TITLE: GCN CIRCULAR NUMBER: 42276 SUBJECT: GRB 251013C: OHP/T193 further optical observations and possible color evolution DATE: 25/10/15 00:11:23 GMT FROM: Christophe Adami at LAM <christophe.adami@lam.fr> 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: We 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). The afterglow is well detected in the two bands and we measured the following preliminary magnitudes: g’ = 21.11 +/- 0.10 mag (AB) r’ = 21.29 +/- 0.07 mag (AB) This 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. The 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). We 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 View this GCN Circular online at https://gcn.nasa.gov/circulars/42276. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...