On 2020 May 25 the TNS (Transient Names Server) was informed about a possible supernova in NGC 5363. This detection received the designation AT2020leu. When I checked the location of AT2020leu I found several things odd.

The coordinates given for the possible supernova were about 8 arcminutes southeast of NGC 5363 and way off this galaxy and even way off of LEDA 49602, the galaxy nearest to the given coordinates. The coordinates marked a spot where no galaxy was visible and even the faintest outskirts of a nearby galaxy could not be assumed. It seemed to me a location somewhere in nowhere.

I then checked the discovery report and found that the discoverer described the position of his discovery to be 6.6 arcseconds southwest of the core of NGC 5363. That is way off the location to which the coordinates on the TNS pointed.

So I started to image NGC 5363. When I checked my first image I found indeed a star close to the galaxy core that could be a supernova. But hold on a second! I took an image of this galaxy group in 2018 (with no moonlight) and compared the new images to my older one. The star was already there in my older image and is a forground star that could be easily mistaken for a supernova.

The discoverer reported that he had estimated the brightness of his discovered "supernova" by comparing it to neighbouring stars on WIKISKY and DSS. When he used images of photographic sky surveys to compare his image to, why did he not notice this star on the survey images? I can only guess. The star is so close to the galaxy center that it cannot be seen on the photographic plates of the POSS 1 and POSS 2 (the latter were used for the DSS) because the galaxy center is overexposed and a big white blob that ishiding the star. The images of the newer (digital) SDSS show the star but at similar brightness and colour as the galaxy center. One has to look very carefully for it to find it. If you do not know that there is a star you may easily overlook it. Unfortunately, some galaxies offer such traps.

On the right hand side I show my image taken on 2018 May 07.96 UT for comparison. It was taken with the same optics and camera as the new image above. And the star is there and no other star near the galaxy center is visible in the new image than those in the older one.

Images by M. Mrotzek