TITLE: GCN CIRCULAR NUMBER: 44857 SUBJECT: GRB 260604C: OPD1.6m - GRANDMA observations - detection DATE: 26/06/07 13:35:13 GMT FROM: Nelio Sasaki at NEPA-UEA <nsasaki@uea.edu.br> Nélio Sasaki (UEA-Parintins), Wagner Corradi (LNA), Leandro de Almeida (LNA), Felipe Navarete (LNA), S. Antier (OCA/IJCLAB), I. Tosta e Melo (IFN), A. Simon (TshNU of Kyiv), L. Chen (TSU), M. Coughlin (UMN) and P. Hello (IJCLAB) on behalf of Observatório do Pico dos Dias (OPD/LNA/Brazil) and the GRANDMA collaboration, report follow-up observations of GRB 260604C (Fermi Team, GCN Circ. 44822, Gotz et al., GCN Circ. 44823, Chen et al., GCN 44851; Moskvitin et al., GCN 44852) performing photometry of the GRB260604C source with SPARC4 in the Sloan bands (g,r,i,z), using exposures of 40 × 180 s in (g), 120 × 60 s in (r), 120 × 60 s in (i), and 120 × 60 s in (z). The observations began on 2026-06-07T00:18:36.036 UTC, corresponding to JD 2461198.512917. The recentered position of the source is: RA = 14:57:49.54, Dec = +28:49:03.96 The measured magnitudes of the target are: Filter | Mag (AB) g | 22.0938 ± 0.0008 (Magnitude limit: 24.6925) r | 21.7162 ± 0.0005 (Magnitude limit: 23.9245) i | 21.5773 ± 0.0008 (Magnitude limit: 25.1456) z | 21.4418 ± 0.0006 (Magnitude limit: 23.8440) Photometric calibration was performed using Gaia DR3 field stars. For each comparison star, the Gaia magnitude (G) and color (G_BP-G_RP) were transformed to Sloan-type magnitudes (g,r,i,z). The photometric zero point in each filter was then obtained from the difference between the transformed magnitudes from the catalog and the instrumental magnitudes of the comparison stars, and applied to the instrumental magnitude of the target. The transformations in (g,r,i) follow the Gaia-SDSS color relationships of Evans et al. (2018), while the magnitude in (z) was estimated using the (G-g) versus (g-z) relationship of Jordi et al. (2010). The Gaia DR3 catalog is from Gaia Collaboration et al. / Vallenari et al. (2023). The observations are performed under the P7 campaign of GRANDMA to monitor SVOM alerts and LSST alerts from explosive fast transients filtered by BOOM/Babamul and FINK. GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (kilonovacatcher.in2p3.fr). Astrophysics Center for Multimessenger studies in Europe is supported by Skyportal/ICARE (contact camille.douzet@ijclab.in2p3.fr, skyportal.io) View this GCN Circular online at https://gcn.nasa.gov/circulars/44857. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...