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Near-infrared Monitoring of the Accretion Outburst in the MYSO S255-NIRS3

We followed-up the massive young stellar object (MYSO) S255-NIRS3 (=S255-IRS1b) during its recent accretion outburst event in the Ks band with Kanata/HONIR for four years after its burst and obtained a long-term light curve. This is the most complete NIR light-curve of the S255-NIRS3 burst event that has ever been presented. The light curve showed a steep increase reaching a peak flux that was 3.4 mag brighter than the quiescent phase and then a relatively moderate year-scale fading until the last observation, similar to that of the accretion burst events such as EXors found in lower-mass young stellar objects. The behavior of the Ks band light curve is similar to that observed in 6.7 GHz class II methanol maser emission, with a sudden increase followed by moderate year-scale fading. However, the maser emission peaks appear 30-50 days earlier than that of the Ks band emission. The similarities confirmed that the origins of the maser emission and the Ks band continuum emission is common as previously shown from another infrared and radio observations by Stecklum et al. (2016); Caratti o Garatti et al. (2017a); Moscadelli et al. (2017). However, the differences in energy transfer paths, such as the exciting/emitting/scattering structures, may cause the delay in the flux-peak dates.