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Full list of authors: Ahumada, Tomás; Singer, Leo P.; Anand, Shreya; Coughlin, Michael W.; Kasliwal, Mansi M.; Ryan, Geoffrey; Andreoni, Igor; Cenko, S. Bradley; Fremling, Christoffer; Kumar, Harsh; Pang, Peter T. H.; Burns, Eric; Cunningham, Virginia; Dichiara, Simone; Dietrich, Tim; Svinkin, Dmitry S.; Almualla, Mouza; Castro-Tirado, Alberto J.; De, Kishalay; Dunwoody, Rachel; Gatkine, Pradip; Hammerstein, Erica; Iyyani, Shabnam; Mangan, Joseph; Perley, Dan; Purkayastha, Sonalika; Bellm, Eric; Bhalerao, Varun; Bolin, Bryce; Bulla, Mattia; Cannella, Christopher; Chandra, Poonam; Duev, Dmitry A.; Frederiks, Dmitry; Gal-Yam, Avishay; Graham, Matthew; Ho, Anna Y. Q.; Hurley, Kevin; Karambelkar, Viraj; Kool, Erik C.; Kulkarni, S. R.; Mahabal, Ashish; Masci, Frank; McBreen, Sheila; Pandey, Shashi B.; Reusch, Simeon; Ridnaia, Anna; Rosnet, Philippe; Rusholme, Benjamin; Carracedo, Ana Sagués; Smith, Roger; Soumagnac, Maayane; Stein, Robert; Troja, Eleonora; Tsvetkova, Anastasia; Walters, Richard; Valeev, Azamat F., Gamma-ray bursts (GRBs) are among the brightest and most energetic events in the Universe. The duration and hardness distribution of GRBs has two clusters1, now understood to reflect (at least) two different progenitors2. Short-hard GRBs (SGRBs; T90 < 2 s) arise from compact binary mergers, and long-soft GRBs (LGRBs; T90 > 2 s) have been attributed to the collapse of peculiar massive stars (collapsars)3. The discovery of SN 1998bw/GRB 980425 (ref. 4) marked the first association of an LGRB with a collapsar, and AT 2017gfo (ref. 5)/GRB 170817A/GW170817 (ref. 6) marked the first association of an SGRB with a binary neutron star merger, which also produced a gravitational wave. Here, we present the discovery of ZTF20abwysqy (AT2020scz), a fast-fading optical transient in the Fermi satellite and the Interplanetary Network localization regions of GRB 200826A; X-ray and radio emission further confirm that this is the afterglow. Follow-up imaging (at rest-frame 16.5 days) reveals excess emission above the afterglow that cannot be explained as an underlying kilonova, but which is consistent with being the supernova. Although the GRB duration is short (rest-frame T90 of 0.65 s), our panchromatic follow-up data confirm a collapsar origin. GRB 200826A is the shortest LGRB found with an associated collapsar; it appears to sit on the brink between a successful and a failed collapsar. Our discovery is consistent with the hypothesis that most collapsars fail to produce ultra-relativistic jets. © 2021, The Author(s), under exclusive licence to Springer Nature Limited., This work was supported by the GROWTH (Global Relay of Observatories Watching Transients Happen) project funded by the National Science Foundation under PIRE grant No. 1545949. GROWTH is a collaborative project among California Institute of Technology (USA), University of Maryland College Park (USA), University of Wisconsin Milwaukee (USA), Texas Tech University (USA), San Diego State University (USA), University of Washington (USA), Los Alamos National Laboratory (USA), Tokyo Institute of Technology (Japan), National Central University (Taiwan), Indian Institute of Astrophysics (India), Indian Institute of Technology Bombay (India), Weizmann Institute of Science (Israel), The Oskar Klein Centre at Stockholm University (Sweden), Humboldt University (Germany), Liverpool John Moores University (UK) and University of Sydney (Australia). Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington (UW), Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee and Lawrence Berkeley National Laboratories. Operations are conducted by Caltech Optical Observatories, IPAC and UW. The work is partly based on the observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, in the island of La Palma. The material is based on work supported by NASA under award No. 80GSFC17M0002. A.J.C.T. acknowledges all co-Is of the GTC proposal and the financial support from the State Agency for Research of the Spanish MCIU through the 'Center of Excellence Severo Ochoa' award to the Instituto de Astrofisica de Andalucia (SEV-2017-0709). The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant No. 12540303 (PI: Graham). S.M. and J.M. acknowledge support from Science Foundation Ireland under grant No. 17/CDA/4723. R.D. acknowledges support from the Irish Research Council (IRC) under grant GOIPG/2019/2033. Analysis was performed on the YORP cluster administered by the Center for Theory and Computation, part of the Department of Astronomy at the University of Maryland. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center. These results also made use of Lowell Observatory's Lowell Discovery Telescope (LDT), formerly the Discovery Channel Telescope. Lowell operates the LDT in partnership with Boston University, Northern Arizona University, the University of Maryland and the University of Toledo. Partial support of the LDT was provided by Discovery Communications. LMI was built by Lowell Observatory using funds from the National Science Foundation (AST-1005313). M.W.C. acknowledges support from the National Science Foundation with grant No. PHY-2010970. S.A. gratefully acknowledges support from the GROWTH PIRE grant (1545949). Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. E.C.K. acknowledges support from the G.R.E.A.T research environment and the Wenner-Gren Foundations. P.T.H.P. is supported by the research program of the Netherlands Organization for Scientific Research (NWO). H.K. is an LSSTC Data Science Fellow and thanks the LSSTC Data Science Fellowship Program, which is funded by LSSTC, NSF Cybertraining grant No. 1829740, the Brinson Foundation and the Moore Foundation; his participation in the program has benefited this work. S.M. and J.M. acknowledge support from Science Foundation Ireland under grant No. 17/CDA/4723. R.D. acknowledges support from the Irish Research Council (IRC) under grant GOIPG/2019/2033. P.C. is a Swarana Jayanti Fellow and acknowledges support from the Department of Science and Technology via award No. DST/SJF/PSA-01/2014-15). We thank the staff of the GMRT who made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. P.C. is a Swarna Jayanti Fellow and thanks the Department of Science & Technology in India. We thank D. Bhattacharya, A. Vibhute and V. Shenoy for help with the CZTI analysis. V.A.F. acknowledges support from the RFBR 18-29-21030.