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We report the first ground-based transit observation of K2-3d, a 1.5 R_Earthplanet supposedly within the habitable zone around a bright M-dwarf host star,using the Okayama 188 cm telescope and the multi(grz)-band imager MuSCAT.Although the depth of the transit (0.7 mmag) is smaller than the photometricprecisions (1.2, 0.9, and 1.2 mmag per 60 s for the g, r, and z bands,respectively), we marginally but consistently identify the transit signal inall three bands, by taking advantage of the transit parameters from K2, and byintroducing a novel technique that leverages multi-band information to reducethe systematics caused by second-order extinction. We also revisit previouslyanalyzed Spitzer transit observations of K2-3d to investigate the possibilityof systematic offsets in transit timing, and find that all the timing data canbe explained well by a linear ephemeris. We revise the orbital period of K2-3dto be 44.55612 \pm 0.00021 days, which corrects the predicted transit times for2019, i.e., the era of the James Webb Space Telescope, by \sim80 minutes. Ourobservation demonstrates that (1) even ground-based, 2 m class telescopes canplay an important role in refining the transit ephemeris of small-sized,long-period planets, and that (2) a multi-band imager is useful to reduce thesystematics of atmospheric origin, in particular for bluer bands and forobservations conducted at low-altitude observatories.

Ground-based Transit Observation of the Habitable-zone Super-Earth K2-3d