TITLE: GCN CIRCULAR NUMBER: 33704 SUBJECT: GRB 230427A: OSIRIS+/GTC observations DATE: 23/04/28 15:05:51 GMT FROM: J. F. Agui Fernandez at IAA-CSIC feli@iaa.es

J. F. Agui Fernandez (IAA-CSIC), A. de Ugarte Postigo (OCA), C. C. Thoene (ASU-CAS), Fabricio Perez, Alvaro Tejero and A. L. Cabrera Lavers (GTC, IAC) report:

We observed the afterglow of GRB 230427A at ~ 11.3 hours after burst (Page et al. GCN 33687; Takahashi et al. GCN 33690; Hu et al. GCN  33695; Tohuvavohu et al. 33703) with OSIRIS+ at the 10.4m GTC telescope, at the Roque de los Muchachos Observatory (La Palma, Spain). Observations consisted of 3x1200 s spectroscopy with grism R1000B, covering the spectral range between 3700 and 7800 AA under good observing conditions.

From the acquisition image we detect a source at the GRB location and we measure a magnitude of r ~ 24.2 compared to PanSTARRS field stars. We detect a faint trace at GRB location above ~3950A which set a limit for the redshift of z < 3.3.