Abstract
In this paper we describe a uniform analysis of eight transits and eleven secondary eclipses of the extrasolar planet GJ 436b obtained in the 3.6, 4.5, and 8.0 micron bands using the IRAC instrument on the Spitzer Space Telescope between UT 2007 June 29 and UT 2009 Feb 4. We find that the best-fit transit depths for visits in the same bandpass can vary by as much as 8% of the total (4.7 sigma significance) from one epoch to the next. Although we cannot entirely rule out residual detector effects or a time-varying, high-altitude cloud layer in the planet's atmosphere as the cause of these variations, we consider the occultation of active regions on the star in a subset of the transit observations to be the most likely explanation. We reconcile the presence of magnetically active regions with the lack of significant visible or infrared flux variations from the star by proposing that the star's spin axis is tilted with respect to our line of sight, and that the planet's orbit is therefore likely to be misaligned. These observations serve to illustrate the challenges associated with transmission spectroscopy of planets orbiting late-type stars; we expect that other systems, such as GJ 1214, may display comparably variable transit depths. Our measured 8 micron secondary eclipse depths are consistent with a constant value, and we place a 1 sigma upper limit of 17% on changes in the planet's dayside flux in this band. Averaging over the eleven visits gives us an improved estimate of 0.0452% +/- 0.0027% for the secondary eclipse depth. We combine timing information from our observations with previously published data to produce a refined orbital ephemeris, and determine that the best-fit transit and eclipse times are consistent with a constant orbital period. [ABRIDGED]
Highlights
Transiting planet systems have proven to be a powerful tool for studying exoplanetary atmospheres
We find that all parameters are consistent with a constant value over the two-year period spanned by our observations, with the exception of the measured transit depths and times in the 3.6 and 4.5 μm bands
We find that the 3.6 μm radius ratio measured on UT 2009 January 28 is 4.7σ deeper than the value measured on UT 2009 January 9 in this same band, and the 4.5 μm radius ratio from UT 2009 January 30 is 2.9σ deeper than the value measured on UT 2009 January 17
Summary
Transiting planet systems have proven to be a powerful tool for studying exoplanetary atmospheres. Observations of transiting systems have been used to detect the signatures of atomic and molecular absorption features at wavelengths ranging from the ultraviolet to the infrared (e.g., Charbonneau et al 2002; Vidal-Madjar et al 2003; Swain et al 2008; Desert et al 2008; Pont et al 2008a; Linsky et al 2010), sometimes the results have proven to be controversial (e.g., Gibson et al 2011) They have enabled studies of the dayside emission spectra and pressure–temperature profiles of close-in planets (e.g., Charbonneau et al 2005; Deming et al 2005; Knutson et al 2008; Grillmair et al 2008), and they have informed us about their atmospheric circulation (e.g., Knutson et al 2007, 2009a; Cowan et al 2007; Crossfield et al 2010). GJ 436 (Butler et al 2004; Maness et al 2007; Gillon et al 2007a, 2007b; Deming et al 2007; Demory et al 2007; Torres 2007) represents an ideal target, as the primary in this system is an early M star with a K-band magnitude of 6.1
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