IRSF SIRIUS JHKs Simultaneous Transit Photometry of GJ 1214b

We report on the high-precision transit photometry of GJ 1214b in JHK_s bands simultaneously taken with the SIRIUS camera on the Infrared Survey Facility 1.4 m telescope located in Sutherland, South Africa. Our Markov-Chain Monte Carlo analysis shows that observed planet-to-star radius ratios in the JHK_s bands are R_p/R_s,J = 0.11833 ± 0.00077, R_p/R_s,H = 0.11522 ± 0.00079, and R_p/R_s,Ks = 0.11459 ± 0.00099, respectively. The radius ratios are very consistent with previous studies by Bean et al. (2011, ApJ, 743, 92) within 1 σ, while our ratio in the K_s band is shallower than previous measurements in the same band by Croll et al. (2011, ApJ, 736, 78) and inconsistent even at the 4σ level with them. We have no satisfactory explanation for this discrepancy at this point. Our overall results support a flat transmission spectrum in the observed bands, which can be explained by a water-dominated atmosphere or an atmosphere with extensive high-altitude clouds or haze. To explain the discrepancy of the radius ratios, and to determine a definitive atmosphere model for GJ 1214b in the future, more extended transit observations in around the K_s band would be especially important.