TITLE:   GCN CIRCULAR
NUMBER:  40125
SUBJECT: GRB 250407A: LCO detection of a fading optical counterpart
DATE:    25/04/08 21:48:17 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 by Fermi GBM of the bright and likely long Gamma-Ray Burst GRB 250407A (Fermi GBM team, GCN circ. 40104; Preis and Greiner, GCN circ. 40105; and Mukherjee et al., GCN circ. 40114)
and the associated X-ray afterglow by SRG/ART-XC (Molkov et al., GCN circ. 40110), we observed the field with the two Las Cumbres Observatory Global Telescope network (LCOGT) 1-m telescopes, equipped with Sinistro cameras, located at the LCOGT node at Teide Observatory (Tenerife, Spain) in the SDSS g, r, and i filters. The observations started at 2025-04-07 21:06:04 UTC, about 5.29 hours after the Fermi trigger. We detect the optical afterglow in the r and i filters, but not in the g filter, at the position reported by Zheng and Filippenko (GCN circ. 40116) and Ducoin et al. (GCN circ. 40117), that is consistent within the errors with the SRG/ART-XC (Molkov et al., GCN circ. 40110), Swift-XRT (Dichiara et al., GCN circ. 40113) and EP-WXT (Zheng et al., GCN circ. 40119) X-ray afterglow positions. GRB 250407A has also been detected by SVOM/GRM (Wang et al., GCN circ. 40120) and Konus-Wind (Frederiks et al., GCN circ. 40121).
  
A fading of the optical brightness between our LCOGT observations and those of KAIT (Zheng and Filippenko, GCN circ. 40116) and COLIBRÍ/DDRAGO (Ducoin et al., GCN circ. 40117) is detected.
We measure the following preliminary r and i magnitudes and limiting (5-sigma) g-band magnitude, 
calibrated against Pan-STARRS DR2 stars, that are not corrected for Galactic extinction:

Date       |  UT start |  mag  | error | filter | exposure time (sec)  | limiting magnitude (5 sigma)
------------------------------------------------------------------------------------------------------
2025-04-07   21:06:04    19.67    0.11      r            300                        -----
2025-04-07   21:10:26    -----    ----      g            180                        20.45
2025-04-07   22:01:01    19.92    0.20      r            180                        -----
2025-04-07   22:15:32    19.53    0.19      i            180                        -----

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