GCN Circular 41347
Subject
GRB 250812A: NTT spectroscopic redshift z = 2.57
Event
Date
2025-08-13T17:10:45Z (a year ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
J. N. D. van Dalen (Radboud), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), L. Cotter (UCD), M. Fraser (UCD), A. Martin-Carrillo (UCD), A. Aryan (NCU), M. Dennefeld (IAP), G. Pignata (IAI), P. D'Avanzo (INAF/OAB), A. de Ugarte Postigo (LAM), J. Anderson (ESO), T. Müller Bravo (Southampton), T.-W. Chen (NCU), M. Gromadzki (Warsaw), C. Inserra (Cardiff), E. Kankare (Turku), M. Nicholl (QUB), O. Yaron (Weizmann), D. Young (QUB), E. Zimmerman (Weizmann) report on behalf of the ePESSTO+ collaboration:
We observed the optical counterpart of the SVOM/ECLAIRs GRB 250812A (Xin et al., GCN 41322) using the ESO New Technology Telescope (NTT) located in La Silla (Chile), equipped with the EFOSC2 camera. A total of 360 s imaging was secured in Gunn r starting at 2025-08-13 04:28:26 (1.074 days after the trigger).
The optical afterglow reported by He et al. (GCN 41324), Xin et al. (GCN 41326), Rakotondrainibe et al. (GCN 41328), Freeberg et al. (GCN 41331), Siegel et al. (GCN 41344), and Santos et al. (GCN 41346) is well detected in our images with a magnitude r = 19.7 +/- 0.1 AB (calibrated against nearby stars from the Legacy Sky Survey catalog) and not corrected for Galactic extinction.
A 3000 s spectroscopic observation was subsequently obtained starting at 2025-08-13 06:30:20 (1.16 days after the trigger), again using EFOSC2 equipped with grism 13. In a preliminary reduction of the spectrum, the source is well detected across the full spectral range from 3985 to 9315 AA. While the S/N is too low to confidently identify individual metal absorption features, a strong absorption trough is clearly visible at ~4344 AA, which we interpret as a DLA. From this DLA feature we infer a redshift of 2.57 which we suggest to be the redshift of GRB 250812A.
We acknowledge expert support from the NTT operator, Pablo Arias.