TITLE: GCN CIRCULAR NUMBER: 26450 SUBJECT: UKIRT photometry of ZTF19acyldun/AT 2019wrt DATE: 19/12/16 14:26:07 GMT FROM: Dong Xu at NAOC/CAS D. Xu, Z.P. Zhu, B.Y. Yu, H.J. Wang, L. Ge, T.M. Zhang, X. Zhou, C.Z. Cui, Y.F. Xu (NAOC), H.B. Zhao, B. Li (PMO), J.Z. Liu (XAO), J.R. Mao (YNAO), X. Gao (Urumqi No.1 Senior High School) report: We observed the ZTF19acyldun/AT 2019wrt discovered by PTF (Stein et al., GCN 26437), a counterpart candidate of LIGO/Virgo S191213g (GCN 26402), using the UKIRT telescope. Observations started at 09:32:22 UT on 2019-12-16 in H- and K-filters, with ~30min exposure for either filter. Sky was clear and seeing was ~0.9 arcsec. ZTF19acyldun/AT 2019wrt is detected in our stacked H-band and K-band images. The object is extended in the East-West direction, which might be affected by an arm of the background host galaxy. Preliminary photometry gives m(H) ~ 18.70 +/- 0.34 and m(K) ~ 17.82 +/- 0.26, contaminated by the host galaxy and calibrated with nearby 2MASS stars. Compared with the measurement of m(g) = 19.97 +/- 0.17 in 2019-12-13 (GCN 26437), the optical/NIR behaviour is largely not consistent with a decaying afterglow of a short GRB. Other scenarios for S191213g would be more self-consistent, such as a supernova or an emerging kilonova. We thank the great support of the UKIRT staff/observer, in particular Michael Pohlen, Watson Varricatt, and Jessica Stasik.