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We present new X-ray observations obtained with Chandra ACIS-S of the HD189733 system, consisting of a K-type star orbited by a transiting Hot Jupiterand an M-type stellar companion. We report a detection of the planetary transitin soft X-rays with a significantly larger transit depth than observed in theoptical. The X-ray data favor a transit depth of 6-8%, versus a broadbandoptical transit depth of 2.41%. While we are able to exclude several possiblestellar origins for this deep transit, additional observations will benecessary to fully exclude the possibility that coronal inhomogeneitiesinfluence the result. From the available data, we interpret the deep X-raytransit to be caused by a thin outer planetary atmosphere which is transparentat optical wavelengths, but dense enough to be opaque to X-rays. The X-rayradius appears to be larger than the radius observed at far-UV wavelengths,most likely due to high temperatures in the outer atmosphere at which hydrogenis mostly ionized. We furthermore detect the stellar companion HD 189733B inX-rays for the first time with an X-ray luminosity of log LX = 26.67 erg/s. Weshow that the magnetic activity level of the companion is at odds with theactivity level observed for the planet-hosting primary. The discrepancy may becaused by tidal interaction between the Hot Jupiter and its host star.

Transit observations of the Hot Jupiter HD 189733b at X-ray wavelengths