GCN Circular 21566
M. Im, C. Choi, J. Kim, H. M. Lee (SNU), S.-L. Kim (KASI) on behalf of the KU collaboration We analyzed the medium-band data we took for SSS17a at the Siding Spring Observatory at 2017-08-18.4 (UT) in Im et al. (LVC GCN 21563). The preliminary magnitude at the medium-band filter m475 centered at 475nm (Choi & Im JKAS, 50, 71) is m475 = 17.5 +- 0.2 AB mag, somewhat brighter than g ~ 17.9 mag of Wolf et al. (LVC GCN 21560) taken at the same site at a similar time. Considering that m475 is similar to the g-band filter, this prefers a moderate dimming in comparison to g=17.2 +- 0.1 mag at the time of 2017-08-18.1 (UT) from Simon et al. (LVC GCN 21551) to a brightening as suggested by Arcavi et al. (LVC GCN 21565). However, currently, the large uncertainties in the magnitude estimates (mostly due to host galaxy subtraction problem) makes us difficult to rule out no brightening or even a slight brightening of the EM counterpart candidate.