GCN Circular 43184
Subject
EP251220a: VLT/FORS2 spectroscopic redshift z = 2.375
Event
Date
2025-12-22T16:47:08Z (9 months ago)
From
Aishwarya L Thakur at INAF-IAPS, Rome <aishth@outlook.com>
Via
Web form
J. An (NAOC), A. Saccardi (CEA/Irfu), R. A. J. Eyles-Ferris (U. Leicester), D. B. Malesani (DAWN/NBI and Radboud), M. Ferro (INAF-OABr), A. P. C. van Hoof (Radboud), A. L. Thakur (INAF-IAPS), D. Xu (NAOC), report on behalf of the Stargate collaboration:
We observed the optical counterpart of EP251220a (Sánchez Álvarez et al., GCN 43167; Rakotondrainibe et al., GCN 43170; Busmann et al., GCN 43181; van Hoof et al., GCN 43182) detected by the Einstein Probe (Liu et al., GCN 43162) using the ESO VLT UT1 (Antu) equipped with the FORS2 spectrograph. Our observation started at 01:02:04 UT on 2025 December 22 (40.6 hr after the EP trigger). A series of spectra was obtained with the 300V grism covering the range 3500-6500 AA and consisted of 4 exposures of 600 s each.
In the acquisition image (40.46 hr after trigger), we measure for the transient a magnitude r = 21.80 +- 0.05 (AB), calibrated against nearby stars from the Pan-STARRS catalogue, and not corrected for Galactic extinction. This value indicates little decay compared to the other reported optical observations.
In a preliminary reduction of the data, we identify multiple absorption features, including Lyα broad trough likely due to H I, as well as Si II, C II, and Al II, at a common redshift of 2.375, which we thus suggest to be the redshift for this transient. The COLIBRÍ redshift upper limit (Sánchez Álvarez et al., GCN 43167) is thus fully consistent with our spectroscopic measurement.
We acknowledge expert support from the ESO observing staff in Paranal, particularly Juan Carlos Olivares and Maria Jose Rain. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).