A faint, fleeting supernova may be the key to understanding how neutron star duos are born.
Astronomers have spotted what seems to be an ultrastripped supernova: a massive star in its death throes after its outer layers of gas have been siphoned off slowly by a compact companion such as a neutron star or black hole.
“This is the first of its kind: the first ultrastripped supernova that has been observed,” says astronomer Kishalay De of Caltech. Similar supernovas could lead to binary neutron stars like the pair that was caught colliding in 2017, he and his colleagues report in the Oct. 12 Science.
The supernova was spotted exploding in a galaxy about 930 million light-years away from Earth by the robotic intermediate Palomar Transient Factory in October 2014, and was named iPTF 14gqr after that wide-field survey. Most supernovas detonate when a star more than eight times as massive as the sun has burned through all its fuel and can no longer hold itself up against gravity. The star’s core collapses, leaving a dense neutron star behind (SN: 10/13/18, p. 11). Meanwhile, a rebounding explosion ejects the remaining outer layers of gas outward as a bright flare that usually lasts for about 17 to 20 days.
But iPTF 14gqr’s light faded after just seven days. It also emitted unusually little energy for a supernova. That wimpy burst suggested that, although the star was about 10 times the sun’s mass to begin with, it ejected only one-fifth of the mass of the sun when it exploded. The best explanation for the puny mass is that the star had lost much of its material sometime before it died, the researchers say.
That’s exactly what you’d expect for an ultrastripped supernova. Astrophysicist Thomas Tauris of Aarhus University in Denmark predicted such strange supernovas in 2013. A nearby, extremely massive companion, like a neutron star or a black hole, could have stolen the matter away from the massive star (SN Online: 5/31/18). Whatever remained of the star could still explode, but faintly — just like iPTF 14gqr.
“I find that the presented evidence for an ultrastripped supernova is very convincing,” says Tauris, who was not involved in the discovery. “Personally, I am also very excited because these ultrastripped supernovae were first discovered on my computer as a pure theoretical concept, so it is really fantastic that observers can now verify their existence in detail.”
The result of this type of explosion would be a close pair of compact stellar corpses, whether two neutron stars or a neutron star and a black hole. In 2017, astronomers detected gravitational waves emitted from two neutron stars colliding, so researchers know that such duos exist. But it’s unclear how the stars get so close to each other in the first place. The discovery of iPTF 14gqr suggests that they were born close and only got closer.
Editor’s note: This story was updated October 23, 2018, to correct that iPTF 14gqr’s light faded after seven days, not over seven days.