GCN Circular 35698

H. Fausey (GWU), J. T. Palmerio (GEPI/Obs. de Paris & IAP), D. B. Malesani (DAWN/NBI & Radboud Univ.), A. Saccardi (GEPI/Obs. de Paris), L. Izzo (INAF-OACn & DARK/NBI), V. D'Elia (SSDC & INAF-OAR), J. P. U. Fynbo (DAWN/NBI), A. Martin-Carrillo (UCD, Ireland), A. J. Levan (Radboud), N. R. Tanvir (U. Leicester), B. Gompertz (Birmingham) report on behalf of the Stargate collaboration:

We observed the field of GRB 240205B detected by Fermi-GBM (Fermi GBM Team, GCN 35682
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), Swift-BAT (Moss et al., GCN 35683
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), and GECAM (Zhang et al., GCN 35689
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) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-21000 AA, and consist of 2 exposures of 600 s each. The observation starting-time was 00:21:26 UT on 2024 Feb 7 (~26 hr after the trigger). Observations were carried out in twilight, at high airmass 2.3 to 2.5.

Despite the low elevation, in a single r-band image taken with the acquisition camera, we clearly detect the optical afterglow (Lipunov et al., GCN 35684
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; Fernandez-Garcia et al., GCN 35686
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; Dutton et al., GCN 35687
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; Ferro et al., GCN 35694
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; Fernandez-Garcia et al., GCN 35696
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). We measure r' = 19.05 +/- 0.07 (AB) calibrated against two nearby SkyMapper stars.

In a preliminary reduction of the spectra, we detect a continuum down to at least 3200 AA implying z < 1.6 from the lack of damped Lyman-alpha absorption. Multiple absorption features are detectd, consistent with being due to Fe II and Mg II at a common redshift z = 0.824. While the modest S/N does not allow to identify any fine-structure lines at this redshift, due to the lack of unidentified features we consider z = 0.824 the likely redshift of GRB 240205B.

We acknowledge the excellent support from the ESO staff in Paranal in conducting such a challenging observation, in particular Boris Haeussler, Zahed Wahhaj, and Diego Parraguez.