ULTRACAM z'-band detection of the secondary eclipse of WASP-12b

We present z(')-band secondary eclipse photometry of the highly irradiated hot Jupiter WASP-12b using ULTRACAM on the 4.2 m William Herschel Telescope. We measure a decrease in flux of delta = 0.130 +/- 0.013 per cent during the passage of the planet behind the star, which is significantly deeper than the previous measurement at this wavelength (0.082 +/- 0.015 per cent). Our secondary eclipse is best fitted with a mid-eclipse phase, phi, that is compatible with a circular orbit phi = 0.501 +/- 0.002, in agreement with previous results. In combination with existing data, our eclipse depth measurement allows us to constrain the characteristics of the planet's atmosphere, which is consistent with a carbon-rich model, with no evidence for a strong thermal inversion. If the difference in eclipse depth reported here compared to that of Lopez-Morales et al. is of physical origin, as opposed to due to systematics, it may be caused by temporal variability in the flux, due to atmospheric dynamics.