TITLE: GCN CIRCULAR NUMBER: 40263 SUBJECT: GRB 250424A: LCO optical observation DATE: 25/04/27 13:05:34 GMT FROM: ankur ghosh at CAPP, University of Johannesburg ghosh.ankur1994@gmail.com

Ankur Ghosh, Soebur Razzaque (CAPP, University of Johannesburg), Alexander Moskvitin, Yulia Sotnikova (SAO RAS), Naveen Dukiya (ARIES), Rahul Gupta (NASA GSFC) on behalf of a larger collaboration.

We observed the field of the GRB 250424A triggered by Swift (Cenko et al., GCN 40224), AstroSat CZTI (Harsha et al., GCN 40231), Konus-Wind (Ridnaia et al., GCN 40243) and EIRSAT-1 GMOD (McKenna et al., GCN 40249)in B, V, r filters of the 1-meter Sinistro at the Las Cumbres Observatory Global Telescope (LCOGT) node located at Cerro Tololo Interamerican Observatory, Chille. The 1-m Sinistro telescope is equipped with a 4K x 4K CCD (FOV: 26 x 26 arcmin, scale: 0.39 arcsec/pixel).

Observations began on , starting from 2025-04-24, 17.8 hours after the GRB trigger. Observation for later epochs are still going on.

We clearly detect the optical transient (OT) reported by GCNs (Francile et al., GCN 40222; Cenko et al., GCN 40224; Brivio et al., GCN 40225; Becerra et al., GCN 40226; Saccardi et al., GCN 40228; de Wet et al., GCN 40229; Ducoin et al., GCN 40230; and D. Turpin et al., GCN 40240, Dutton et al., GCN Circ. 40241, Siegel et al., GCN 40244) in our B, V, r band images.

------------------------------------------------------------------------------------------------------ |Date| Start JD |t-T0 (hours)| |Exp (sec)| |Filter| |Magnitude| ------------------------------------------------------------------------------------------------------ 2025-04-25 2460790.52730 17.78 1 x 900 V V = 21.31+/- 0.08 2025-04-25 2460790.78105 23.87 1 x 900 B B = 22.07+/- 0.08 2025-04-25 2460790.79155 24.12 1 x 900 r r = 21.04 +/- 0.05 ------------------------------------------------------------------------------------------------------

The field was calibrated against nearby APASS stars, with magnitudes converted using Lupton (2005) equations, and has not been corrected for Galactic extinction.

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