On the Multi-Faceted Synergy between X and Gamma-Ray Astronomies
  When it comes to identifying or to characterizing gamma-ray sources, X-ray
observations are of paramount importance. Correlated X-and-gamma-ray flux
variations are a powerful identification tool, if the gamma-ray source is
unidentified, or an important diagnostic tool to understand the behaviour of an
identified source. Moreover, X-ray observations of non-variable unidentified
gamma-ray sources, both galactic and extragalactic, can unveil interesting
candidate counterparts, narrowing down the search space and improving
significantly the chances for a successful identification. Swift observations
of Fermi gamma-ray-selected pulsar error boxes provide accurate positions of
likely counterparts. This makes it possible both to confirm pulsations and to
improve timing solution, opening up a new synergy between X and gamma ray
observations.
