GCN Circular 37852
Subject
EP241021a: VLT/FORS2 redshift z = 0.75
Date
2024-10-24T10:21:30Z (3 months ago)
From
Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>
Via
Web form
G. Pugliese (API-UvA), D. Xu (NAOC), L. Izzo (INAF-OACn & DARK/NBI), A. J. Levan (Radboud Univ. and Warwick Univ.), Z.P. Zhu (NAOC), D. B. Malesani (DAWN/NBI and Radboud Univ.), V. D'Elia (ASI/SSDC and INAF/OAR), P. G. Jonker (Radboud Univ.), A. Martin-Carrillo (UCD), B. P. Gompertz (Birmingham), A. Rossi (INAF/OAS), A. Saccardi (GEPI/Obs. de Paris) report on behalf of the Stargate collaboration:
We observed the optical counterpart of the fast X-ray transient EP 241021a (Hu et al., GCN 37834) reported by Fu et al. (GCN 37840, 37842), Li et al. (GCN 37844, 37846), Ror et al. (GCN 37845), Zheng et al. (GCN 37849), Moskvitin & Spiridonova (GCN 37850). We used the FORS2 spectrograph mounted on the ESO VLT UT1 (Antu), equipped with the 300V grism and no order-sorting filter. Two spectra by 20 min each were secured, with mean epoch 2024 Oct 24.229 UT (about 3.01 days after the trigger). Seeing conditions were excellent at 0.5", as measured in the acquisition image.
In the acquisition image (taken 2.998 days after the trigger), we measure a preliminary magnitude r = 22.06 +- 0.05 (AB, calibrated against nearby objects from Pan-STARRS).
Continuum is detected at least across the range 4600-8600 AA. A strong emission line is visible at 6519 AA, which we identify as due to [O II] 3728 AA at z = 0.75. This interpretation is confirmed by the detection of a doublet in absorption at 4897 AA, which matches well Mg II at z = 0.748.
We note the persistently flat light curve of the optical counterpart (Fu et al., GCN 37840, 37842; Li et al., GCN 37844, 37846; Ror et al., GCN 37845; Moskvitin & Spiridonova, GCN 37850; our own acquisition measurement), which is unusual for a GRB afterglow over such a long time range. We encourage further photometric monitoring at all wavelengths.
We acknowledge the expert support from the ESO observing staff at Paranal, in particular Martina Baratella.