M. Gonzalez-Bañuelos, M. Kopsacheili (ICE), P. D'Avanzo (INAF-OAB), D. B. Malesani (DAWN/NBI and Radboud univ.), L. Galbany, T. Müller-Bravo (ICE), C. Gutierrez (IEEC/ICE-CSIC), M. Dennefeld (IAP), A. Gkini (Stockholm), N. Ihanec (Warsaw and ING), J. Anderson (ESO), T.-W. Chen (NCU), M. Gromadzki (Warsaw), C. Inserra (Cardiff), E. Kankare (Turku), M. Nicholl (QUB), O. Yaron (Weizmann), D. Young (QUB), report on behalf of the ePESSTO+ collaboration:
We observed the candidate counterpart of the short GRB 231117A (Laha et al., GCN Circ. 35071; Navaneeth et al., GCN Circ. 35072; Cattaneo et al., GCN Circ. 35075; Svinkin et al., Gin Circ. 35079; Cheung et al. GCN Circ. 35081; Dafcikova et al. GCN Circ. 35095) under the advanced Public ESO Spectroscopic Survey for Transient Objects (ePESSTO+; see Smartt et al. 2015, A&A, 579, 40; http://www.pessto.org). The observations were performed with the ESO New Technology Telescope (NTT) at La Silla equipped with the EFOSC2 instrument in spectroscopic mode, starting on 2023 Nov 18 at 00:22:34 UT (i.e. 21.3 hr after the burst) and using grism 13 (covering the wavelength range 3985-9315 AA).
The 1.5" slit was oriented in order to cover both the optical afterglow and the host galaxy candidate positions (Yang et al., GCN Circ. 35083; Rastinejad et al., GCN 35087; Gompertz & Ackley, GCN 35088). However, the observations were carried out under poor seeing conditions (1.7") and in the acquisition image the two objects cannot be distinguished.
The spectrum shows prominebt emission features, which we identify as [O II], [O III] and Halpha at a common redshift z = 0.257, in agreement with the findings of Ahumada et al. (GCN Circ. 35093). No clear absorption features are detected.