{
  "bibcode": "2017GCN.20680....1S",
  "body": "P. Schady, J. Greiner (both MPE Garching), and S. Steinmassl (TU Munich),\nreport:\n\nWe observed the field of the optical transient reported by MASTER \n(Podesta et al. 2017, GCN #20650) of GRB 170209A (Fermi/GBM trigger 508295323;\nRoberts & Meegan 2017, GCN #20652) simultaneously in g'r'i'z'JHK with GROND \n(Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at \nESO La Silla Observatory (Chile).\n\nObservations started at 03:16 UT on 2017-02-10, 26 hrs after the GRB trigger,\nresulting in 36 min of total exposures in g'r'i'z' and  30 min in JHK.\nThey were performed at an average seeing of 2.3\" and at an airmass of 1.1.\nA second epoch observation was started at 03:03 UT on 2017-02-15, around 6 days\nafter the GRB trigger, resulting in 72  min of total exposures in g'r'i'z' and \n60 min in JHK. Seeing conditions were better, around 1.3\".\n\nWe detect two sources close to the MASTER position: the fainter one, to the\nNorth-West, has faded by more than 1.2 mag, while the second source (to the\nSouth-West) stayed constant within the errors.\n\nThe position of the fading object is\nRA  (2000.0)  =  07:23:07.19 \nDecl.(2000.0) = -52:14:45.5\nwith an error of +-0.3\", and about 1.5 arcsec from the MASTER position.\n\nWe find the following magnitudes of the fading object:\n     epoch 1               epoch 2\ng'  > 23.5 mag            > 24.6 mag\nr'  = 23.4 +- 0.1 mag     > 24.6 mag\ni'  = 23.3 +- 0.3 mag     > 24.2 mag\nz'  = 23.1 +- 0.3 mag     > 24.2 mag\nJ   > 21.3 mag            > 21.7 mag\nH   > 21.0 mag            > 21.3 mag\nK   > 19.3 mag            > 19.7 mag\n\nThe close match with the MASTER object, the fading and the powerlaw-like \nr'i'z colors confirm the nature of this object as the afterglow of \nGRB 170209A, as suggested also by Hurley et al. 2017, GCN #20656 based \npurely on the positional coincidence.\n\nGiven magnitudes are calibrated against GROND zeropoints and 2MASS field stars\nand are not corrected for the Galactic foreground extinction corresponding to\na reddening of E(B-V)= 0.12 mag in the direction of the burst (Schlafly & \nFinkbeiner 2011).\n\nWe thank the support astronomer, S. Ciceri, for the support of the observations.",
  "circularId": 20680,
  "createdOn": 1487176168000,
  "email": "jcg@mpe.mpg.de",
  "subject": "GRB 170209A: GROND confirmation of MASTER afterglow",
  "submitter": "Jochen Greiner at MPI  <jcg@mpe.mpg.de>",
  "eventId": "GRB 170209A"
}