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:
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.