TITLE: GCN CIRCULAR NUMBER: 30771 SUBJECT: GRB 210905A: VLT/X-shooter spectroscopic redshift DATE: 21/09/05 06:06:26 GMT FROM: Andrea Rossi at INAF <andrea.rossi@inaf.it> N. Tanvir (Univ. Leicester), A. Rossi (INAF-OAS), D. Xu (NAOC), Z.P. Zhu (NAOC,HUST), L. Izzo (DARK/NBI), D. A. Kann (HETH/IAA-CSIC), and A.J. Levan (Radboud) report on behalf of the Stargate collaboration: We observed the location of the afterglow (Strausbaugh & Cucchiara, GCN 30769, 30770) of GRB 210905A (Sonbas et al., GCN 30765) 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 by 1200 s each. Our observations started on 2021 September 05 at 02:44 UT, ~2.53 hours after the GRB detection. In the acquisition image obtained between 2.53 and 2.73 hours after the burst, the afterglow is well detected at I and z-bands, but not in the r-band, confirming the red color already noted by Strausbaugh & Cucchiara. We measure the following AB magnitudes: r>23.5 I=20.3+-0.1 z=19.4+-0.1 Spectroscopy of the source reveals a continuum well detected in the red part of visible arm. A clear break is detected around 9000AA which we interpret as the Lyman-alpha break at z = 6.318, and explains the strong r-z color as due to Lyman dropout. Other lines such as Fe II, Al II, C IV and Si II are visible and display a double absorption profile, which is possibly due to different intervening systems at z1 = 6.312 and z2 = 6.318. We thus conclude that z=6.318 is the redshift of GRB 210905A. Further analysis is undergoing. We acknowledge support from the ESO observing staff at Paranal, in particular Israel Blanchard, Michael Abdul-Masih, and Matias Jones.