TITLE: GCN CIRCULAR NUMBER: 39905 SUBJECT: GRB 250327B: OHP/T193 spectroscopic observations and redshift confirmation DATE: 25/03/28 12:49:52 GMT FROM: Benjamin Schneider at MIT bschn@mit.edu
M. Dennefeld (IAP/CNRS/Sorbonne U.), B. Schneider (LAM), C. Adami (LAM/Pytheas/AMU), E. Le Floc'h (CEA/Irfu), A. Saccardi (CEA/Irfu), S. Vergani (CNRS, Obs. de Paris, LUX), S. Basa (Pytheas/OHP/LAM) report on behalf of the MISTRAL-GRB collaboration:
We observed the SVOM/ECLAIRs GRB 250327B (Bouchet et al., GCN 39888) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager in red setting.
We obtained 1x60 s in the r’, i’, z’ and Y bands starting at 22:44:30 UT on 2025-03-27 (T0+1.55 h after the trigger). The optical afterglow reported by Moskvitin et al. GCN 39889; Xin et al. GCN 39890; O’Neill et al. GCN 39891; Malesani et al. GCN 39893; Lian et al. GCN 39894; Shrestha et al. GCN 39896; Ducoin et al., GCN39897; Perley et al., GCN 39902; and Kumar et al., GCN 39904 is well detected at r = 17.25 +/- 0.05 mag (AB). The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitude is not corrected for Galactic extinction.
In addition, we obtained 3x15 min of spectroscopic observations covering from 6400 to 9950 AA starting at 2025-03-27 22:52:55 UT (T0+1.69 h). In the stacked spectrum, we clearly detect multiple absorption features, which we interpret as being due to Si II, Si II*, C IV, Fe II, Fe II*, Al II, Al III, Mg I, Ni II* at a common redshift of z = 3.038 +/- 0.005. The detection of fine-structure lines robustly associates this system to the high-energy transient. This value is also consistent with the value reported by Malesani et al. (GCN 39893).
We acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Stéphane Favard.
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