Photographing a supernova in the process of erupting is notoriously difficult to do because it is impossible to predict the exact moment of the phenomenon, and its very first burst of light. In fact, the chances of witnessing such an event is akin to winning a cosmic-scale lottery in which the odds are at least 100 million to one. Yet, despite the monumental odds, and it having never been done in the past, this is exactly what was accomplished by Victor Buso, an amateur astronomer from Rosario, Argentina.
The image he managed to capture shows the spiral galaxy NGC 613 which is located about 70 million light-years away in the constellation Sculptor, and the Type IIb supernova that Buso was fortunate enough to observe erupting on September 20, 2016.
According to reports, Buso was testing a new camera while observing NGC 613 with his 16-inch telescope, and being eager to see how well the camera worked with this particular telescope, he examined a series of short exposure pictures he had taken immediately after taking them. To his surprise, however, the series of exposures showed a previously invisible pinpoint of light close to the galaxy that got progressively brighter with each successive exposure.
From the reports, it is not clear how, or when astronomer Melina Bersten and other astronomers at the Instituto de Astrofísica de La Plata learned of the observation, but all agreed that Buso had caught the first light of the supernova that is now designated SN 2016gkg.
Follow-up investigations over the next two months by UC-Berkeley astronomer Alex Filippenko, using the Lick Observatory’s 3-meter Shane telescope as well as the two ten-meter Keck telescopes in Maunakea, Hawaii, produced seven spectra of the supernova that allowed investigators to deduce the nature of the progenitor star. From these studies, it emerged that the optical light Buso had captured derives from a process known as “shock breakout”, which happens when a supersonic pressure wave from the rebounding core of the star meets relatively dense gas at the star’s surface. This process creates an enormous amount of heat, which in turn, creates a burst of visible light that brightens rapidly before fading away.
Moreover, based on detailed analysis of the obtained spectra and complex theoretical modeling, Alex Filippenko and his team of researchers found that the progenitor star had a mass of about 20 times that of the Sun just before it exploded. However, their report also points out that the star had probably lost as much as 75% of its mass to a companion in a process known as “mass transfer” over a relatively short period prior to the star’s destruction.
Full details on the discovery of SN 2016gkg, as well as a detailed report describing the follow-up investigation and observations can be found in the February 21, 2018 edition of the journal Nature.