GCN Circular 40776
Subject
GRB 250617B: VLT/X-shooter spectroscopic redshift z = 1.396
Event
Date
2025-06-18T10:49:27Z (20 days ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
G. Corcoran (UCD), M. Garnichey (LUX-Paris Obs.), B. Schneider (LAM), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Saccardi (CEA/Irfu), S. D. Vergani (LUX-Paris Obs.), report on behalf of a larger collaboration:
We observed the field of GRB 250617B detected by Swift (Page et al., GCN 40760) and Fermi (Fermi GBM team, GCN 40763; Bala & Meegan, GCN 40766), using the ESO VLT UT3 (Melipal) with the X-shooter. Our spectra cover the wavelength range 3000-25000 AA, and consist of 4 exposures of 900 s each. The observation mid time was 2025 Jun 18.375 UT (11.96 hr after the GRB).
In images taken with the acquisition camera, we clearly detect in the g, r, and z bands the optical counterpart reported by Page et al. (GCN 40760), Moskvitin et al. (GCN 40761), Jelinek et al. (GCN 40762), Grossan et al. (GCN 40771), Ma et al. (GCN 40772), Schneider et al. (GCN 40774) and Antier et al. (GCN 40775). We measure a preliminary magnitude:
r = 21.30 +/- 0.04 (AB) at mid time 11.21 hr after trigger.
The photometric calibration was performed using nearby stars from the Pan-STARRS catalog and the magnitude is not corrected for Galactic extinction.
In a preliminary reduction of the spectra, we detect a continuum over the entire covered wavelength range. From the detection of multiple absorption lines, which we interpret as Mg II doublet (2796, 2804) and Fe II, we infer a redshift of z = 1.396. At a consistent redshift, we also detect emission lines due to [O II] doublet and H-alpha from the GRB host galaxy.
We acknowledge excellent support from the ESO observing staff in Paranal.