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

Abstract

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 $\sim$1.01 times the planetary radius for an atmosphere of atomic Hydrogen and Helium (including ions), and produces a maximum depth of $\sim$2.1% at $\sim$$\pm46$~min from the center of the planetary transit on the geometrically thick and optically thin corona. We compute numerically in the 0.25--2keV energy band that this maximum depth is only of $\sim$1.6% at $\sim$$\pm47$~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.