TITLE:   GCN CIRCULAR
NUMBER:  37858
SUBJECT: EP241021a: GTC OSIRIS+ spectroscopy of the optical counterpart
DATE:    24/10/25 00:23:54 GMT
FROM:    Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>

I. Pérez-Fournon (IAC and ULL), N.C. Sun (UCAS), W. Li (NAOC), F. Poidevin (IAC and ULL),
D.S. Aguado (IAC and ULL), Y. Wang (NAOC), A. Cabrera-Lavers (GRANTECAN and IAC), J.A. Acosta-Pulido, A. López-Oramas, D. Nespral (all IAC and ULL), Z. Niu (NAOC), and F. Acero (CEA Saclay and IAC)

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.