GCN Circular 22840
Patricia Schady (MPE Garching) reports: We observed the field of GRB 180624A (Swift trigger 844192; Gibson et al., GCN #22832) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile). Observations started at 04:42 UT on 2018-06-25, 14.9 hrs after the GRB trigger. They were performed under poor seeing conditions and at an average airmass of 1.5. A source is visible within the 1.5" enhanced Swift-XRT error circle reported by GOAD et al. (GCN #22835), and consistent with the position of the optical afterglow candidate reported by Guidorzi et al. (GCN #22839). The source appears extended and most of the flux in the JHK bands is to the NW of the afterglow error circle. Based on a total of 22 min exposure time in g'r'i'z', we estimate preliminary AB magnitudes of g' = 21.5 +/- 0.1 mag r' = 20.2 +/- 0.1 mag i' = 19.9 +/- 0.1 mag z' = 19.6 +/- 0.1 mag A fit to the GROND optical SED implies a photometric redshift of z_phot=3.75+/-0.10. Given magnitudes are calibrated against SDSS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.04 in the direction of the burst (Schlafly & Finkbeiner 2011). We thank Markus Rabus for the excellent support from La Silla.