NASA's recently retired Kepler space telescope was famous for its ability to spot thousands of exoplanets. But this year, it presented a mysterious observation of a supernova.
Kepler offered scientists another chance to look at the light coming from before, during, and after a supernova. This "exquisite" data, in the words of the scientists who wrote about it, deepened the mystery about why these kinds of type 1a supernovae shine so brightly shortly after they blow, and what causes their final explosion.
Scientists were first alerted to the presence of a supernova, now called 2018oh, via a suite of five telescopes worldwide (called the All Sky Automated Survey for SuperNovae) which provided the images obtained on February 4, 2018.
Luckily, the galaxy in which the supernova occurred, called UGC 478, was part of the Kepler space telescope's galaxy-observing campaign. Scientists were then able to combine Kepler's sensitive data with specialised observations of the supernova's colour using other telescopes, including the Dark Energy Camera and the Pan-STARRS1 telescope.
It might sound strange — why was an exoplanet hunter looking at distant galaxies? Kepler repeatedly takes wide images of the same spots in its survey, and can observe thousands of galaxies at a time. Moreover, it's meant to be sensitive to small changes in the amount of light emitted by objects, as it usually looks for the dim light changes caused planets passing in front of distant stars.
2018oh is a type 1a supernova, based on the way its light looks. Models show that these supernovae come from a pair of stars in which one, a white dwarf, sucks up a lot of mass from the other before exploding. This is the fourth type 1a supernova spotted in a Kepler survey yet, and it's the closest and brightest with the most precise data.
A host of researchers recently released cleaned-up analyses of the supernova. Two such results found that the graph of the "light curve," or amount of light over time, didn't appear as an even curve. Instead, it had a upward straight line piece, where the supernova shone brighter than expected for five days, before curving upward, hinting that two different processes were happening.
They also noted that the supernova appeared bluer than others.
So, what was causing the two-part brightening of this supernova? There are several ideas. Maybe the light interacted with the nearby star, which offered fuel for the explosion. Or maybe there was a helium shell on the surface of the white dwarf, which provided more ignition material. Or maybe there was a hunk of radioactive nickel clumped unevenly in the star, which would have added light to the signature. Further observations with the Swift X-ray telescope seemed to rule out interactions with dust surrounding the exploding star.
One researcher we spoke to thinks it's too early for an answer — and that maybe it's time to go back to the drawing board. "Theory people need to go back to the books and model the different ways that this early emission could be produced," Maximillian Stritzinger, a physicist at Aarhus University in Denmark who has studied the colours emitted by these type 1a supernovae, told Gizmodo.
The supernova joins a list of difficult-to-describe type 1as, according to the studies. But there's hope yet. Kepler may still have observed more supernovae, and that data may further help scientists solve the mystery.