TITLE: GCN CIRCULAR NUMBER: 28727 SUBJECT: GRB200826A: GMOS-N detection of a supernova bump DATE: 20/10/21 18:40:54 GMT FROM: Tomas Ahumada at U. of Maryland Tomas Ahumada (UMD), Leo Singer (NASA GSFC), Harsh Kumar (IITB), and Simeon Reusch (DESY) report on behalf of the ZTF and GROWTH collaborations: We imaged position of ZTF20abwysqy/AT2020scz (Ahumada et al. GCN 28295), the afterglow of GRB 200826A (GCN 28284) with the Gemini Multi-Object Spectrograph (GMOS-N) mounted on the Gemini-North 8-meter telescope on Mauna Kea, on 2020-09-23 and 2020-10-10. Each epoch consisted of 14 r- and i- band 200s exposures. We reduced and co-added the images from each epoch using DRAGONS, a Python-based data reduction platform provided by the Gemini Observatory. We subtracted the coadded images of the two epochs using HOTPANTS. A source is detected on 2020-09-23 at the position of the afterglow with i = 23.9 AB mag and r > 23.5 AB mag. At host galaxy's redshift of z = 0.7481 (GCN 28319) and assuming Planck 2015 cosmological parameters, the absolute magnitude of the GMOS-N detection is M_i = -19.5 AB mag. At this time, a kilonova would have an absolute magnitude of M_i >~ -14.5 AB mag whereas a SN1998bw-like supernova would have an absolute magnitude of M_i ~ -19 AB mag. Since the GMOS-N detection is consistent with a supernova but not a kilonova, we conclude that GRB 200826A was powered by a collapsar rather than a neutron star merger. We thank the Gemini Observatory Director for awarding this observing time and the Gemini staff for help with planning and for executing the observations. -- Tomas Ahumada (he/him) Ph.D. Student Department of Astronomy University of Maryland, College Park NASA Goddard Space Flight Center, Code 661 B.Sc. Astronomy, Pontificia Universidad Católica de Chile