TITLE: GCN CIRCULAR NUMBER: 40071 SUBJECT: EP250404a: LCO optical counterpart detection DATE: 25/04/05 00:52:37 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), D. Aguado, A. López-Oramas, D. Nespral (IAC and ULL), N.C. Sun (UCAS), W. Li, Y. Wang, and Z. Niu (NAOC)
We report on Las Cumbres Observatory Global Telescope network (LCOGT) observations of the X-ray transient EP250404a (Hu et al., GCN circ. 40051), that is temporally and spatially consistent with the likely long GRB 250404A, detected by Fermi/GBM (Fermi GBM team, GCN 40050; and Mukherjee et al., GCN circ. 40067).
We observed the field of EP250404a with the two LCOGT 1-m telescopes, equipped with Sinistro cameras, located at the LCOGT node at Teide Observatory (Tenerife, Spain) in the SDSS i, g, and r filters, starting at 2025-04-04 20:27:26 UTC, about 6.12 hours after the EP trigger. An uncatalogued source is clearly detected in the three images at the position of the optical counterpart first reported by Jiang et al. (GCN circ. 40052) and also detected by Perez-Garcia et al. (GCN circ. 40053), Du et al. (GCN circ. 40058), Zhu et al. (GCN circ. 40061), Konno et al. (GCN circ. 40063), Odeh et al. (GCN circ. 40064), and He et al. (GCN circ. 40069). A redshift of z = 2.63 has been reported by Zhu et al. (GCN circ. 40061).
We measure the following preliminary magnitudes, calibrated against Pan-STARRS DR2 stars, that are not corrected for Galactic extinction:
Date | UT start | mag | error | filter | exposure time (sec) --------------------------------------------------------------------- 2025-04-04 20:27:26 20.11 0.22 i 300 2025-04-04 22:01:01 20.65 0.16 g 300 2025-04-04 22:05:55 20.38 0.11 r 300
This work makes use of observations from the Las Cumbres Observatory global telescope network (LCOGT observing programme IAC2025A-009, SGLF).
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