A supernova collided with its companion star, a white dwarf, revealing secrets of these powerful events. White dwarfs are the remains of stars, once about the size of our own sun, which shed most of their material, leaving behind a stellar corpse approximately the size of the Earth.
Type 1a supernovae were thought by some astronomers to result when material from one star in a binary system loses material to its white dwarf companion. At a critical mass, this gas erupts in a massive explosion that can briefly become brighter than its entire host galaxy. This is known as the single-degenerate model. Other astronomers held the opinion, called the double-degenerate model, that these events occur in systems hosting a pair of white dwarfs that collide, triggering the explosion.
Thousands of these events have been witnessed in other galaxies by astronomers over the last few decades. However, mysteries still surround the process by which they form due to the rarity of the events in our own galaxy and the faint images of white dwarfs in other galaxies as seen from Earth.
The iPTF camera installed on the 48-inch Samuel Oschin Telescope at Mount Palomar examines roughly 5 percent of the visible sky each night, searching for any changes in brightness.
A Type 1a supernova, designated iPTF14atg, was detected in the galaxy IC831 on May 3, 2015. This family of stars sits roughly 300 million light years away from our own galaxy. As soon as astronomers saw evidence of a supernova, they alerted other researchers who trained a network of observatories to study the event. One of these observatories was the Swift telescope, which examines the universe in ultraviolet light.
"My colleagues and I spent many sleepless nights on designing our system to search for luminous ultraviolet emission from baby Type 1a supernovae. As you can imagine, I was fired up when I first saw a bright spot at the location of this supernova in the ultraviolet image. I knew this was likely what we had been hoping for," Yi Cao, a graduate student at Caltech who is also a member of the iPTF team, said.
The ultraviolet observations seemed to suggest that this supernova was formed from the accumulation of matter from a larger companion falling onto a white dwarf, supporting the single-degenerate model for Type 1a supernovae. However, collisions could still be responsible for supernovae in other sytems, researchers found.
Type 1a supernovae are often used as "standard candles" to measure distances to galaxies, because their luminosity was thought to be consistent around the universe.
"By calibrating the relative brightness of Type 1a supernovae to several percent accuracy, astronomers were able to use them to discover the acceleration of the universe," Daniel Kasen from the Lawrence Berkeley National Laboratory said.
Analysis of the ultraviolet pulse from this Type 1a supernova was published in the journal Nature.
