GCN Circular 39809
Subject
EP250321a: VLT/X-shooter redshift z = 4.368
Date
2025-03-21T09:49:05Z (5 days ago)
From
Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>
Via
Web form
Z. P. Zhu (NAOC), B. Schneider (LAM), A. Saccardi (CEA/Irfu), D. Xu (NAOC), V. D’Elia (SSDC & INAF-OAR), A. J. Levan (Radboud and Warwick), D. B. Malesani (DAWN/NBI & Radboud), P. T. O’Brien (Leicester), J. T. Palmerio (CEA/Irfu), S. D. Vergani (CNRS, Obs. Paris/LUX), report on behalf of the Stargate collaboration:
We observed the optical and NIR counterpart (Fu et al., GCN 39804; Perez-Garcia et al., GCN 39805; Brivio et al., GCN 39807) of the Einstein Probe transient EP250321a (Hu et al., GCN 39800) using the ESO/VLT UT3 (Melipal) equipped with the X-shooter spectrograph.
Observations were conducted in rapid response mode (RRM) using the X-shooter spectrograph, and were requested as soon as we became aware of the optical counterpart. In a 10-s r-band exposure taken with the acquisition camera (102.8 min after the EP trigger), the counterpart is well detected with a magnitude r = 20.2 +/- 0.1 (AB, calibrated against a single Pan-STARRS object). Comparing with the report by Fu et al. (GCN 39804), our measurement indicates fading and confirms this object as the optical afterglow of EP250321a.
Our spectra, covering the wavelength range 3000-21000 AA, consist of 2 exposures of 600 s each. The observation mid time was 2025 Mar 21.336 UT (1.91 hr after the EP trigger).
In a preliminary reduction of the spectra, we clearly detect a continuum down to ~4930 AA, and a broad trough at ~6525 AA. These two features are consistent with Lyman limit and Lyman alpha, respectively. From the detection of a plethora of narrow absorption features, including S II, Si II, Si II*, O I, OI*, C II, C II*, Ni II, Si IV, C IV, Fe II, Fe II*, Al II, Al III, Mg II, Mg I, we infer a redshift of z = 4.368. The detection of fine-structure lines robustly confirms the physical association between this absorption system and the high-energy transient. We thus conclude that EP250321a was at z = 4.368.
We wish a happy March equinox to the whole GCN community. We acknowledge excellent support from the ESO observing staff in Paranal, in particular Cecilia Bustos, Enrico Congiu, Israel Blanchard, Julien Drevon, and Miguel Lopez.