TITLE: GCN CIRCULAR NUMBER: 42871 SUBJECT: GRB 251126A: OHP/T193 optical observations and tentative spectroscopic redshift DATE: 25/11/28 14:22:37 GMT FROM: Christophe Adami at LAM <christophe.adami@lam.fr> C. Adami (LAM/Pytheas/AMU), A. Saccardi (CEA/Irfu), B. Schneider (LAM), M. Dennefeld (IAP), E. Le Floc'h (CEA-Saclay) report on behalf of the MISTRAL GRB collaboration: We carried out observations of the Swift GRB 251126A (Caputo et al., GCN 42843) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 3 exposures of 3 min in the r-band at a midtime of 2025-11-26 22:58 UT corresponding to T-T0 = 3.79 hours. The afterglow reported by Swain et al. (GCN 42844), Lipunov et al. (GCN 42847), Fu et al. (GCN 42848), Reguitti et al. (GCN 42849), Broens et al. (GCN Circ. 42850), Angulo et al. (GCN 42855), Pulido-Torres et al. (GCN 42856), Gupta et al. (GCN 42857), Cotter et al. (GCN 42858), Seki et al. (GCN 42859), Volnova et al. (GCN 42861), Breeveld and Caputo (GCN 42863), Gupta et al. (GCN 42864), Burkhonov et al. (GCN 42866), Maksut et al. (GCN 42868) and Moskvitin et al. (GCN 42870) is very well detected in r’ with a preliminary magnitude of : r’ = 20.1 +/- 0.13 We also obtained 1 exposure of 600s in the g-band at a midtime of 2025-11-27 00:38 UT. The afterglow is only barely visible with an upper magnitude limit of : g’ > 21.9 (S/N=3) The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the STDWeb/STDPipe tools (Karpov 2025), is in the AB system, and is not corrected for Galactic extinction. In addition, a spectrum covering the wavelength range 4200-8000 AA was secured with the MISTRAL spectroscopic blue setting, starting at 23:10 UT (midtime = T0+4.5h) with a total exposure time of 2x1800 s. In a preliminary reduction of the spectrum, a faint trace is detected down to ~5600 AA (in the flux calibrated spectrum), where a spectral break is observed despite the low S/N. If due to Lyman alpha, this would correspond to a redshift of z~3.6. Our spectroscopic value is in agreement with the photometric redshift reported by Angulo et al. (GCN 42855). Given the low S/N over the continuum, we caution that this redshift solution should be considered tentative. We acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Stephane Favard, and the SOPHIE observer Felix Savoure. View this GCN Circular online at https://gcn.nasa.gov/circulars/42871. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...