GCN 44383: GRB 260421B: LCO optical counterpart detection

I. Pérez-Fournon (IAC and ULL), F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, 
I. Correa-Plasencia, E. Lekaroz-Urriza, M. Quintana-Ansaldo (all ULL), A. López-Oramas (IAC and ULL), and D. Aguado (IAC and ULL)

Following the detection of the long GRB 260421B by Fermi GBM (Fermi GBM team, GCN  circ. 44360; Preis & Greiner, GCN circ. 44361; and Mukherjee, GCN circ. 44382) and by SVOM ECLAIRs, GBM, and MXT (Zhao et al., GCN circ. 44362), we observed the field with one of the two Las Cumbres Observatory (LCO) 1-m telescopes equipped with Sinistro cameras located at the LCO node at Siding Spring Observatory near Coonabarabran, New South Wales, Australia. The observation, a single exposure of 200 sec in the SDSS r' filter, started on 2026-04-21 at 13:40:22 UT, about 9.45 hours after the Fermi trigger. The optical counterpart, first reported by He et al. (GCN circ. 44363), is detected in our image  with an AB magnitude of  21.55 +/- 0.21, calibrated against PanSTARRS-1 DR2 stars and not corrected for Galactic extinction. Other optical detections or upper limits have been reported by Sánchez Álvarez et al. (GCN circ. 44364), Turpin et  al. (GCN circ. 44365), Wu et al. (GCN circ. 44366), Lipunov et al. (GCN circ. 44367), Schneider et al. (GCN circ. 44368, redshift of z = 2.115), and Li et al. (GCN circ. 44377).
 
This work makes use of observations from the Las Cumbres Observatory global telescope network (LCO program IAC2026A-011, SGLF and Superluminous Supernovae surveys).

This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5).