  The Advanced Telescope for High Energy Astrophysics (Athena) is the X-ray
observatory large mission selected by the European Space Agency (ESA), within
its Cosmic Vision 2015-2025 programme, to address the "Hot and Energetic
Universe" scientific theme (Nandra et al. 2013), and it is provisionally due
for launch in the early 2030s. The Square Kilometer Array (SKA) is the next
generation radio observatory and consists of two telescopes, one comprised of
dishes operating at mid frequencies (SKA1-MID) and located in South Africa, and
the other comprised of Log-Periodic antennas operating at low radio frequencies
(SKA1-LOW), which will be located in Australia (Braun et al. 2017). The
scientific commissioning of the radio telescope is planned to begin in
2021-2022. The SKA-Athena Synergy Team (SAST) has been tasked to single out the
potential scientific synergies between Athena and SKA. The astrophysical
community was involved in this exercise primarily through a dedicated
SKA-Athena Synergy Workshop, which took place on April 24-25, 2017 at SKAO,
Jodrell Bank, Manchester. The final result of the synergy exercise, this White
Paper, describes in detail a number of scientific opportunities that will be
opened up by the combination of Athena and SKA, these include: 1. the Cosmic
Dawn; 2. the Evolution of black holes and galaxies; 3. Active galaxy feedback
in galaxy clusters; 4. Non-thermal phenomena in galaxy clusters; 5. Detecting
the cosmic web; 6. Black-hole accretion physics and astrophysical transients;
7. Galactic astronomy: stars, planets, pulsars and supernovae.
