Detection of Thermal Emission of XO-2b: Evidence for a Weak Temperature Inversion

We estimate flux ratios of the extrasolar planet XO-2b to its host star XO-2 at 3.6, 4.5, 5.8, and 8.0 mu m with Infrared Array Camera on the Spitzer Space Telescope to be 0.00081 +/- 0.00017, 0.00098 +/- 0.00020, 0.00167 +/- 0.00036, and 0.00133 +/- 0.00049, respectively. The fluxes provide tentative evidence for a weak temperature inversion in the upper atmosphere, the precise nature of which would need to be confirmed by longer wavelength observations. XO-2b substellar flux of 0.76 x 10(9) erg cm(-2) s(-1) lies in the predicted transition region between atmospheres with and without upper atmospheric temperature inversion.