The electromagnetic probe has proven to be a very efficient way to access the $3-$dimensional structure of the nucleon, particularly thanks to the exclusive Compton processes. We explore the hard photoproduction of a large invariant mass diphoton in the kinematical regime where the diphoton is nearly forward and its invariant mass is the hard scale enabling to factorize the scattering amplitude in terms of generalized parton distributions. We calculate unpolarized cross sections and the angular asymmetry triggered by a linearly polarized photon beam.Comment: 8 pages, 9 figures. Version 2, as to be published in Phys Rev D96, contains minor improvements including a brief comparison of different GPD models and a brief discussion of the very high energy behavior of the cross sectio

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We correct the expression for the γN → γγN 0 amplitude in the generalized Bjorken regime, where the diphoton is nearly forward and its invariant mass is the hard scale enabling to factorize the scattering amplitude in terms of generalized parton distributions. This amplitude has a very peculiar and interesting analytic structure, while the cross section estimates are only slightly changed.

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