Supernova (sort of) in M51
On January 22, 2019, the ATLAS telescopes on Hawaii discovered a supernova, and coded it AT2019abn. In itself not a unique phenomenon. The Asteroid Terrestrial-impact Last Alert System, or ATLAS, project is designed to be an early warning signal for asteroids that might be heading for Earth. For this purpose, two very fast wide-field telescopes are in operation. Both 50cm f/2.0 telescopes are equipped with an array of CCD sensors. The Field of View of each telescope is 7.4º. With exposures of 30s each, the whole night sky up to magnitude 20 can be photographed by one telescope in one night. With two telescopes, each area of the sky can be photographed twice… each night.
Anything that moves or changes in brightness, will be picked up by ATLAS. While the system is designed to monitor asteroids, ‘by-products’ are the discovery of comets and supernovae. In total already more than 7000 supernovae have been observed. But the interesting aspect of AT2019abn was that it was in one of the spiral arms of M51, or Whirlpool galaxy, an often photographed object by amateur astrophotographers.
Back in 2018 I had made an image of M51. So in theory, making another image now in 2019 and comparing that with the one from 2018 should allow for the ‘discovery’ of this supernova. Rather than going through the trouble of making a colour image, the decision was made to only do a monochrome comparison. So 3.5h of luminance data were collected on April 13 and 14. And this was compared to 4h of luminance data from 2018. It took a while to have both images processed a similar way and for reasons of clarity the images were inverted. But then it was possible to compare, and there was the supernova!
Researchers collected more data on this supernova, and it was eventually concluded that it was not a real supernova, but a so-called Intermediate-Luminosity Red Transient (ILRT). This phenomenon can occur in double-star systems where the stars come close enough to develop a single envelope, and a lot of plasma is emitted. The cooling down of the plasma and re-joining of electrons and protons to atoms sends out a lot of red light with brightness somewhere between a nova and a supernova. A scientific paper by Jencson et al describing this supernova-like event can be found here.
DeepSky imaging usually involves very static objects, with changes impossible to observer, because they are taking place on such an astronomic scale. So to see a ‘dynamic’ event like this happen, even if not a real supernova, is very nice to be able (and lucky enough) to have photographed.
This image has been published on Astrobin.