A Search for Methane in the Atmosphere of GJ 1214b via GTC Narrow-Band Transmission Spectrophotometry

We present narrow-band photometric measurements of the exoplanet GJ 1214b using the 10.4 m Gran Telescopio Canarias (GTC) and the OSIRIS instrument. Using tuneable filters we observed a total of five transits, three of which were observed at two wavelengths nearly simultaneously, producing a total of eight individual light curves, six of these probed the possible existence of a methane absorption feature in the 8770 - 8850 {\AA} region at high resolution. We detect no increase in the planet-to-star radius ratio across the methane feature with a change in radius ratio of $\Delta$R = -0.0007 $\pm$ 0.0017 corresponding to a scale height (H) change of -0.5 $\pm$ 1.2 H across the methane feature, assuming a hydrogen dominated atmosphere. We find a variety of water and cloudy atmospheric models fit the data well, but find that cloud-free models provide poor fits. These observations support a flat transmission spectrum resulting from the presence of a high-altitude haze or a water-rich atmosphere, in agreement with previous studies. In this study the observations are predominantly limited by the photometric quality and the limited number of data points (resulting from a long observing cadence), which make the determination of the systematic noise challenging. With tuneable filters capable of high resolution measurements (R ~ 600 - 750) of narrow absorption features, the interpretation of our results are also limited by the absence of high resolution methane models below 1 $\mu$m.