TITLE: GCN CIRCULAR NUMBER: 40953 SUBJECT: GRB 250704A: VLT/X-shooter afterglow discovery and spectroscopic redshift z = 1.091 DATE: 25/07/04 14:56:37 GMT FROM: Benjamin Schneider at MIT <bschn@mit.edu> B. Schneider (LAM), A. Saccardi (CEA/Irfu), M. Garnichey (LUX-Paris Obs.), G. Corcoran (UCD), V. D’Elia (ASI/SSDC), M. De Pasquale (Univ. Messina), D. B. Malesani (DAWN/NBI and Radboud), A. L. Thakur (INAF-IAPS), S. D. Vergani (LUX-Paris Obs.) report on behalf of the Stargate collaboration: We observed the field of GRB 250704A detected by SVOM/ECLAIRs (Cao et al., GCN 40934) with 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 600 s each. The observation mid time was 2025 Jul 4.41 UT (6.10 hr after the GRB). In images taken with the acquisition camera, we clearly detect a source inside the error circle of the Swift/XRT X-ray afterglow (Osborne et al., GCN 40950) at coordinates (J2000): RA = 01:08:46.69 Dec = -17:19:25.87 The source is well detected in the g, r, and z bands, and we measure a magnitude r = 20.48 +- 0.05 AB, at a mid time Jul 4.39 UT (5.59 hr after trigger). The photometric calibration is preliminary as it was performed using a single nearby star from the Pan-STARRS catalog. This source is significantly brighter than existing archival imaging, and we thus conclude that this is the optical afterglow of GRB 250704A. In a preliminary reduction of the spectra, we detect a continuum over the entire covered wavelength range. From detection in the blue down to 3080 AA, and the lack of hydrogen absorption, we set a redshift upper limit z < 1.53. Furthermore, we detect multiple absorption lines, which we interpret as Al II, Fe II and the Mg II doublet, at a common redshift of z = 1.091. Three intervening Mg II absorbers are also detected at z = 0.65, z = 0.71, and z = 0.91. No emission lines are observed across the spectrum. Due to the lack of detection of fine-structure transitions, z = 1.091 is strictly a lower limit to the GRB redshift, but the absence of unidentified features and the reasonable S/N over the whole continuum suggest z = 1.091 as the actual value of the redshift. We acknowledge excellent support from the ESO observing staff in Paranal, in particular Robert Klement, Diego Parraguez, and Leonel Rivas. View this GCN Circular online at https://gcn.nasa.gov/circulars/40953. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...