TITLE:   GCN CIRCULAR
NUMBER:  41490
SUBJECT: EP250821a: VLT X-shooter spectroscopic redshift z = 0.577
DATE:    25/08/22 09:01:58 GMT
FROM:    nrt3@le.ac.uk

J. An (NAOC), V. Abril-Melgarejo (LUX-Paris Obs.), R. A. J. Eyles-Ferris (U. Leicester), D. Xu (NAOC), A. L. Thakur (INAF-IAPS), A. J. Levan (Radboud U.), P. G. Jonker (Radboud U.), D. B. Malesani (DAWN/NBI and Radboud), S. R. Oates (U. Lancaster), G. Corcoran (UCD), A. Martin-Carrillo (UCD), V. D’Elia (ASI/SSDC), N. R. Tanvir (U. Leicester) report on behalf of the Stargate collaboration:

We observed the location of the optical counterpart and coincident likely host galaxy (Dichiara et al., GCN 41469; Li et al. GCN 41470; Yao et al. GCN 41485) of the X-ray transient EP250821a detected by Einstein Probe (Hu et al. GCN 41459; Liang et al., GCN 41467), using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.

Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 600 s each. Observations started on 2025-08-22 at approximately 04:10 UT (20.9 hr after the EP trigger). In a preliminary reduction of the spectra, we detect absorption lines of Fe II (fine structure), Mg II, Ca II, Na I, and emission lines of [O II], [O III], H alpha, H beta, H gamma, [N II], [S II], from which we infer a redshift of z = 0.577. We note that our spectroscopic redshift measurement is roughly consistent with the photometric redshift value of 0.64 +/- 0.06 provided for the catalogued object in Legacy Survey that was noted by Li et al. (GCN 41470).

From the X-shooter acquisition imaging we estimate a magnitude of r = 20.9 at 2025-08-22 03:41 UT, approximately 20.4 hr post-trigger. Given the Legacy Survey host magnitude of r = 21.79 +/- 0.02, this suggests a significant transient contribution at this epoch.

We acknowledge excellent support from the ESO observing staff in Paranal, in particular Celia Desgrange, Lorena Faundez, Camila de Sa Freitas. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).