Computation of the transmitted and polarized scattered fluxes by the exoplanet HD 189733b in X-rays

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Thousands of exoplanets have been detected, but only one exoplanetary transit was potentially observed in X-rays from HD189733A. What makes the detection of exoplanets so difficult in this band? To answer this question, we run Monte-Carlo radiative transfer simulations to estimate the amount of X-ray flux reprocessed by HD189733b. Despite its extended evaporating-atmosphere, we find that the X-ray absorption radius of HD189733b at 0.7keV, the mean energy of the photons detected in the 0.25--2 keV energy band by XMM-Newton, is 1.01 times the planetary radius for an atmosphere of atomic Hydrogen and Helium (including ions), and produces a maximum depth of 2.1% at ±46 min from the center of the planetary transit on the geometrically thick and optically thin corona. We compute numerically in the 0.25--2keV energy band that this maximum depth is only of 1.6% at ±47 min from the transit center, and little sensitive to the metal abundance assuming that adding metals in the atmosphere would not dramatically change the density-temperature profile. Regarding a direct detection of HD189733b in X-rays, we find that the amount of flux reprocessed by the exoplanetary atmosphere varies with the orbital phase, spanning between 3--5 orders of magnitude fainter than the flux of the primary star. Additionally, the degree of linear polarization emerging from HD 189733b is <0.003%, with maximums detected near planetary greatest elongations. This implies that both the modulation of the X-ray flux with the orbital phase and the scattered-induced continuum polarization cannot be observed with current X-ray facilities.