{
  "createdOn": 1760487083012,
  "body": "C. Adami (LAM/Pytheas/AMU), B. Schneider (LAM), E. Le Floc'h (CEA/Irfu), N.A. Rakotondrainibe  (LAM), V. Buat (LAM/Pytheas/AMU), S. Basa (Pytheas/OHP/LAM), M. Dennefeld (IAP) report on behalf of the MISTRAL GRB collaboration:\n\nWe carried out further observations of the GRB 251013C optical and NIR afterglow (Palmerio et al., GCN 42223 ; Perez-Garcia et al., GCN 42225 ; Konno et al., GCN 42226 ; Martin-Carrillo et al., GCN 42227 ; Masi et al., GCN 42228 ; Palmerio et al., GCN 42229 ; Moskvitin et al., GCN 42230 ; Gompertz et al., GCN 42231 ; Lipunov et al., GCN 42235 ; Garnavich, GCN42240; Watson et al., GCN 42241 ; Lopez-Camara et al., GCN 42242 ; Zhang et al., GCN 42248; Brosio et al., GCN 42251; Reguitti et al., GCN 42253 ; Quintana-Ansaldo et al., GCN 42254; Maksut et al., GCN 42256; Ghosh et al., GCN 42261 ; Quadri et al., GCN 42262 ; Adami et al. GCN 42266 ; Peretto et al., GCN 42267 ; Ruocco et al., GCN 42269 ; Calapai, GCN 42275) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 3 exposures of 10min followed by 1 exposure of 5min in the r-band, corresponding to T-T0 = 27.89 hr (mid-time). In addition, we obtained 2 exposures of 10min followed by 1 exposure of 15min in the g-band, corresponding to T-T0 = 28.71 hr (mid-time).\n\nThe afterglow is well detected in the two bands and we measured the following preliminary magnitudes:\n\ng’ = 21.11 +/- 0.10 mag (AB)\nr’ = 21.29 +/- 0.07 mag (AB)\n\nThis may suggest a color evolution compared to e.g. previous g-r values reported by Palmerio et al. (GCN 42229): 0.31 +/- 0.01 at T-T0 ~ 2.05 hr. In comparison, our preliminary values seem to indicate g’-r’ = -0.18 +/- 0.12 at T-T0 ~ 28.3 hr.\n\nThe photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitudes are not corrected for Galactic extinction. We used the STDWeb/STDPipe tools (Karpov 2025).\n\nWe acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Jean Balcaen and the SOPHIE observers J.B. Salomon and J.L. Halbwachs\n\n",
  "format": "text/plain",
  "submittedHow": "web",
  "circularId": 42276,
  "subject": "GRB 251013C: OHP/T193 further optical observations and possible color evolution",
  "bibcode": "2025GCN.42276....1A",
  "submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
  "eventId": "GRB 251013C"
}