TITLE:   GCN CIRCULAR
NUMBER:  40993
SUBJECT: EP250704a / GRB 250704B: end of optical plateau phase and rapid fading of the counterpart
DATE:    25/07/06 20:50:24 GMT
FROM:    Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>

J. An (NAOC), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), X. Liu (NAOC), A. K. Haris-Kiss (Helsinki Univ.), M. Korpi-Lagg (Aalto Univ.), S. Wedemeyer (Univ. Oslo), A. Casasbuenas Corral (IAC), report on behalf of a larger collaboration:

We performed further observations of the field of the EP250704a (Li et al., GCN 40941) also detected by SVOM (GRB 250704B; Wang et al., GCN 40940), using the StanCam camera mounted on the Nordic Optical Telescope (NOT). We obtained 3x300 s exposures in the Bessel-R and I bands, starting at 2025-07-05 at 23:49:00 UT (~39.54 hr after the EP trigger). 

In our stacked image, the optical afterglow detected by COLIBRÍ (Schneider et al., GCN 40942), BOOTES-5 (Perez-Garcia et al., GCN 40957), Pan-STARRS (Gillanders et al., GCN 40958), SVOM/VT (Xin et al., GCN 40960), GROWTH-India (Mohan et al., GCN 40962), SAO RAS (Moskvitin et al., GCN 40963), JinShan (Liu et al., GCN 40965), VLT/X-shooter (An et al., GCN 40966), VLT/HAWK-I (Yang et al., GCN 40970), NOT (Martin-Carrillo et al., GCN 40971), FTW (Busmann et al., GCN 40974) and GSP (Li et al., GCN 40975), the optical counterpart is detected at a preliminary magnitude of:

i = 21.56 +/- 0.14 (AB),

calibrated against nearby Pan-STARRS sources and not corrected for Galactic extinction. 

Compared to our previous measurement (Martin-Carrillo et al., GCN 40971), the new magnitude indicates a rapid decay (effective power-law index ~ 1.8), in stark contrast with the flat / rising behavior observed over the first ~16 hr after the trigger (e.g., Gillanders et al., GCN 40958; Yang et al., GCN 40970; Martin-Carrillo et al., GCN 40971; Busmann et al., GCN 40974).