TITLE: GCN CIRCULAR NUMBER: 21584 SUBJECT: LIGO/Virgo G298048: GROND photometry of candidate optical counterpart reveals brightening in the NIR DATE: 17/08/19 01:58:14 GMT FROM: Philip Wiseman at MPE/Garching 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.