TITLE:   GCN CIRCULAR
NUMBER:  21566
SUBJECT: LIGO/Virgo G298048: Moderate dimming or no brightening of the optical counterpart candidate?
DATE:    17/08/18 17:15:43 GMT
FROM:    Myungshin Im at Seoul National U  <myungshin.im@gmail.com>

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.