TITLE: GCN CIRCULAR NUMBER: 21632 SUBJECT: LIGO/Virgo G298048: Continued monitoring of the optical counterpart candidate DATE: 17/08/21 15:49:41 GMT FROM: Myungshin Im at Seoul National U M. Im, C. Choi, J. Kim, Lim, G. H. M. Lee (SNU), S.-L. Kim, C.-U. Lee (KASI) on behalf of the KU collaboration We continued to monitor the strong optical counterpart candidate of LIGO/Virgo G298048 (GCNs 210505, 21509, 21513), SSS17a/DLT17ck(Coutler et al. GCN 21529; Allam et al. GCN 21530; Yang et al. GCN 21531; Arcavi et al. GCN 21538; also see notes below for a list of optical/NIR observation reports). The observation was carried out using three KMTNet 1.6m telescopes and other small telescopes (Im et al. GCNs 21563, 21566, 21597) at three sites , the Siding Spring Observatory (SSO), the South African Astronomical Observatory (SAAO), and the Cerro-Tololo Inter-American Observatory (CTIO). We imaged the object with B,V,R,I,i,z, filters and medium-band images at 825, 875, 925, 975 nm. We confirm that the object is fading in optical and its continuum shape has been becoming redder as time goes on (e.g., Cenko et al. GCN 21572; Lyman et al. GCN 21582; Wiseman et al. GCN 21584; Chambers et al. GCN 21590; Malesani et al. GCN 21590; Chen et al. GCN 21608). A part of the data from several sites are presented below. UT V R I Site 08-18.72 17.8(0.1) 17.4(0.1) 17.3(0.1) SAAO 08-18.99 18.3(0.1) 17.6(0.1) 17.4(0.1) CTIO 08-19.72 19.4(0.1) 18.3(0.1) 17.9(0.1) SAAO 08-19.98 19.8(0.1) 18.4(0.1) 18.2(0.1) CTIO The observations and further analysis of the data are ongoing. Notes: It is remarkable that more than 40 optical/NIR follow-up imaging/spectroscopic observations of the optical counterpart candidates have been reported in three days since the first discovery report of the optical counterpart. For reference, we list them here (let me know if I am missing any). They are, Coutler et al. (GCN 21529, 21567), Allam et al. (GCN 21530), Yang et al. (GCNs 21531, 21579), Melandri et al. (GCN 21532, 21596), Arcavi et al. (GCNs 21538, 21565, 21581), Nicholl et al. (GCN 21541, 21580, 21585), Tanvir et al. (GCN 21544), Lipunov et al. (GCNs 21546, 21570, 21587), Drout et al. (GCN 21547), Evans et al. (GCN 21550), Simon et al. (GCN 21551), Singer et al. (GCN 21552), Chambers et al. (GCN 21553, 21590, 21617), Wolf et al. (GCN 21560), Yoshida et al. (GCN 21561, 21586), Im et al. (GCNs 21563, 21566, 21597), Cenko et al. (GCN 21572), Lyman et al. (GCN 21582), Kilpatrick et al. (GCN 21583), Wiseman et al. (GCN 21584), Malesani et al. (GCN 21591), D’Elia et al. (GCN 21592), Wiersema et al. (GCN 21594), Tominaga et al. (GCN 21595), Valenti et al. (GCN 21606), Chen et al. (GCN 21608), Pozanenko et al. (GCN 21618), Nakaoka et al. (GCN 21623), Castro-Tirado et al. (GCN 21624), Coward et al. (GCN 21627).