TITLE: GCN CIRCULAR NUMBER: 39859 SUBJECT: GRB 250322A: VLT/X-shooter redshift confirmation of the putative host galaxy DATE: 25/03/24 11:41:00 GMT FROM: Yu-Han Yang at University of Rome Tor Vergata yyang@roma2.infn.it

Yu-Han Yang (U Rome), Eleonora Troja (U Rome), Rosa Becerra (U Rome), Massine El Kabir (U Rome) report on behalf of the ERC BHianca team:

We observed the bright galaxy within the localization of GRB 250322A (Gupta et al., GCN 39835, Goad et al. GCN 39841, Martin-Carillo et al. GCN 39842) with the X-Shooter spectrograph on the ESO VLT UT3 (Melipal). Observations began at T+32.2 hours and obtained a total of 2x600s spectra at an average airmass of about 1.2 and seeing of 0.6”.

We detect a bright continuum in the VIS and NIR arms and identify multiple emission lines, including H_alpha, H_beta, NII, OII, SII, at a common redshift of 0.4215±0.0005, consistent with measurement by Fong et al. (GCN 39852). We estimated that the chance coincidence between the bright galaxy and the XRT localization is <2% (Bloom et al. 2002), supporting it as a likely host galaxy of the GRB.

We thank the staff at the VLT, for the rapid execution of these observations.

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