{
  "bibcode": "2017GCN.21050....1B",
  "body": "Jan Bolmer (ESO, Chile), Helmut Steinle and Patricia Schady (both MPE Garching) report:\n\nWe observed the field of GRB 170428A (trigger=750298, Beardmore et al., GCN #21042)\nsimultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at\nthe 2.2 m MPG telescope at the ESO La Silla Observatory (Chile).\n\nObservations started at 09:57 UT on 2017-04-28, 43 min after the GRB trigger, and they\nwere performed at an average seeing of 1.2'' and at an average airmass of 2.2. We detect an\nuncatalogued source within the XRT error circle reported by Beardmore et al. (GCN #21047) \nat the position of RA, Dec = 330.07823, 26.91584, which is equivalent to:\n\nRA (J2000): 22h 00m 18.78s\nDec (J2000): +26d 54' 57.0''\n\nwith an uncertainty of 0.3 arcsec.\n\nBased on combined images with 17 min of total integration time in g'r'i'z' and 19 min in JHK\nat a mid-time of 10:15 UT on 2017-04-28, this source has the following preliminary\nmagnitudes (all in the AB system):\n\ng' = 21.4 +/- 0.1 mag\nr' = 21.3 +/- 0.1 mag\ni' = 21.1 +/- 0.1 mag\nz' = 21.1 +/- 0.1 mag\nJ = 20.7 +/- 0.2 mag\nH = 20.9 +/- 0.4 mag\nK > 20.0 mag\n\nThe given magnitudes are derived based on calibrating the images against SDSS in the optical\nand 2MASS field stars in the NIR bands. They are not corrected for the Galactic foreground\nextinction corresponding to a reddening of E_(B-V)= 0.05 in the direction of the burst (Schlafly\n& Finkbeiner 2011).\n\nAs noted by Xu et al. (GCN #21048), there already exists a single source with a brightness of\nr ~ 22.2 mag, presumably the host galaxy, in the SDSS and Pan-STARRS archival data. We\ntherefore likely observed the NIR/optical afterglow of the short GRB 170428A.",
  "circularId": 21050,
  "createdOn": 1493428509000,
  "email": "jan@bolmer.de",
  "subject": "GRB 170428A: GROND detection of the afterglow",
  "submitter": "Jan Bolmer at MPE/Garching  <jan@bolmer.de>",
  "eventId": "GRB 170428A"
}