GCN Circular 22845

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
Not found
 
 
) 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
Not found
 
 
), 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
Not found
 
 
). 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
Not found
 
 
). 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.