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GCN Circular 30771

Subject
GRB 210905A: VLT/X-shooter spectroscopic redshift
Date
2021-09-05T06:06:26Z (3 years ago)
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.
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