TITLE: GCN CIRCULAR NUMBER: 30927 SUBJECT: IceCube-20210922A: DESNU Optical counterpart candidate from 5 nights of DECam observations DATE: 21/10/08 21:17:22 GMT FROM: Robert Morgan at U. of Wisconsin-Madison <robert.morgan@wisc.edu> Robert Morgan (U of Wisconsin-Madison), Nora Sherman (U of Michigan), Alyssa Garcia (U of Michigan), Ken Herner (Fermilab), Julian Beas-Gonzalez (Fermilab) Kathy Vivas (NSF�s NOIR Lab), Alfredo Zenteno (NSF�s OIR Lab), Dillon Brout (Harvard U), Yujin Yang (Korea Astronomy and Space Institute), Byeongha Moon (Korea Astronomy and Space Institute). We triggered the Dark Energy Camera (DECam) at Cerro Tololo Inter-American Observatory in Chile on the localization area of the GOLD neutrino event detected by IceCube (IceCube-20210922A, GCN 30862). We covered 100% of the localization area in the r,i, and z bands on nights 0, 1, 2, 8, and 13 following the trigger to a maximum 10-sigma r-band depth of 23.4 mag. We determined interesting candidates from our observations by selecting sources not present in archival DECam images, requiring a detection with signal-to-noise > 5.0 on two separate nights, and requiring that the candidate have multiple autoscan (Goldstein 2015) score > 0.5 for image artifact rejection. We also removed objects listed as variable stars in the GAIA DR2 catalog, and visually inspected the images. We find one candidate counterpart: TNS ID | DESNU NAME | RA | DEC | MAG_r 2021aaue | desnu-936243-a | 61.225968 | -4.434543 | 23.41 +/- 0.16 The candidate is positioned on the border of the 90% localization area, assuming a 2D Gaussian distribution. Preliminary analysis of the candidate light curve indicates a steady fading rate of 0.05 mag per day starting two nights after the neutrino trigger. Analysis of this candidate is ongoing and spectroscopic characterization is encouraged. We note that Morgan et al. 2019 (arXiv: 1907.07193) found a detection of 1 temporally coincident core-collapse SN to be consistent with the expectation value of the supernova background in observations with similar depth and area covered. We also report the non-detection of the JCMT/SCUBA2 source from GCN 30882 in our observations.