{
  "bibcode": "2009GCN..9901....1O",
  "body": "F. Olivares, P. Afonso, T. Kruehler, J. Greiner  (all MPE Garching), report \non behalf of the GROND team:\n\nThe field of GRB 090904B (Swift trigger 361831, D'Elia et al., GCN #9881, \nalso GBM trigger 273720255/090904058, Goldstein, GCN #9895) was observed \nsimultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, \n405) mounted at the 2.2 m ESO/MPI telescope at La Silla Observatory \n(Chile). Observations started at 01:28 UT, on September 4, 2009, 4 minutes \nafter the GRB trigger. The field was observed during a second epoch, \nstarting at 01:28 UT, on September 5, 2009, 24.07 hours after the GRB \ntrigger. Image subtraction between the two epochs reveals a fading source \non the edge of the enhanced Swift/XRT error circle (Osborne et al., GCN \n#9887). The position of the afterglow is\n\nRA (J2000.0) = 17h 36m 44.59s\nDEC (J2000.0) = -25d 12' 46.5\"\n\nwith an uncertainty of 0.5\" in each coordinate.\n\nThe afterglow is located roughly 1.1\" South/East of the 2MASS/USNO star \nreported by REM (Covino et al., GCN #9882) and MOA-II (Nishimoto et al., \nGCN #9892), and both sources are strongly blended in the GROND images of \n1.0\" seeing.\n\nAt a midtime of 6.5 minutes after the burst, we measure rough preliminary \nAB magnitudes of the optical afterglow of r' ~ 22.2, i ~ 20.6 and z' ~ 19.7.\n\nCalibrations were done against GROND zero points, and given magnitudes are \nnot corrected for the significant Galactic extinction corresponding to a \nreddening of E(B-V) = 1.76 mag (Schlegel et al. 1998). The object is \ndetected in all bands except g', which, given the large Galactic foreground \nextinction, does not constrain the redshift.",
  "circularId": 9901,
  "createdOn": 1252328048000,
  "email": "kruehler@mpe.mpg.de",
  "subject": "GRB 090904B: GROND detection of the optical afterglow",
  "submitter": "Thomas Kruehler at MPE/MPI  <kruehler@mpe.mpg.de>",
  "eventId": "GRB 090904B"
}