Just in time for its 25th birthday, the Hubble Space Telescope has discovered something never observed before: a double stellar conflagration that falls in between novae and supernovae in brightness.
Novae, meaning new, are stars that suddenly brighten. The name comes from the fact that, before telescopes, stars too faint to see would suddenly become visible, appearing to represent new stars. We now know novae occur when material from a companion star falls on the surface of a white dwarf, leading to a burst of light that lasts weeks or months.
Supernovae are tens of thousands of times brighter. While some occur through a very different process, others have a similar trigger to novae. However, in novae just a small section of the white dwarf's surface explodes, whereas in Type Ia supernovae carbon fusion occurs in the core. They also fade over a more consistent period.
So it was puzzling when Hubble witnessed an event in Eridanus whose brightness was ten times fainter (allowing for distance) than any supernovae, but still a hundred times brighter than the brightest nova. The outburst also faded within two weeks, fast for a nova and unheard of for a supernova.
Then, just to make things really puzzling, Hubble witnessed something similar seven months later, but this time in the infrared. The observations were made in January and August last year, and presented at a symposium celebrating Hubble's anniversary.
“The combination of properties is puzzling,” says Dr. Mario Livio, an astrophysicist at the Space Telescope Science Institute in Maryland, to Nature. “I thought about a number of possibilities, but each of them fails.”
The event occurred 7.8 billion light-years away. Although we can see supernovae at that distance, this fainter outburst would have been very difficult to pick up were it not for intervening galaxies that act as a gravitational lens, making the explosion, and the galaxy in which it occurred, much brighter than would otherwise be the case.
One event that is around the right brightness and time period is a kilonova, which occurs when two neutron stars collide. However, kilonovas are very rare events.
“We’d be very lucky to see one such event,” says Dr. Steven Rodney of Johns Hopkins University. Two in the same place so soon after each is ridiculously unlikely. One possible explanation is that what was seen was the same event, with the infrared light delayed in transmission to us. Rodney says early evidence suggests this is unlikely, but he is not yet willing to rule the possibility out. He has also left open the theory that this is the first example of a whole new class of events.