Ia explosions have a very similar spectrum, but are substantially fainter. Astronomers have speculated that these represent incomplete explosions, created by similar conditions to normal Type Ias, but leaving some of the precursor star behind. LP 40-365 looks to be the first good evidence we have to back this theory up, making it a truly wonderful find.
The explosion gave LP 40-365 its estimated speed of 546 kilometers per second (340 miles per second), enough to ensure it will eventually escape gravitational pull of the galaxy, and heated it to hundreds of thousands of degrees. The dwarf fragment has now cooled to a temperature of about 10,000 Kelvin – still hotter than the Sun, despite the fact that it is not generating any new energy. Ferrario told IFLScience there are wide error bars on the rate of cooling for an object so much smaller than other white dwarfs, but the explosion happened sometime between 5 and 50 million years ago.
As to what would cause a Ia explosion to not reach completion, no one knows, and Ferrario said the authors will attempt to model the event. Since they are fainter than other supernovae, there are probably a lot of Iax events we don't see, so estimating how common they are compared to the complete version is also not yet possible.
Ferrario doubts there are other large fragments from the explosion that produced LP 40-365 to be found, but thinks the companion star that triggered its explosion probably also flew off at high speed. Finding it, she added would allow us to establish the time and place of the location, but will be like looking for the proverbial needle in a haystack. Not only would the companion presumably be traveling in roughly the opposite direction and therefore much further away than LP 40-365's current distance of 970 light years from Earth, but we have only a faint idea of where to look.