TITLE: GCN CIRCULAR NUMBER: 35832 SUBJECT: GRB 240225B: VLT/X-shooter redshift DATE: 24/02/29 10:36:12 GMT FROM: Andrea Rossi at INAF <andrea.rossi@inaf.it> B. Schneider (MIT), G. Pugliese (API), A. Rossi (INAF/OAS), J. Palmerio (GEPI/Obs. de Paris & IAP), D. B. Malesani (DAWN/NBI and Radboud Univ.), Z. Zhu (NAOC), D. Xu (NAOC), B. P. Gompertz (Birmingham), A. Saccardi (GEPI/Obs. de Paris) report on behalf of the Stargate collaboration: We observed the field of the MAXI/GSC GRB 240225B (Nakajima et al., GCN 35796; see also Joshi et al. 2024, GCN 35798; Kawakubo et al., GCN 35811) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 1200 s each. The observation mid-time is 02:29:56 UT on 2024 Feb 29 (~3.3 days after the MAXI trigger). In grz images taken with the acquisition camera on Feb 29 01:37:53 UT, we clearly detect the optical afterglow (Gompertz et al., GCN 35805; Liu et al., GCN 35812; Malesani et al., GCN 35819; Wise et al., GCN 35820; Gompertz et al., GCN 35824; Pankov et al., GCN 35826; Moskvitin et al., GCN 35828; Mo et al., GCN 35829; Ror et al., GCN 35830; Sasada et al., GCN 35831), for which we measure a preliminary AB magnitude r ~ 21 (the absolute calibration is uncertain due to the low number of Pan-STARRS calibrators in our small FoV). In a preliminary reduction of the spectra, we clearly detect a continuum over the entire wavelength range. From detection of multiple absorption features, which we interpret as being due to Fe II, Mg II, AlIII, Ca II, and Fe II*, we infer a common redshift of z = 0.946. We conclude this is the redshift of the burst. We also detect emission lines ([O II] and [O III] doublets, and H_alpha) at a consistent redshift, which we interpret as being due to the GRB host galaxy. As noted by Gompertz & Malesani (GCN 35824), the host galaxy is also detected in the Legacy Survey with r ~ 24.2 and a photometric redshift z ~ 0.9, which is consistent with the value we measured. We acknowledge expert support from the ESO staff in Paranal, in particular Francesca Lucertini, Israel Blanchard and Thomas Rivinius. View this GCN Circular online at https://gcn.nasa.gov/circulars/35832. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...