TITLE: GCN CIRCULAR NUMBER: 34753 SUBJECT: GRB 230918B: VLT/X-shooter classification of AT 2023tbf (GOTO23aky) as a dwarf nova DATE: 23/09/20 22:32:36 GMT FROM: Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl> A. Saccardi (GEPI/Paris Obs.), L. Izzo (INAF/OACn & DARK/NBI), J. T. Palmerio (GEPI/Paris Obs. & IAP), D. B. Malesani (Radboud Univ. & DAWN/NBI), P. D'Avanzo (INAF-OABr), V. D’Elia (ASI-SSDC & INAF-OAR), M. Della Valle (INAF/OAC), D. H. Hartmann (Clemson Univ.), K. E. Heintz (DAWN/NBI), P. Jakobsson (U. of Iceland), L. Kaper (U. of Amsterdam), G. Leloudas (DTU Space), A. J. Levan (Radboud Univ.), A. Martin-Carrillo (UCD), B. Schneider (MIT), S. Schulze (OKC), A. de Ugarte Postigo (OCA), and K. Wiersema (U. of Hertfordshire) report on behalf of the Stargate collaboration: We observed the transient AT 2023tbf (GOTO23aky; Gompertz et al., GCN 34738), spatially and temporally consistent with the Fermi GBM GRB 230918B (Fermi GBM team; GCN 34734). We used the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph, starting on 2023 September 20.093 UT. Our data cover the wavelength range 3000-24800 AA and consist of 4 exposures of 600 s each. In the acquisition image, taken on 2023 Sep 20.086 UT, we measure for the transient a magnitude r = 19.64 +- 0.05 AB (calibrated against a single nearby star from the SkyMapper catalog). A strong continuum is detected in the blue end of the spectrum, with a decreasing signal towards the VIS and NIR arms. The UVB shows several broad undulations, not seen in GRB afterglow spectra; in the VIS and NIR arms, the continuum is smoother. The source shows narrow absorption from Ca H and K a z = 0, as well as broad features in correspondence of the Balmer lines, at least H-beta to H-eta, all at z = 0. No features are identified at z > 0. Comparison with template spectra (e.g. Morales-Rueda & Marsh, 2002, MNRAS, 332, 814) allows us to classify AT 2023tbf as a dwarf nova. We thus conclude that AT 2023tbf is not a GRB afterglow and is not associated with GRB 230918B. We acknowledge excellent support from the observing staff at Paranal, in particular Ditte Slumstrup, Xavier Haubois, Francisco Caceres and Paulina Venegas. View this GCN Circular online at https://gcn.nasa.gov/circulars/34753. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...