularly throw off layers of gas. This one is at least 33 times the mass of the sun. Throwing off gas is normal for the type. Mimicking a supernova on a schedule is not.

How does a hidden black hole fake a supernova?

A team led by Mojgan Aghakhanloo of the University of Virginia needed X-ray data taken during an outburst. They got time on NASA's Chandra X-ray Observatory in March 2026, and Chandra caught a strong burst of X-rays just as the star flared again. Earlier Chandra looks at the galaxy, in 2001 and 2002, had shown nothing odd.

The X-rays point to an unseen companion, either a neutron star or a stellar-mass black hole, on an elliptical 113-day orbit. Both are leftovers of massive stars that really did explode, so this was once a pair of stars. Each time the companion swings closest, its gravity drags gas off the blue star. That gas forms a hot, fast-spinning disk that gives off X-rays.

Those X-rays heat the dense cloud the blue star had earlier shed, and the cloud glows in visible light. "The dense gas surrounding the system is heated by the X-rays and then glows in the optical," said Nathan Smith of the University of Arizona. The team calculates the gas is pulled in faster than a neutron star can safely take, known as the Eddington limit.

AT 2016blu is also bright enough to count as an ultra-luminous X-ray source, a rare class whose power source is still unclear. Whether impostors and these sources are linked is too early to say. Only one other impostor, SN 2010da, shows similar X-rays, and its companion is a red supergiant. The work was published on 9 November in The Astrophysical Journal.