TITLE: GCN CIRCULAR NUMBER: 43657 SUBJECT: GRB 260208A: Zwicky Transient Facility Follow-Up of a Fermi GRB (Trigger 792220053) DATE: 26/02/09 03:44:52 GMT FROM: Tomas Ahumada <tomas.ahumada@noirlab.edu> Tomas Ahumada (NOIRLab), Robert Stein (UMD), Theophile du Laz (CIT), Igor Andreoni (UNC), Michael Coughlin (UMN), Mansi Kasliwal (CIT), Shreya Anand (Stanford), report on behalf of the ZTF collaboration: We observed the localization region of the long GRB 260208A (trigger 792220053.24286 / 260208214, Fermi GBM team, GCN 43635) detected by the Gamma-Ray Burst Monitor (GBM) on the Fermi satellite with the Palomar 48 inch telescope equipped with the 47 square degree Zwicky Transient Facility (ZTF) camera. We obtained a series of g- and r-band images covering 130 square degrees beginning at 2026-02-08T10:43:17 (5.6 hours after the burst trigger time). This corresponds to 93% of the probability enclosed in the Earth-occultation corrected GRB localization map. Each exposure was 300 seconds, reaching g-band and r-band median depths of 21.2 mag and 21.2 mag respectively. The images were processed in real-time through the ZTF reduction and image subtraction pipelines at IPAC (Masci et al. 2019). We queried the ZTF alert stream using Kowalski (Duev et al. 2019) through Fritz (Coughlin et al. 2023). We required at least 2 detections separated by at least 15 minutes to select against moving objects. Furthermore, we cross-match our candidates with the Minor Planet Center to flag known asteroids, reject stellar sources (Tachibana and Miller 2018), and apply machine learning algorithms (Mahabal et al. 2019). We require that no spatially coincident ZTF alerts were issued before the detection time of the GBM trigger. We detect the afterglow candidate AT 2026ciw (Podesta et al. GCN 43640, Wortley et al. GCN 43641) as ZTF26aafswob. This is the only source that passed our filtering scheme (Ahumada et al. 2022). The most recent ZTF visit to the field was 3 days prior to the detection. The ZTF detections are reported in the following table: Time (MJD) | Filter | Magnitude 61079.44815970 | g | 19.05 ± 0.05 61079.52940970 | r | 19.11 ± 0.05 This Circular is 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 Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; and OKC, University of Stockholm, Sweden. Operations are conducted by Caltech's Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA. View this GCN Circular online at https://gcn.nasa.gov/circulars/43657. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...