IRSF SIRIUS JHKs Simultaneous Transit Photometry of GJ 1214b

Abstract

Abstract We report on the high-precision transit photometry of GJ 1214b in JHK$ _{\rm 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$ _{\rm s}$ bands are $ R_{\rm p}/R_{{\rm s},J}$$ =$ 0.11833 $ \pm$ 0.00077, $ R_{\rm p}/R_{{\rm s},H}$$ =$ 0.11522 $ \pm$ 0.00079, and $ R_{\rm p}/R_{{\rm s},K_{\rm s}}$$ =$ 0.11459 $ \pm$ 0.00099, respectively. The radius ratios are very consistent with previous studies by Bean et al. (2011, ApJ, 743, 92) within 1 $ \sigma$ , while our ratio in the K$ _{\rm s}$ band is shallower than previous measurements in the same band by Croll et al. (2011, ApJ, 736, 78) and inconsistent even at the 4 $ \sigma$ 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$ _{\rm s}$ band would be especially important.