{
  "bibcode": "2009GCN..9480....1R",
  "body": "A. Rossi (Tautenburg), F. Olivares, T. Kruehler and J. Greiner (all\nMPE Garching) report on behalf of the GROND team:\n\nWe observed the field of GRB 090531B (Swift trigger 353728, Cummings et al. \n2009, GCN #9461) simultaneously in g'r'i'z'JHK with GROND \n(Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPI/ESO telescope \nat La Silla Observatory (Chile).\n\nObservations started on 1 June 2009 at 00:03 UT, 5.5 hours after the burst and \ncontinued until 00:50 UT under mediocre seeing (~1.5), cirrus and Moon light.\nA second epoch started at 04:33 UT under better conditions, consisting of \n72 min exposure in g'r'i'z' and 60 min in JHK.\n\nWe find three objects within the XRT error circle (Sbaruffatti et al. 2009,\nGCN #9463), and another three at the outer circumference of the error circle\n(see finding chart on http://www.mpe.mpg.de/~jcg/grb090531B.html). Within\n+-0.2 mag, none of these objects shows any variability between 5.5 and 10 hrs \npost-burst, and thus are most likely foreground objects.\n\nIn empty regions of the error circle, the first epoch yields the following \nupper limits (all AB):\n\ng' > 22.9\nr' > 22.6\ni' > 22.1\nz' > 22.8\nJ  > 20.1\nH  > 19.8\nK  > 19.2\n\nwhich were obtained using the USNO and 2MASS field stars as reference.\nHowever, given the poor conditions and preliminary photometric calibration,\na variable object might be hidden behind one of the foreground objects,\nand masked in the difference image due to the distorted PSF from the first\nepoch. In that case, the upper limits would be about 2 mag brighter.\nNo correction has been applied for the strong foreground reddening of\nE(B-V)=1.11 mag (Schlegel et al. 1998).",
  "circularId": 9480,
  "createdOn": 1243943904000,
  "email": "jcg@mpe.mpg.de",
  "subject": "GRB 090531B: GROND observation",
  "submitter": "Jochen Greiner at MPI  <jcg@mpe.mpg.de>",
  "eventId": "GRB 090531B"
}