VLT/X-shooter classification of AT 2023tbf (GOTO23aky) as a dwarf nova

ATel #16250; A. Saccardi (GEPI/Paris Obs.), L. Izzo (INAF/OACn and DARK/NBI), J. T. Palmerio (GEPI/Paris Obs. and IAP), D. B. Malesani (Radboud Univ. and DAWN/NBI), P. D'Avanzo (INAF-OABr), V. D’Elia (ASI-SSDC and 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)
on 21 Sep 2023; 16:41 UT
Credential Certification: Daniele Malesani (d.malesani@astro.ru.nl)

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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 34738). 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 Å 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-β to H-η, 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.