TITLE: GCN CIRCULAR NUMBER: 39564 SUBJECT: EP250302a: VLT/MUSE optical redshift z = 1.131 DATE: 25/03/03 09:11:58 GMT FROM: Daniele B. Malesani at IMAPP / Radboud University d.malesani@astro.ru.nl

L. Izzo (INAF/OACn and DARK/NBI), R. A. J. Eyles-Ferris (Univ. Leicester), D. Xu (NAOC), A. Saccardi (CEA/Irfu), B. Schneider (LAM), D. B. Malesani (DAWN/NBI and Radboud Univ.), J. T. Palmerio (CEA/Irfu), N. R. Tanvir (Univ. Leicester), P. G. Jonker (Radboud Univ.), S. D. Vergani (CNRS - Paris Observatory/LUX), A. J. Levan (Radboud Univ. and Warwick Univ.), report on behalf of the Stargate collaboration:

We observed the optical/NIR counterpart (Zhu et al., GCN 39550; Busmann et al., GCN 39551; Leonini et al., GCN 39553; Wu et al., GCN 39555; Xin et al., GCN 39558; Adami et al., GCN 39560; Becerra et al., GCN 39562) of the fast X-ray transient EP250302a (Dai et al., GCN 39556) using the ESO VLT UT4 (Yepun) equipped with the MUSE integral-field spectrograph.

The observation mid-epoch is 2025 March 3.193 UT (13.03 hr after the initial high-energy trigger), the total exposure time is 76.5 min, and the covered wavelength range is 4750-9300 AA.

The optical counterpart (Zhu et al., GCN 39550) is well detected in the wavelength-stacked “white light” image, as well as in continuum spectroscopy. We also observe several clear absorption features, which we interpret as due to Mg I (2853), Mg II (2804, 2796), and Fe II (2600, 2587, 2383, 2374, 2344), all at a common redshift z = 1.131.

Given the overall good S/N of the spectrum, and the fact that there are no features left un-identified, we consider z = 1.131 to be most likely the redshift of EP250302a.

Our spectra do not cover the blue region where the z = 0.549 features reported by Yang et al. (GCN 39561) would fall, so we cannot confirm the presence of that system, which could be an intervening absorber. No emission lines at z = 0.549 appear in our data.

Thanks to the large MUSE field of view (1' x 1'), further to the optical transient, two neighboring bright galaxies were covered in our observation, including the one noted by Busmann et al. (GCN 39551) and Yang et al. (GCN 39561). For them, we measure the following redshifts:

1) RA = 11:18:05.26, Dec = +33:35:05.6: z = 0.127 from Na I D, Mg b, Hbeta, G band absorption; 2) RA = 11:18:01.03, Dec = +33:35:30.8: z = 0.127 from Halpha, [N II] and [S II] emission.

We conclude that these objects are not physically associated with EP250302a.

We thank Malte Busmann and Brendan O’Connor for sharing a finding chart of the field, and the ESO observing staff in Paranal, in particular Claudia Cid, Monika Petr-Gotzens, and Jonathan Smoker, for efficiently carrying out these observations.

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