Abstract

The Cherenkov Telescope Array, CTA, will be the major global observatory for very high energy gammarayastronomyoverthenextdecadeandbeyond. ThescientificpotentialofCTAisextremelybroad: from understanding the role of relativistic cosmic particles to the search for dark matter. CTA is an explorer of theextremeuniverse,probingenvironmentsfromtheimmediateneighbourhoodofblackholestocosmic voids on the largest scales. Covering a huge range in photon energy from 20 GeV to 300 TeV, CTA will improve on all aspects of performance with respect to current instruments. Wider field of view and improved sensitivity will enable CTA to survey hundreds of times faster than previous TeV telescopes. The angular resolution of CTA will approach 1 arc-minute at high energies — the best resolution of any instrument operating above the X-ray band — allowing detailed imaging of a large number of gammaray sources. A one to two order-of-magnitude collection area improvement makes CTA a powerful instrument for time-domain astrophysics, three orders of magnitude more sensitive on hour timescales than Fermi-LAT at 30 GeV. The observatory will operate arrays on sites in both hemispheres to provide full sky coverage and will hence maximize the potential for the rarest phenomena such as very nearby supernovae, gamma-ray bursts or gravitational wave transients. With 99 telescopes on the southern site and 19 telescopes on the northern site, flexible operation will be possible, with sub-arrays available for specific tasks. CTA will have important synergies with many of the new generation of major astronomical and astroparticle observatories. Multi-wavelength and multi-messenger approaches combining CTA data with those from other instruments will lead to a deeper understanding of the broad-band non-thermal properties of target sources, elucidating the nature, environment, and distance of gamma-ray emitters. Details of synergies in each waveband are presented. The CTA Observatory will be operated as an open, proposal-driven observatory, with all data available on a public archive after a pre-defined proprietary period (of typically one year). Scientists from institutions worldwide have combined together to form the CTA Consortium. This Consortium has prepared a proposal for a Core Programme of highly motivated observations. The programme, encompassing approximately 40% of the available observing time over the first ten years of CTA operation, is made up of individual Key Science Projects (KSPs), which are presented in the subsequent chapters. The science cases have been prepared over several years by the CTA Consortium, with community input gathered via a series of workshops connecting CTA to neighbouring communities. A major element of the programme is the search for dark matter via the annihilation signature of weakly interacting massive particles (WIMPs). The strategy for dark matter detection presented here places the expected crosssection for a thermal relic within reach of CTA for a wide range of WIMP masses from ∼200 GeV to 20 TeV. This makes CTA extremely complementary to other approaches, such as high-energy particle collider and direct-detection experiments. CTA will also conduct a census of particle acceleration over a wide range of astrophysical objects, with quarter-sky extragalactic, full-plane Galactic and Large Magellanic Cloud surveys planned. Additional KSPs are focused on transients, acceleration up to PeV energies in our own Galaxy, active galactic nuclei, star-forming systems on a wide range of scales, and the Perseus cluster of galaxies. All provide high-level data products which will benefit a wide community, and together they will provide a long-lasting legacy for CTA. Finally, while designed for the detection of gamma rays, CTA has considerable potential for a range of astrophysics and astroparticle physics based on charged cosmic-ray observations and the use of the CTA telescopes for optical measurements.