TITLE:   GCN CIRCULAR
NUMBER:  45868
SUBJECT: EP261006a / AT 2026aetp: VLT/X-shooter spectroscopic redshift z = 0.1853
DATE:    26/10/08 21:49:44 GMT
FROM:    Gregory Corcoran  at University College Dublin  <gregory.corcoran@ucdconnect.ie>

G. Corcoran (UCD), A. Saccardi (CEA/Irfu), J. An (NAOC), A. J. Levan (Radboud and Warwick), R. A. J. Eyles-Ferris (U. Leicester), D. Xu (NAOC), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), report on behalf of the Stargate Collaboration:

We observed the optical counterpart AT 2026aetp (Eyles-Ferris et al., GCN 45831; Wu et al., GCN 45852; Bochenek et al., GCN 45853; Chen et al., GCN 45854) of the Einstein Probe fast X-ray transient EP261006a (Lao et al., GCN 45826) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.

A spectrum covering the wavelength range 3000-21000 AA was secured, with a total exposure time of 3x900 s in each of the UVB, VIS, and NIR arms. Our observations started on 2026 October 8 at 05:20:28 UT (1.48 days after the EP-WXT trigger).

In a preliminary reduction of the spectra, we detect a clear blue continuum over the entire wavelength range, consistent with the spectrum described by Jiang et al. (GCN 45833). From the detection of multiple narrow absorption features, consistent with Mg II, Mg I, and Ca II, we infer a redshift of z = 0.1853 (in excellent agreement with the Legacy Survey DR 10 photometric redshift of z = 0.1954 +/- 0.0109).

While the blue continuum is consistent with a tidal disruption event origin, we do not detect any significant emission features that would allow us to confirm this classification or propose an alternative one. Further photometric and spectroscopic observations are encouraged.

We acknowledge excellent support from the ESO staff in Paranal, in particular Celia Desgrange and Felipe Gaete.