TITLE: GCN CIRCULAR NUMBER: 40267 SUBJECT: EP250427a / GRB 250427A: LCO optical counterpart detection DATE: 25/04/27 16:30:09 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)

Following the detection of the fast X-ray transient EP250427a (Wang et al., GCN circ. 40257), detected also by Fermi-GBM, GRB 250427A (Ravasio et al., GCN circ. 40262), we observed the field with one of the two Las Cumbres Observatory Global Telescope network (LCOGT) 1-m telescopes, equipped with Sinistro cameras, located at the LCOGT node at McDonald Observatory (Texas) in the SDSS r filter. The observation started at 2025-04-27 10:10:47 UTC, about 6.53 hours after the EP-WXT trigger. An uncatalogued source is clearly detected at the optical counterpart position first reported by Perez-Garcia et al. (GCN circ. 40259) and with other optical and near-infrared detections reported by Liu et al. (GCN circ. 40260), Becerra et al. (GCN circ. 40261), Brivio et al. (GCN circ. 40264), Chornock et al. (redshift of z = 1.519, GCN circ. 40265), and Saccardi et al. (redshift of z = 1.520, GCN circ. 40266).

We measure the following magnitude, calibrated against Pan-STARRS DR2 stars, that is not corrected for Galactic extinction:

Date | UT start | mag | error | filter | exposure time (sec) | ------------------------------------------------------------------------ 2025-04-27 10:10:47 18.55 0.10 r 180

This work makes use of observations from the Las Cumbres Observatory global telescope network (LCOGT observing programme IAC2025A-009, SGLF).

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