The strange behavior of a star that briefly resembled a supernova has seemingly been explained. It appears the star in question has a neutron star as a companion, a highly unusual stellar combination.

If children wish to be superheroes, one can hardly blame stars for impersonating supernovae, but it can be frustrating for astronomers. Supernova imposters flare brightly, but do not explode like true supernovae. These imposters usually have detectable companion stars, and we suspect even those without a known sidekick have one whose influence is triggering the supernova-resembling flare. 

Dr Breanna Binder of the University of Washington has tackled the challenge of a particularly unusual imposter, SN 2010da. It lies in the galaxy NGC300, approximately 6 million-light years away, and briefly became so bright that it was detected by an amateur astronomer who noticed something unusual in the popular galaxy. After SN 2010da faded, however, the star remained visible to professional telescopes. A true supernova would disappear entirely, leaving either a black hole or neutron star in its place. 

Where SN 2010da differs from other imposters is in the intensity of radiation picked up by the Chandra X-ray telescope four months after its detection. "There was just this massive amount of X-rays coming from SN 2010da, which you should not see coming from a supernova impostor," Binder said in a statement. 

The X-ray radiation that appears to be coming from SN 2010da is 100 times as powerful as that seen from any star of its type. Neutron stars have been seen to be this bright in terms of X-rays, but they look very different to SN 2010da in almost every other way.

Her conclusion was that SN 2010da's flare was triggered by an accompanying neutron star, which appears to undergo flares of its own. This explains everything we have observed, but it is also astonishing.

^{The supernova impostor SN 2010da is circled in green and the X-ray emission indicated by a white cross in this image. Royal Astronomical Sociey/Breanna Binder/NASA}

"If this star's companion truly is a neutron star, that would mean that the neutron star was once a giant, massive star that underwent its own supernova explosion in the past,” Binder said, "The fact that this supernova event didn't expel the other star, which is 20 to 25 times the mass of our sun, makes this an incredibly rare type of binary system."

Companion stars sometimes survive a supernova's force to stay in orbit, but this is uncommon, particularly at ranges close enough to trigger flare-ups.

Binder found most stars in the region are either 5 million or 30 million years old, suggesting two bursts of star formation.

"Most stars that are as massive as these usually live 10 to 20 million years, not 30 million," said Binder. "The most massive, hottest stars can form, grow, swell, explode and leave a neutron star emitting X-rays in about 5 million years."

Consequently she has concluded that SN 2012da and its companion formed 5 million years ago, and the companion must have approached the upper end of stellar masses.

Such a system provides us with an outstanding opportunity to learn about the extreme end of stellar behavior.