TITLE: GCN CIRCULAR NUMBER: 44392 SUBJECT: EP250512a / GRB 250512B: VLT/X-shooter host galaxy identification and redshift z = 1.773 DATE: 26/04/22 15:48:38 GMT FROM: Nusrin Habeeb at University of Leicester <nh312@leicester.ac.uk> B. Schneider (LAM), A. Saccardi (CEA/Irfu), G. Corcoran (UCD), S. D. Vergani (LUX-Paris Obs.), D. B. Malesani (DAWN/NBI and Radboud), J.-L. Atteia (IRAP), N. Habeeb (U. Leicester), R. Brivio (INAF/OAB), report on behalf of the Stargate collaboration: Underneath the position of the optical/NIR afterglow (Li et al., GCN 40443; Xin et al., GCN 40444; Busmann et al., GCN 40447) of EP250512a / GRB 250512B (EP/WXT: Zhao et al., GCN 40437; Yang et al., GCN 40448; SVOM/ECLAIRs: Maggi et al., GCN 40439; Konus-Wind: Svinkin et al. GCN 40460), an object is well detected in archival imaging taken using the MegaCam instrument on the CFHT in the broad "gri" filter (Bannister et al. 2016, doi:10.3847/0004-6256/152/3/70). The object magnitude is about 24.5 AB mag. Given the spatial coincidence, this is likely the host galaxy of EP250512a / GRB 250512B. Starting at 01:53:33 UT on 2026 April 20, i.e. about 342.6 days after the burst, we obtained four spectra of 1200 s each using the X-shooter instrument installed on the ESO VLT UT3 (Melipal), covering the wavelength range 3000-21000 AA. Our spectroscopy reveals a single, confidently detected emission line at 13,889 AA. Interpreting this feature as [O III] 5007 at z ~ 1.773, we note the presence of a second, fainter emission corresponding to the expected position of the [O II] doublet. At this redshift, Halpha falls in a region of very poor transparency, and no detection is expected. We note that the precise position of the [O III] emission is also affected by atmospheric opacity, which adds some uncertainty to the redshift (of the order of 0.001). Alternative interpretations of the line at 13,889 AA are disfavored because we would then expect to see other lines in regions of good atmospheric transparency, which are absent in our spectrum. We acknowledge excellent support from the ESO staff in Paranal, in particular Maria del Mar Carretero Castrillo, Francisco Caceres, Luciano Sbordone, and Thomas Rivinius. View this GCN Circular online at https://gcn.nasa.gov/circulars/44392. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...