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.