Complement larger X-ray missions, and observations at other wavelengths, providing context for better understanding of various phenomena. Advances in spatial, spectral, polarimetric, and temporal resolution must be supported through the Explorer Program to help build the framework for future progress. No specific mission recommendations are made as the Explorer Program is carried out through an open science peer selection process. We fully endorse this program and would like to see more frequent opportunities made available. Relative to building new missions, the operation costs of existing missions that provide sustaining science are low.
International collaborations are an important means of leveraging.
Theory Need to provide theoretical understanding before missions are flown to assure that missions return valuable data. Need predictions of what will be seen to scope sensitivity and resolution requirements. Image the event horizons of black holes, study the infall of matter, and view the formation of jets. Directly image black holes, neutron stars, white dwarfs, and their immediate environments in the Milky Way. At this resolution, ripples due to gravity waves will be visible.