{
  "bibcode": "2017GCN.20843....1S",
  "body": "Patricia Schady (MPE Garching) reports:\n\nFollowing on from the GROND (Greiner et al. 2008, PASP 120, 405)\nobservations of GRB 170306A on the 2017-03-06 (Schady et al., GCN #20819),\nI re-observed the field simultaneously in g'r'i'z'JHK with GROND at on\n2017-03-10, 4 days after the GRB trigger. Observations were performed at\nan average seeing of 1.4\" and at an average airmass of 1.7.\n\nOne of the three sources detected with GROND (Schady et al., GCN #20819)\nwithin the refined XRT error circle (D�Avanzo et al., GCN #20828) is no\nlonger seen in the latest GROND observations down to the following 3 sigma\nlimiting magnitudes (all in AB system):\n\ng� > 22.6 mag\nr� >  23.5 mag\ni� > 22.3 mag\nz� > 22.4 mag\nJ > 20.4 mag\nH > 19.9 mag\nK > 18.9 mag\n\nThis source is therefore the likely optical/NIR afterglow of GRB 170306A.\nThe position of the afterglow candidate is\n\nRA (J2000)    = 17:32:16.69\nDec (J2000) =  -44:44:55.5\n\nwith a positional uncertainty of 0.3\" in both coordinates.\n\nBased on combined images with 36 min of total integration time in g'r'i'z'\nand 30 min in JHK at a mid-time of 09:33 UT on 2017-03-06, the afterglow\ncandidate had the following preliminary magnitudes (all in AB system):\n\ng' = 21.6 +/- 0.1 mag\nr� = 20.4 +/- 0.1 mag\ni� = 20.0 +/- 0.1 mag\nz' = 19.9 +/- 0.1 mag\nJ = 19.1 +/- 0.1 mag\nH = 18.4 +/- 0.1 mag\nK = 18.5 +/- 0.2 mag\n\nThe given magnitudes are derived based on images calibrated against GROND\nzero points in the optical and 2MASS field stars in the NIR bands. They\nare not corrected for the Galactic foreground extinction corresponding to\na reddening of E_(B-V)= 0.36 in the direction of the burst (Schlafly &\nFinkbeiner 2011).\n\nI acknowledge the excellent support provided by the Chilean observer at\nthe telescope, and the support astronomer, Sam Kim, in obtaining these\ndata. I also thank David Buckley for some useful email exchanges that in\npart led to the second epoch of GROND observations.",
  "circularId": 20843,
  "createdOn": 1489151722000,
  "email": "pschady@mpe.mpg.de",
  "subject": "GRB 170306A: GROND afterglow detection",
  "submitter": "Patricia Schady at MPE/Swift  <pschady@mpe.mpg.de>",
  "eventId": "GRB 170306A"
}