GCN Circular 45829

A. Saccardi (CEA/Irfu), B. Schneider (LAM), N. Habeeb (U. Leicester), D. B. Malesani (DAWN/NBI and Radboud), S. D. Vergani (LUX-Paris Obs.), M. Garnichey (LUX-Paris Obs.), J. An (NAOC), report on behalf of the Stargate collaboration:

We observed the optical counterpart (Zheng et al., GCN 45825
Loading...
 
 ) of the SVOM/ECLAIRs GRB 261007A (Goto et al., GCN 45824
Loading...
 
 ), also detected by EP/WXT (Wen et al., GCN 45828
Loading...
 
 ), using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. 

In a 3x15 s r-band exposure taken with the acquisition camera (1.21 hr after the SVOM trigger), the counterpart is detected with a magnitude r = 18.53 +/- 0.04 (AB), calibrated against Pan-STARRS and not corrected for Galactic extinction.

A spectrum covering the wavelength range 3000-21000 AA was secured, with a total exposure time of 2x600 s in each of the UVB, VIS, and NIR arms. Our observations started at 07:03:33 on 2026 October 7 (1.36 hr after the SVOM trigger).

In a preliminary reduction of the spectra, we clearly detect a continuum in each arm, and a broad trough at ~3770 AA, consistent with Lyman alpha. From the detection of multiple narrow absorption features, including N V, S II, Si II, Si II*, O I, O I*, Ni II, C II, C II*, C IV, Si IV, Fe II, Al II, Al III, Cr II, Zn II, Fe II*, Ni II*, Mn II, Mg II, Mg I, and Ca II, we infer a redshift of z = 2.101. The detection of fine-structure lines also confirms the physical association between this absorption system and the high-energy transient. We thus securely conclude that GRB 261007A / EP261007a is at z = 2.101.

We also detect several intervening systems, in particular a sub-DLA at z = 1.79 (Lya, Mg II, Si II, Fe II) and a system at z = 1.83 showing many absorption lines (C IV, Si IV, Fe II, Al II, Al III, Mg II, Mg I).

We acknowledge excellent support from the ESO staff in Paranal.