The mystery of a supernova imposter

A team astronomers discovered a supernova imposter that, oddly, was giving off a fierce amount of X-rays.

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Astronomers have solved the mystery of an ‘imposter’ supernova that was found generating strong X-ray signals.

In May 2010 an amateur astronomer looked at galacy NGC300 and discovered what appeared to be a supernova. These are explosions caused when some massive stars come to the end of their life.

Once the star explodes, it usually leaves behind a black hole or a collapsed, dense stellar core called a neutron star. But when astronomers looked at where the supernova had been some weeks later, they found to their surprise that it was still there.

“Most supernovae are visible for a short time and then - over a matter of weeks - fade from view,” says Breanna Binder, part of the team that studied the star.

“SN 2010da is what we call a 'supernova impostor' - something initially thought to be a supernova based on a bright emission of light, but later to be shown as a massive star that for some reason is showing this enormous flare of activity.”

In September 2010, Binder used NASA’s Chandra X-ray Observatory to look at NGC300 and found a massive amount of X-rays coming from the object, which shouldn’t be the case for a supernova imposter. In fact, the X-rays were in line with those that would be produced by a neutron star.

In 2014, the team looked at the star again with Chandra and the Hubble Space Telescope and found that X-rays were still being produced. This led Binder to conclude that the supernova impostor is probably paired with a neutron star.

“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,” says Binder. “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.”

The team looked at the stars in the region surrounding the binary system and found that most were created in two bursts; one 30 million years ago and one less than 5 million years ago.

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

As a result, Binder believes the X-ray signals must be coming from a neutron star paired with the supernova imposter in a binary system. The X-rays could be caused as material from the impostor is transferred to the neutron star companion, and are in fact a sign of the neutron star being ‘turned on’ for the first time.

“That would mean that this is a really rare system at an early stage of formation,” said Binder, “and we could learn a lot about how massive stars form and die by continuing to study this unique pairing.”


Inset image: Image captured by Breanna Binder’s group using the Hubble Space Telescope. SN 2010da is circled in green and its X-ray emission is indicated by a white cross.
Credit: Reproduced from a Royal Astronomical Society publication. Breanna Binder/NASA/Royal Astronomical Society
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