GCN Circular 40966
Subject
EP250704a / GRB 250704B: VLT/X-shooter spectroscopic redshift z = 0.661
Event
Date
2025-07-05T06:15:41Z (3 days ago)
From
J. An <jiean0813@foxmail.com>
Via
Web form
J. An (NAOC), D. B. Malesani (DAWN/NBI and Radboud), D. Xu, X. Liu, Z.P. Zhu (NAOC), B. Schneider (LAM), M. De Pasquale (Univ. of Messina), V. D’Elia (SSDC & INAF-OAR), A. L. Thakur (INAF/IAPS), D. Pieterse (Radboud), A. J. Levan (Radboud and Warwick), N. R. Tanvir (Univ. Leicester) report on behalf of the Stargate collaboration:
We observed the optical counterpart of EP250704a detected by EP/WXT (Li et al., GCN 40941), also likely by SVOM/GRM as GRB 250704B (Wang et al., GCN 40940), 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 1200 s each. The observation mid time was 2025-07-05 03:51:50.164 UT (19.58 hr after the GRB).
In the acquisition camera, the optical counterpart (first discovered by Schneider et al., GCN 40942) is well detected in the Sloan r- band, and we measure a magnitude r ~ 20.0 AB, at a mid time Jul 5.13 UT (18.8 hr after trigger), calibrated against nearby Pan-STARRS DR2 stars.
In a preliminary reduction of the spectra, we detect a continuum over the entire covered wavelength range (down to ~3100 AA at the blue end). From the detection of multiple absorption lines, which we interpret as due to the Mg II doublet, several Fe II lines and Mg I, we infer a redshift of z = 0.661. No emission lines are detected at this redshift.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Cedric Ledoux, Diego Parraguez, and Robert Klement.