  Modern X-ray observatories yield unique insight into the astrophysical time
domain. Each X-ray photon can be assigned an arrival time, an energy and a sky
position, yielding sensitive, energy-dependent light curves and enabling
time-resolved spectra down to millisecond time-scales. Combining those with
multiple views of the same patch of sky (e.g., in the Chandra and XMM-Newton
deep fields) so as to extend variability studies over longer baselines, the
spectral timing capacity of X-ray observatories then stretch over 10 orders of
magnitude at spatial resolutions of arcseconds, and 13 orders of magnitude at
spatial resolutions of a degree. A wealth of high-energy time-domain data
already exists, and indicates variability on timescales ranging from
microseconds to years in a wide variety of objects, including numerous classes
of AGN, high-energy phenomena at the Galactic centre, Galactic and
extra-Galactic X-ray binaries, supernovae, gamma-ray bursts, stellar flares,
tidal disruption flares, and as-yet unknown X-ray variables. This workshop
explored the potential of strategic X-ray surveys to probe a broad range of
astrophysical sources and phenomena. Here we present the highlights, with an
emphasis on the science topics and mission designs that will drive future
discovery in the X-ray time domain.
