TITLE: GCN CIRCULAR NUMBER: 39918 SUBJECT: GRB 250329A: VLT/X-shooter redshift z = 2.921 DATE: 25/03/29 08:38:33 GMT FROM: Antonio Martin-Carrillo at UCD,Space Science Group antonio.martin-carrillo@ucd.ie
B. Schneider (LAM), G. Corcoran (UCD), E. Le Floc’h (CEA), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), N. R. Tanvir (Leicester), report on behalf of the Stargate collaboration:
We observed the optical counterpart (Watson et al., GCN 39915, Wang et al. GCN 39917) of the SVOM/ECLAIRs GRB 250329A (Wang et al., GCN 39916) using the ESO/VLT UT3 (Melipal) equipped with the X-shooter spectrograph.
In a 10-s r-band exposure taken with the acquisition camera (1.58 hr after the SVOM trigger), a source is well detected at the location of the counterpart reported by Watson et al., GCN 39915 with a magnitude r = 21.9 +/- 0.1 (AB, calibrated against a single Pan-STARRS object). Our measurement is consistent with Wang et al. GCN 39917 and confirms the fast fading nature of this source and its likely association with GRB 250329A.
Our spectra, covering the wavelength range 3000-21000 AA, consist of 4 exposures of 600 s each. The observation mid time was on 2025 Mar 29.2611 UT (1.97 hr after the SVOM trigger). In a preliminary reduction of the spectra, a break is visible at ~4775 AA, which we identify as due to HI Lyman-alpha absorption. From the detection of multiple narrow absorption features, including OI, Si II, C II, Si IV, C IV, Fe II, Al II, Al III, Mg I, Ni II*, Fe II*, Mg II, Ca II, we infer a redshift of z = 2.921.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Martina Baratella, Felipe Gaete, Rob van Holstein, and Vincent Megevand. We would like to thank especially the two visitors at UT3, Helmut Wiesemeyer and Mario van den Ancker for their patience while our observation was being taken.
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