{
  "bibcode": "2017GCN.21608....1C",
  "body": "T.-W. Chen, P. Wiseman, J. Greiner, and P. Schady (all MPE Garching) \nreport on behalf of the GROND team:\n\nWe observed the field of the transient source SSS17a in NCG 4993 \n(RA = 13:09:48.09 DEC = -23:22:53.35, discovered by Coulter et al. \nGCN #21529), possibly associated with LIGO/Virgo trigger G298048, \nsimultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,\nPASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla \nObservatory (Chile).\n\nObservations started at 23:15 UT on August 19th 2017. They were \nperformed at an average seeing of 1.8\" and at an average airmass \nof 1.4.\n\nWe detected a single point source at the location of the transient, \nalthough the strong host contribution hinders photometric accuracy.\n\nBased on the first 11.7 min of total exposures in g'r'i'z' and \n13.2 min in JHK, we estimate preliminary magnitudes using aperture\nphotometry (all in AB system) of\n\ng' = 19.8 +/- 0.2 mag, \nr' = 18.1 +/- 0.1 mag, \ni' = 18.0 +/- 0.1 mag, \nz' = 16.8 +/- 0.1 mag, \nJ = 16.4 +/- 0.2 mag, \nH = 16.6 +/- 0.2 mag, and \nK = 16.6 +/- 0.2 mag.\n\nWe confirm that the transient significantly declines in optical \nbrightness (Yoshida et al. GCN #21586, Chambers et al. GCN #21590,\nTominaga et al. GCN #21595, Melandri et al. GCN #21596, Im et al. \nGCN #21597, Valenti et al. GCN #21606), but the transient keeps \nflat in z'JHK-band brightness for 1 day compared to our previous \nreport (Wiseman et al. GCN #21584).\n\nGiven magnitudes are calibrated against Pan-STARRS1 field stars \nas well as 2MASS field stars and are not corrected for the expected \nGalactic foreground extinction corresponding to a reddening of \nE_(B-V)= 0.1 mag in the direction of the candidate (Schlafly & \nFinkbeiner 2011).\n\nWe acknowledge the excellent help in obtaining these data from \nAngela Hempel on La Silla.",
  "circularId": 21608,
  "createdOn": 1503204561000,
  "email": "jchen@mpe.mpg.de",
  "subject": "LIGO/Virgo G298048: GROND observations of candidate optical counterpart",
  "submitter": "Ting-Wan Chen at PESSTO  <jchen@mpe.mpg.de>",
  "eventId": "LIGO/Virgo G298048"
}