{
  "bibcode": "2021GCN.30574....1N",
  "body": "A. Nicuesa Guelbenzu, S. Klose, S. Schmidl (all TLS Tautenburg)\nand A. Rau (MPE Garching) report:\n\nWe observed the field of GRB 210731A (Swift trigger 1062336; Troja et\nal., GCN 30568) simultaneously in g'r'i'z'JHK with GROND mounted at\nthe 2.2m MPG telescope at ESO La Silla Observatory (Chile).\n\nObservations started on 01-AUG-2021 at 02:33 UT, about 4.2 hours after\nthe GRB trigger. They were performed at an average seeing of 1.0\narcsec and at an average airmass of 1.1.\n\nAt a meantime of 2:46 UT, for the GRB afterglow (de Wet et al,\nGCN 30570; Kuin et al., GCN 30572) we measure the following AB\nmagnitudes:\n\ng' = 18.71 +/- 0.01 mag,\nr' = 18.44 +/- 0.01 mag,\ni' = 18.19 +/- 0.01 mag,\nz' = 18.01 +/- 0.01 mag,\nJ  = 17.66 +/- 0.02 mag,\nH  = 17.38 +/- 0.02 mag, and\nK  = 17.14 +/- 0.15 mag.\n\nOptical magnitudes are calibrated against the Pan-STARRS catalog,\nNIR magnitudes against 2MASS stars in the field.\n\nAfter correcting for a Galactic foreground reddening of E(B-V)=0.09\nmag (Schlegel et al. 1998), the SED is a power-law with no evidence\nfor extinction by dust in the GRB host galaxy.\n\nWe thank Sam Kim for excellent support and for performing the\nobservations.",
  "circularId": 30574,
  "createdOn": 1627835196000,
  "email": "ana@tls-tautenburg.de",
  "subject": "GRB 210731A: GROND observations",
  "submitter": "Ana Nicuesa at TLS Tautenburg  <ana@tls-tautenburg.de>",
  "eventId": "GRB 210731A"
}