{
  "submittedHow": "web",
  "bibcode": "2025GCN.40953....1S",
  "body": "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:\n\nWe 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).\n\nIn 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):\n\nRA = 01:08:46.69\nDec = -17:19:25.87\n\nThe 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.\n\nIn 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.\n\nDue 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.\n\nWe acknowledge excellent support from the ESO observing staff in Paranal, in particular Robert Klement, Diego Parraguez, and Leonel Rivas.",
  "eventId": "GRB 250704A",
  "createdOn": 1751640997387,
  "circularId": 40953,
  "format": "text/plain",
  "subject": "GRB 250704A: VLT/X-shooter afterglow discovery and spectroscopic redshift z = 1.091",
  "submitter": "Benjamin Schneider at MIT <bschn@mit.edu>"
}