According to astronomers at the University of Washington, what was believed to be a supernova (named SN 2010da), is instead a binary star system with a neutron star that just turned on for the first time.
The study encompasses observations analyzed from 2007 to 2014, allowing astronomers to gain a clear picture of the mysterious object. The findings were published in the Monthly Notices of the Royal Astronomical Society.
In May of 2010, astronomers discovered what appeared to be a supernova (a dieing star ending its life in a massive explosion) in the galaxy NGC 300 about 6 million light years away from Earth. A supernova has been known to shine for weeks after detonation, but to the surprise of scientists, this one still persisted even months after the fact.
“Most supernovae are visible for a short time and then—over a matter of weeks—fade from view,” said Breanna Binder, lead author of the study and a researcher at the University of Washington’s Department of Astronomy.
“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 the past, most supernova impostors have been shown to be massive binary star systems – where two stars orbit each other – similar to the Milky Way’s Eta Carinae. Astronomers think the occasional flare ups could be attributed to instabilities caused by the companion, much like we’ve seen throughout the history of Eta Carinae.
The story was thought to be over as scientists settled on the binary star system explanation, until months later when puzzling X-ray emissions were observed using NASA’s Chandra X-ray Observatory.
“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.
The surprise X-ray emissions lead to these follow up observations with NASA’s Hubble Space Telescope and Chandra. Scientists were able to observe the system and study the strange X-ray behavior. Astronomers concluded that unlike any other supernova impostor reported to date, SN 2010da is probably paired with a neutron star – the extremely dense leftover core of a massive supernova.
“A neutron star at this location would be surprising, since we already knew that this star was a supernova impostor—not an actual supernova. 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. 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,” Binder explained.
Most interesting of all, the team looked over sky surveys from 2007 and 2008 and found no X-ray emission in the area of SN 2010da. Subsequently, astronomers think that the observations in 2010 could have been the neutron star “turning on” for the first time, completing its formation. The X-ray emissions were likely produced from companion material funneling toward the neutron star for the first time.
“That would mean that this is a really rare system at an early stage of formation, and we could learn a lot about how massive stars form and die by continuing to study this unique pairing,” Binder concluded.