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Its source has not been fully identified.","posed_since":"","precise":"Hard X-ray self-seeded FELs show a broadband pedestal around the seeded line containing a sizable fraction of the energy. Determine the fractions due to residual SASE noise, sideband instability, electron-beam microbunching from the accelerator, and seed-crystal effects, and the minimum achievable pedestal fraction.","problem_ref":null,"references":"","settled_by":"Spectral measurements with controlled microbunching and undulator taper, compared with 3D FEL simulations using measured electron phase space.","status_note":"","title":"Origin of the spectral pedestal in self-seeded hard X-ray 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