{
  "bibcode": "2010GCN.10874....1U",
  "body": "Adria Updike (Clemson Univ.), Ana Nicuesa (Tautenburg Obs.), Marco Nardini, \nThomas Kruehler and Jochen Greiner (all MPE Garching) report on behalf of \nthe GROND team:\n\nWe observed the field of GRB 100621A (Swift trigger 425151; Ukwatta et  \nal., GCN #10870) simultaneously in g'r'i'z'JHK with GROND (Greiner et  \nal. 2008, PASP 120, 405) mounted at the 2.2 m MPI/ESO telescope at La  \nSilla Observatory (Chile). Observations started at 03:07 UT on June 21st, \n4 minutes after the GRB trigger, and continued for 3 hours. They were \nperformed at an average seeing of 1.5\" under clear sky conditions.\n\nIn stacked images of 6 min total integration time in g'r'i'z' and 8 min in JHK,\ncentered at 03:20 UT, we detect a bright, uncatalogued NIR source within the \nSwift/XRT error circle (Evans et al. GCN #10872) at the following coordinates:\nRA (2000.0)  =  21h 01m 13.08s\nDEC (2000.0) = -51d 06' 22.5\"\nwith an error of +-0.3\". This coincides with the enhanced XRT position\n(Evans et al. 2010, GCN #10873).\n\nIn the first hour the source faded by about 1 mag in the JHK bands, \nthus we identify this object as the GRB afterglow of GRB 100621A.\n\nOur preliminary photometry centered at 03:45 UT is (in AB  \nmagnitudes):\n\ng' = 22.7+-0.4 \t\nr' = 21.6+-0.1\ni' = 20.5+-0.1 \t\nz' = 19.7+-0.1\nJ  = 17.3+-0.1\nH  = 16.3+-0.1, and\nK  = 15.8+-0.1\n\nThese magnitudes were derived by calibrating the images against the GROND \nzeropoints and 2MASS field stars and are not corrected for the Galactic \nforeground extinction corresponding to a reddening of E_(B-V)= 0.03 in the \ndirection of the burst (Schlegel et al. 1998).\n\nWe note that there are indications of this source in the DSS2 images.\nAlso, there is no fading in the g' band, suggesting that this is the\nunderlying host galaxy at a redshift smaller than about 3. The extremely \nred colour then is likely due to severe intrinsic dust extinction. \n\nWe encourage spectroscopic observations.",
  "circularId": 10874,
  "createdOn": 1277114216000,
  "email": "jcg@mpe.mpg.de",
  "subject": "GRB 100621A: GROND observation of bright NIR afterglow",
  "submitter": "Jochen Greiner at MPI  <jcg@mpe.mpg.de>",
  "eventId": "GRB 100621A"
}