TITLE: GCN CIRCULAR NUMBER: 40043 SUBJECT: GRB 250403A: LCO optical counterpart detection DATE: 25/04/04 10:34:53 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias ipf@iac.es
I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, I. Correa-Plasencia (ULL), and A. López-Oramas (IAC and ULL)
We report on Las Cumbres Observatory Global Telescope network (LCOGT) observations of GRB 250403A, detected by Fermi GBM (Fermi GBM team, GCN circ. 40025), SVOM/ECLAIRs (Julakanti et al., GCN circ. 40026), and Swift XRT (Dichiara et al., GCN circ. 40037)
We observed the field of GRB 250403A with two LCOGT 1-m telescopes, equipped with Sinistro cameras, located at the LCOGT node at Sutherland Observatory (South Africa) in the SDSS r, i, g, and u filters, starting at 2025-04-03 22:00:46 UTC, about 6.76 hours after the Fermi and SVOM triggers. A source is clearly detected in the four filters at the position of the optical counterpart first reported by GOTO (Starling et al., GCN circ. 40028) and detected also by WU et al. (GCN circ. 40029), Du et al. (GCN circ. 40031), Li et al. (GCN circ. 40032), Julakanti et al. (GCN circ. 40034), Ghosh et al. (GCN circ. 40039), and Jiang et al. (GCN circ. 40041).
The r, i, and g images were calibrated against Pan-STARRS DR2 stars and the u image was calibrated against stars from the Gaia DR3 synthetic photometry catalog generated from the Gaia BP/RP mean spectra (Gaia Collaboration, 2022). We measure the following preliminary magnitudes, that are not corrected for Galactic extinction:
Date | UT start | mag | error | filter | exposure time (sec) --------------------------------------------------------------------- 2025-04-03 22:00:46 20.34 0.08 r 300 2025-04-03 23:00:56 20.39 0.08 i 300 2025-04-03 23:08:52 20.97 0.08 g 300 2025-04-04 00:14:01 21.44 0.16 u 600
We note that a faint source, morphologically classified as point-like, is detected in the Legacy Surveys (LS) DR10 images at the position of the optical counterpart of GRB 250403A, with coordinates RA, Dec (J2000) = 194.6157, -24.3795, and magnitudes in the LS DR10 catalog of g = 24.16, r = 24.17, i = 23.88, and z = 24.34.
This work makes use of observations from the Las Cumbres Observatory global telescope network (LCOGT observing programme IAC2025A-009, SGLF).
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