GCN Circular 21584

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

We observed the field of the of the transient source SSS17a in NCG 4993 
(RA =
13:09:48.09 DEC = -23:22:53.35, Coulter et al. GCN #21529, Allam et al. 
GCN
#21530, Yang et al. GCN #21531, Melandri et al. GCN #21532), 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 18th 2017. They were 
performed at an
average seeing of 1.8" and at an average airmass of 1.4.

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

Based on the first 13.8 min of total exposures in g'r'i'z' and 12.4 min 
in JHK, we
estimate preliminary magnitudes (all in AB system) of

g' = 17.9 +/- 0.1 mag,
r' = 17.2 +/- 0.1 mag,
i' = 17.2 +/- 0.1 mag,
z' = 16.8 +/- 0.1 mag,
J = 16.6 +/- 0.2 mag,
H = 16.9 +/- 0.2 mag, and
K = 16.8 +/- 0.2 mag.

There appears to be little change in the optical brightness of the 
source: we
compare our magnitudes to the  g = 17.76 mag and r = 17.20 mag from 14 
hours
previous,reported by SkyMapper (Wolf et al. GCN #21560); and to the i,z 
= 17.25
mag of Pan-STARRS (Chambers et al. GCN #21553) and z = 17.3 from HSC-z 
(Yoshida
et al. GCN #21561) from 19 hours previous.

On the other hand, the object has brightened significantly in the NIR 
over the
last 24 hours. We compare the J band to the value of 17.5 mag (AB) from
VISTA/VIRCAM reported by Tanvir et al. (GCN #21544), while the K band 
(now 14.9
in Vega) is much brighter than the 16.9 mag (Vega) reported by Singer 
et al.
(GCN #21552).

Given magnitudes are calibrated against Pan-STARRS zeropoints/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).

After the Milky Way extinction correction, we fitted our colour SED 
assuming a
black body (no K-correction applied), which indicates a temperature of 
5333 +/-
243 K.