The most energetic phenomena occurring in the Universe are probed by X-ray radiation. Supernovae explosions in the Milky Way and outer galaxies shock the interstellar medium to million degrees temperatures. Stellar-mass black holes and neutron stars release high-energy photons as they accrete gas in binary systems. Supermassive black holes at galaxy centres densely populate the X-ray sky as they grow up to a few billion solar masses over cosmic time. Finally, the Universe is filled by hot gas, with half of the baryons spread out in filamentary large-scale structures at hundreds of thousands degrees temperatures, and plasmas at even higher temperatures being trapped in the potential wells of galaxy groups and clusters.
Two flagship X-ray missions, ESA's XMM-Newton and NASA's Chandra, are constantly advancing our understanding of the hot and energetic Universe since two decades, and new discoveries are brought by many other international X-ray observatories. The scientific programs based on these and past X-ray missions are usually complemented by observations taken over the entire electromagnetic spectrum. A multi-wavelength approach is by now essential to address the most pressing scientific questions arising from X-ray observations and to test theoretical models. In 2017 we witnessed the first detection of the electromagnetic counterpart, including X-rays, of a gravitational wave. In 2018, a neutrino emitted by an accreting supermassive black hole has been detected. The era of multi-messenger astronomy has officially begun.
This series of decadal X-ray conferences in Bologna has been devised to review the progresses brought by an almost continuous flow of X-ray data, putting them in a multi-wavelength context. We expect presentations summarising the most relevant breakthrough from XMM-Newton and Chandra in their 20th anniversary of operation, as well as from all other high-energy missions on orbit. We also intend to stimulate discussion on the directions that X-ray astronomy should take in the next decade(s) and, in particular, about the role of future X-ray missions (e.g., eROSITA, Athena, XRISM, Arcus, Theseus, eXTP) and of their synergies with current and future leading facilities (e.g., Euclid, JWST, WFIRST, SKA, ELT, TMT, LISA) that will be operational in the era of multi-messenger astronomy.
The main scientific issues that will be addressed are:
Multi-messenger and transient astronomy
Compact and diffuse sources in galaxies and in the Galactic Center
Hot and diffuse baryons
Active Galactic Nuclei: accretion physics and evolution across cosmic time