TITLE:   GCN CIRCULAR
NUMBER:  21608
SUBJECT: LIGO/Virgo G298048: GROND observations of candidate optical counterpart 
DATE:    17/08/20 04:49:21 GMT
FROM:    Ting-Wan Chen at PESSTO  <jchen@mpe.mpg.de>

T.-W. Chen, P. Wiseman, J. Greiner, and P. Schady (all MPE Garching) 
report on behalf of the GROND team:

We observed the field of the transient source SSS17a in NCG 4993 
(RA = 13:09:48.09 DEC = -23:22:53.35, discovered by Coulter et al. 
GCN #21529), possibly associated with LIGO/Virgo trigger G298048, 
simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla 
Observatory (Chile).

Observations started at 23:15 UT on August 19th 2017. They were 
performed at an average seeing of 1.8" and at an average airmass 
of 1.4.

We detected a single point source at the location of the transient, 
although the strong host contribution hinders photometric accuracy.

Based on the first 11.7 min of total exposures in g'r'i'z' and 
13.2 min in JHK, we estimate preliminary magnitudes using aperture
photometry (all in AB system) of

g' = 19.8 +/- 0.2 mag, 
r' = 18.1 +/- 0.1 mag, 
i' = 18.0 +/- 0.1 mag, 
z' = 16.8 +/- 0.1 mag, 
J = 16.4 +/- 0.2 mag, 
H = 16.6 +/- 0.2 mag, and 
K = 16.6 +/- 0.2 mag.

We confirm that the transient significantly declines in optical 
brightness (Yoshida et al. GCN #21586, Chambers et al. GCN #21590,
Tominaga et al. GCN #21595, Melandri et al. GCN #21596, Im et al. 
GCN #21597, Valenti et al. GCN #21606), but the transient keeps 
flat in z'JHK-band brightness for 1 day compared to our previous 
report (Wiseman et al. GCN #21584).

Given magnitudes are calibrated against Pan-STARRS1 field stars 
as well as 2MASS field stars and are not corrected for the expected 
Galactic foreground extinction corresponding to a reddening of 
E_(B-V)= 0.1 mag in the direction of the candidate (Schlafly & 
Finkbeiner 2011).

We acknowledge the excellent help in obtaining these data from 
Angela Hempel on La Silla.