EP241021a: GTC OSIRIS+ spectroscopy of the optical counterpart

We report on GTC OSIRIS+ spectroscopy of the fast X-ray transient EP241021a, that was discovered by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Hu et al., GCN 37834) and detected also by the EP Follow-up X-ray Telescope (FXT) (Wang et al., GCN 37848). We observed the optical counterpart of EP241021a reported by Fu et al. (GCN 37840) with the OSIRIS+ instrument of the Gran Telescopio Canarias (GTC) and its R500R grism. Three spectra of 1800 sec each were obtained starting at 2024-10-24T01:28:47 UT (about 2.848 days after the trigger) in excellent observing conditions. The combined reduced spectrum shows a high S/N continuum and narrow emission lines from the likely host galaxy of [O II] 3727 AA , Hbeta, and [O III] 4959, 5007 AA at a redshift of z = 0.75, in agreement with the results of Pugliese et al. (GCN 37852) using VLT/FORS2 at shorter wavelengths. The optical counterpart has also been detected by other teams in different imaging programmes (Fu et al., GCN 37842; Li et al., GCN 37844; Ror et al., GCN 37845; Li et al., GCN 37846; Zheng et al., GCN 37849; and Moskvitin and Spiridonova, GCN 37850). This EP transient has not been detected in Fermi-GBM observations (Burns et al., GCN 37855). Based on observations made with the Gran Telescopio Canarias (GTC), installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma. We acknowledge the excellent support from the GTC observing staff.