TITLE: GCN CIRCULAR NUMBER: 30923 SUBJECT: IceCube-210922A: DESI Observations DATE: 21/10/05 15:40:33 GMT FROM: Antonella Palmese at Fermilab <palmese@fnal.gov> Antonella Palmese (UC Berkeley), Segev BenZvi (U of Rochester), Stephen Bailey (LBNL), Tamara Davis (U of Queensland), Alex Kim (LBNL), Martin Landriau (LBNL), David Moutard (Wayne State University), Adam Myers (U of Wyoming), Peter Nugent (LBNL), Anand Raichoor (LBNL), Edward Schlafly (LLNL), David Schlegel (LBNL), Umut Demirbozan (IFAE), John Della Costa (SDSU), on behalf of the DESI Transients and Low-z Cosmology Working Group report: We observed the location of IceCube 210922A (GCN 30862) using the Dark Energy Spectroscopic Instrument (DESI), a multi-object spectrograph with 5000 fibers installed on the Mayall 4m telescope at Kitt Peak National Observatory. On the night of September 23 2021, we recorded spectra of 249 galaxies in the 90% CL of the IceCube event, which represent ~20% of all DESI Bright Galaxy Sample (BGS) galaxies in the region, a sample of galaxies complete down to r-band apparent magnitude of 19.5. Each fiber covers a circular region of 1.5” in diameter centered on the core of the galaxy. Observations started at 02:48 MST and lasted a total of 2200s. We searched for core-collapse SNe using a Convolutional Neural Network algorithm (DESITRIP - DESI Transients Identification Pipeline) optimized to identify transients on top of galaxy emission in DESI spectra. We also searched for Tidal Disruption Events based on spectral lines. Observations were deeply affected by the vicinity to the Moon, so that our searches are mostly based upon the r and z arm observations, where the mean SNR per 0.8 Ang reached in the spectra was 8 and 10 after calibration, respectively. The mean SNR was 3 in the b arm. We did not find significant evidence for the presence of an interesting optical counterpart in the DESI spectra. A list of the positions of the observed galaxies is available at https://data.desi.lbl.gov/public/transients/icecube/IC210922A_DESI210923.txt<https://data.desi.lbl.gov/desi/public/transients/icecube/IC210922A_DESI210923.txt>