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.