TITLE: GCN CIRCULAR NUMBER: 22845 SUBJECT: GRB 180624A: LBT/MODS redshift measurement DATE: 18/06/25 18:25:52 GMT FROM: Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst A. Rossi (INAF-OAS), A. Gargiulo (INAF/IASF Milano), D. B. Malesani (DAWN/NBI and DARK/NBI), S. D. Vergani (GEPI/Obs. Paris), V. D'Elia (ASDC), S. Campana (INAF-OAB), report on behalf of the CIBO collaboration: We observed the field of GRB 180624A (Swift trigger 844192; Gibson et al., GCN 22832) with the Multi-Object Double Spectrographs (MODS) instrument mounted on the 2x8.4-m LBT telescope (Mt Graham, AZ, USA). Images were obtained starting at 10:15 UT on 2018-06-25, 20.5 hr after the GRB trigger. Inspection of the 60 s acquisition image reveals a point source within the Swift/XRT enhanced error circle (Evans et al., GCN 22835), at coordinates: RA(J2000) = 21:12:23.378 Dec(J2000)= -02:20:17.30 with an uncertainty of 0.2" in each coordinate, which is consistent with the source reported by Guidorzi et al. (GCN 22839). The source has magnitude r' = 20.6 +- 0.1 (AB), calibrated against r'-band SDSS field stars, and it is not visible in Pan-STARRS images, thus clearly singling it out as the afterglow. Spectroscopy of the source was obtained for a total of 2x600 s, covering the wavelength range 3200-10000 AA. A broad trough is well visible centered around 4700 AA, which we identify as a DLA feature at z ~ 2.86. Detection of several absorption features, which we interpret as due to, among others, Si II, C IV, Al II, Al III, Fe II, allows to precisely measure the redshift to be z = 2.855, which we consider to be the redshift of GRB 180624A. We also note the presence of a close-by and fainter galaxy, which is also visible in the Pan-STARRS images, 2.5" NW of the afterglow, at position: RA(J2000) = 21:12:23.244 Dec(J2000)= -02:20:15.76 possibly consistent with the extended source reported by Schady (GCN 22840). Given the measured redshift, we do not consider this object as related to the GRB, as it would have an unreasonable brightness and large separation at z = 2.8. No correction for the expected Galactic foreground extinction, corresponding to a reddening of E_(B-V) = 0.05 mag in the direction of the burst (Schlafly & Finkbeiner 2011), has been applied. We acknowledge the excellent support from the LBT-INAF staff, particularly Olga Kuhn and Steve Allanson in obtaining these observations.