TITLE: GCN CIRCULAR NUMBER: 43020 SUBJECT: GRB 251205A: ZTF and ATLAS optical afterglow detections DATE: 25/12/07 09:28:50 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es> I. Pérez-Fournon (IAC and ULL), J. Basurto Merino, P.G. Berdayes, A. Caballero-Almagro, A. Cerón, M. Contreras, F. Díaz-Segado, T. Ferrer-Laviña, B. Gandolfi, V. Ghiraldo, J. Hernández Fung, L. Juliá-Maroto, E. Lekaroz-Urriza, M. Manzano García, E. Mejía-Martínez, J. Prieto Polo, M. Pulido-Torres, M. Quintana-Ansaldo, A. Schenone-Zanuzzi, A. Selezneva, T. Tundidor Rodríguez, E. Urquijo-Rodríguez (all ULL) and M. Abdul-Masih (IAC and ULL). We report on ZTF and ATLAS detections of the optical afterglow of the Swift GRB 251205A (Lanava et al., GCN Circ. 43005; Beardmore et al., GCN Circ. 43007; Dichiara et al., GCN Circ. 43017; and Krimm et al., GCN Circ. 43018) at a spectroscopic redshift of z = 1.100 (de Ugarte Postigo et al., GCN Circ. 43008). Both ZTF and ATLAS detected the optical afterglow of GRB 251205A during standard observations of these time-domain surveys. ZTF first detected this transient (ZTF25acgbeof) on 2025-12-06 at 12:16:12 UT at a ZTF g-band magnitude of g-ZTF = 19.168 +/- 0.169. It was reported to the Transient Name Server by Rehemtulla et al. (AT 2025afws, TNS Astronomical Transient Report No. 279483). Using the ATLAS forced-photometry server (Shingles et al. 2021, Transient Name Server AstroNote 2021-7), we find three ATLAS detections, as reported in the following table of ZTF and ATLAS photometry: MJD mag (AB) error survey filter 61015.511250 19.168 0.169 ZTF g 61015.543854 18.788 0.105 ZTF r 61015.625035 19.062 0.279 ATLAS orange 61015.628805 19.225 0.321 ATLAS orange 61015.646040 18.889 0.229 ATLAS orange These ZTF and ATLAS detections are consistent with other reported UV and optical detections: Lanava et al. (GCN Circ. 43005), Lipunov et al. (GCN Circ. 43006), de Ugarte Postigo et al. (GCN Circ. 43008), Malesani et al. (GCN Circ. 43009), O'Neill et al. (GCN Circ. 43010), Mandarakas et al. (GCN Circ. 43011), Pankov et al. (GCN Circ. 43012), Bochenek and Perley (GCN Circ. 43015), Klingler et al. (GCN Circ. 43016), and Patil et al. (GCN Circ. 43019). The ZTF coordinates of ZTF25acgbeof / AT 2025afws are RA (J2000) = 13:21:05.324, Dec (J2000) = 29:55:43.64, consistent within the errors with the optical and X-ray positions of the GRB 251205A afterglow reported by Lanava et al. (GCN Circ. 43005), Lipunov et al. (GCN Circ. 43006), Beardmore et al. (GCN Circ. 43007), de Ugarte Postigo et al. (GCN Circ. 43008), and Klingler et al. (GCN Circ. 43016). This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5). View this GCN Circular online at https://gcn.nasa.gov/circulars/43020. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...