Science with the Cherenkov Telescope Array

The following sections are included:

• Executive Summary
• Authors
• Acknowledgements
• Contents

Ground-based gamma-ray astronomy is a young field with enormous scientific potential. The possibility of astrophysical measurements at teraelectronvolt (TeV) energies was demonstrated in 1989 with the detection of a clear signal from the Crab nebula above 1 TeV with the Whipple 10 m imaging atmospheric Cherenkov telescope (IACT). Since then, the instrumentation for, and techniques of, astronomy with IACTs have evolved to the extent that a flourishing new scientific discipline has been established, with the detection of more than 150 sources and a major impact in astrophysics and more widely in physics. The current major arrays of IACTs, H.E.S.S., MAGIC, and VERITAS, have demonstrated the huge physics potential at these energies as well as the maturity of the detection technique. Many astrophysical source classes have been established, some with many well-studied individual objects, but there are indications that the known sources represent the tip of the iceberg in terms of both individual objects and source classes. The Cherenkov Telescope Array (CTA) will transform our understanding of the high-energy universe and will explore questions in physics of fundamental importance. As a key member of the suite of new and upcoming major astroparticle physics experiments and observatories, CTA will exploit synergies with gravitational wave and neutrino observatories as well as with classical photon observatories. CTA will address a wide range of major questions in and beyond astrophysics, which can be grouped into three broad themes…

CTA will have important synergies with many of the new generation of astronomical and astroparticle observatories. As the flagship VHE gamma-ray observatory for the coming decades, CTA plays a similar role in the VHE waveband as the SKA in radio, ALMA at millimetre, or E-ELT/TMT/GMT in the optical wavebands, providing excellent sensitivity and resolution compared to prior facilities. At the same time, the scientific output of CTA will be enhanced by the additional capabilities provided by these instruments (and vice-versa). MWL and MM studies using CTA provide added value to the science cases in two main ways…

Over the lifetime of CTA, most of the available observation time at both of the CTA sites will be divided into open time, based on scientific merit and awarded by a Time Allocation Committee to Guest Observer proposers, and a Core Programme consisting of a number of major legacy projects. Smaller fractions of observation time will be allocated to Director’s Discretionary time and host reserved time. The Core Programme, corresponding to approximately 40% of the total observation time over the first 10 years of CTA operations, will be comprised of Key Science Projects (KSPs) to be carried out by the CTA Consortium. Here, we introduce the KSPs; in the following chapters, they are described in more detail…

The existence of dark matter as the dominant gravitational mass in the universe is by now well established, but the detailed nature of dark matter is still unknown. Multiple hypotheses endure as to the character of dark matter, and for the most popular models discussed the Cherenkov Telescope Array (CTA) has a unique chance of discovery. In the form of weakly interacting massive particles (WIMPs), dark matter particles can self-annihilate, converting their large rest masses into other Standard Model particles, including gamma rays. Indirect detection from such annihilations provides a unique test of the particle nature of dark matter, in situ in the Cosmos. Observations of the gamma rays provide the probe for the “indirect” detection of dark matter employed by CTA. In the standard thermal history of the early universe, the annihilation cross-section has a natural value, the “thermal cross-section”, which provides the scale for the sensitivity needed to discover dark matter in this way. Particular models for WIMPs such as supersymmetry (SUSY) and theories with extra dimensions give predictions for gamma-ray energy spectra from the annihilations which are essential ingredients towards the predictions of the sensitivity of the indirect searches. Another vital ingredient in the CTA sensitivity predictions is the distribution of dark matter in the targets observed for the search…

The Galactic Centre Key Science Project is comprised of a deep exposure of the inner few degrees of our Galaxy, complemented by an extended survey to explore the regions not yet covered by existing very high-energy (VHE) instruments at high latitudes to the edge of the bulge emission. A schematic representation is shown in Figure 5.1, with details of the observation strategy and possible options given in Section 5.2…

Astronomical surveys of the Galaxy provide essential, large-scale data sets that form the foundation for Galactic science at all photon energies. Here, we introduce a Galactic Plane Survey (GPS) for CTA that will fulfil a number of important science goals, including: (1) providing a census of Galactic very high-energy (VHE) gamma-ray source populations, namely supernova remnants (SNRs), pulsar-wind nebulae (PWNe), and binary systems, through the detection of hundreds of new sources, substantially increasing the Galactic source count and permitting more advanced population studies, (2) identifying a list of promising targets for follow-up observations, such as new gamma-ray binaries and PeVatron candidates, to be carried out by the Consortium within the Key Science Projects (KSPs) or to be proposed through the Guest Observer (GO) programme, (3) determining the properties of the diffuse emission from the Galactic plane, (4) producing a multi-purpose, legacy data set, comprising the complete Galactic plane at very high energies, that will have long-lasting value to the entire astronomical and astroparticle physics communities, and (5) discovering new and unexpected phenomena in the Galaxy, such as new source classes and new types of transient and variable behaviour…

The Large Magellanic Cloud (LMC) is a unique galaxy hosting extraordinary objects, including the star-forming region 30 Doradus (the most active star-forming region in the local group of galaxies), the super star cluster R136 (an exceptional cluster with a large concentration of very massive O and Wolf-Rayet stars), supernova SN1987A (the closest supernova in modern times), and the puzzling 30 Dor C superbubble (a rare superbubble with non-thermal emission). As a satellite of the Milky Way, it is one of the nearest star-forming galaxies, and a very active one; it has one-tenth of the star formation rate of the Milky Way, distributed in only about two percent of its volume. This activity is attested by more than 60 supernova remnants (SNRs), dozens to hundreds of HII regions, and bubbles and shells observed at various wavelengths, all of which promise fruitful gamma-ray observations. The LMC is seen nearly face-on at high Galactic latitude, and hence source confusion, line of sight crowding, and interstellar absorption do not hamper these studies, in contrast to the case for the plane of our Galaxy. It is therefore a unique place to obtain a significantly-resolved global view of a star-forming galaxy at very high energies. In addition, the distance to the LMC is known to the few percent level, thus allowing precise luminosity measurements to be made, something which is often very difficult for Galactic sources…

This Key Science Project (KSP) consists of a blind survey of 25% of the total sky. The survey is aimed primarily at extragalactic science with the main objective to construct an unbiased VHE extragalactic source catalogue with an integral sensitivity limit of ∼6mCrab above 125 GeV. At the moment there are about 60 extragalactic sources seen with the imaging atmospheric Cherenkov telescopes (IACTs) H.E.S.S., MAGIC, and VERITAS, most of these being BL Lacertae (BL Lac) objects. In addition there are five radio galaxies, six flat spectrum radio quasars (FSRQs) and two starburst galaxies (NGC 253 and M82). The sample is, however, strongly biased since most of observations were motivated by high averaged flux at lower frequencies in optical, X-ray or gamma-ray wavebands. Moreover, about half of the detections have been made when alerted that the sources were in a flaring state. A second objective of this KSP is to provide a high resolution map of the extragalactic sky at gamma-ray energies between 50GeV and 10TeV. A third objective is to search for unexpected and serendipitous VHE phenomena over a large portion of the sky. The area covered by the Extragalactic Survey will connect to the Galactic Plane Survey (GPS) so that no Galactic latitude is left un-surveyed…

The universe hosts a diverse population of astrophysical objects, within our Galaxy and beyond, that explode or flare up in dramatic and unpredictable fashion across the electromagnetic spectrum and over a broad range of timescales spanning milliseconds to years. Collectively designated “transients”, many are known to be emitters of high energy gamma rays and are also potential sources of non-photonic signals that include cosmic rays, neutrinos, and/or gravitational waves (GWs). They are of great scientific interest, being associated with catastrophic events involving relativistic compact objects such as neutron stars (NSs) and black holes (BHs) that manifest the most extreme physical conditions in the universe. However, their dynamic nature has often hindered detailed observational characterisation and robust physical understanding. One of the key strengths of CTA is the unprecedented sensitivity in VHE gamma rays for transient phenomena and short timescale variability [18], which can revolutionise our knowledge of cosmic transients. Its relatively large field of view (FoV) is also a crucial asset in discovering transient events on its own, as well as in following up alerts of transients issued by monitoring instruments…

Cosmic rays are primarily energetic nuclei which fill the Galaxy and carry on average as much energy per unit volume as that in starlight, in interstellar magnetic fields, or in motions of interstellar gas [361]. The confinement time of cosmic rays within the Galaxy is of the order of ∼10 million years [361], and this implies that cosmic-ray sources must provide∼10⁴¹ erg/s in the form of accelerated particles in order to maintain the cosmic-ray intensity at the observed level. Moreover, the spectrum of cosmic rays observed at Earth is largely dominated by protons up to the so called knee in the spectrum at an energy of a few PeV. Above that energy, the differential power-law spectrum steepens from ∼ E^−2.7 to ∼ E^−3.0, where E is the particle energy, and its chemical composition becomes heavier [362]. Thus, the search for cosmic-ray sources has been focused on objects able to satisfy these requirements, i.e. powerful cosmic-ray proton PeVatrons…

Cosmic rays are believed to be an important regulator of the star formation process. It is hence important to understand where cosmic rays are being accelerated, how they propagate, and where they interact in the interstellar medium (ISM). While travelling through the ISM, cosmic rays interact with the ambient gas and radiation fields to produce gamma rays, which trace directly the parental cosmic-ray population. Gamma rays are thus among the best tools to study cosmic-ray properties in star-forming environments. CTA observations of star-forming systems over six orders of magnitude in the star-formation rate (SFR) will help to unveil the relationship between high-energy particles and the star-formation process. The study of individual star-forming regions and star-forming galaxies will furthermore help to disentangle source-specific properties from global ones. By studying the gamma-ray emission in these systems, we can measure the fraction of interacting high-energy particles as a function of the SFR and hence investigate to which extent cosmic rays, magnetic fields, and radiation are in equipartition…

VHE observations of active galaxies harbouring supermassive black holes (SMBHs) and ejecting relativistic outflows represent a unique tool to probe the physics of extreme environments, including accretion physics, jet formation, interaction of the black hole magnetosphere with the accretion disk corona, relativistic interaction processes, and general relativity. The same observations also allow us to search for signatures of ultra high-energy cosmic rays (UHECRs) and to characterise the evolution and differentiation (through intrinsic diversity or interaction within the host galaxy) of some of the brightest cosmic sources through space and time. The use of gamma-loud active galactic nuclei (AGN) as beacons provides insights into the cosmological evolution of star and galaxy formation through constraints on photon fields and magnetic fields along the line of sight. In addition, the study of VHE signals from extragalactic sources has a strong impact on the search for new fundamental physics…

Galaxy clusters are expected to be reservoirs of cosmic rays accelerated by structure formation processes, galaxies, and active galactic nuclei (AGN). The detection of diffuse synchrotron radio emission in several clusters confirms the presence of cosmic-ray electrons and magnetic fields permeating the intra-cluster medium (ICM). While there is no direct proof for proton acceleration yet, gamma rays can prove it as cosmic-ray protons can yield high-energy (HE) gamma-ray emission through neutral pion decay. Additionally, some of the galaxies hosted in clusters could be detected in gamma rays individually, if harbouring AGN, or via their summed output, e.g., in the case of star-forming galaxies. Finally, about 80% of the mass of clusters is in the form of dark matter, and therefore they are considered prime targets for indirect dark matter searches…

Although designed as a gamma-ray observatory, CTA is a powerful tool for a range of other astrophysics and astroparticle physics. For example, CTA can make precision studies of charged cosmic rays in the energy range from ∼100 GeV up to PeV energies, and it can be used as an instrument for optical intensity interferometry, to provide unprecedented angular resolution in the optical for bright sources. Below, we briefly summarise these possibilities. Most of the topics we discuss can be explored in parallel with gamma-ray data-taking, without interfering with the major science operations of CTA. Those studies (such as intensity interferometry) which require specific observations can likely make use of bright moonlight time, thus enhancing the CTA science return without negative impact on the key science goals.

The expectations for the scientific capabilities of CTA presented in this document are based on sensitivity and response functions derived from detailed Monte Carlo simulations of the CTA observatory. The simulations model extensive air showers initiated by high-energy (HE) primary particles (gamma rays or background cosmic-ray nuclei and electrons), the Cherenkov light production by the shower particles, and the measurement processes of imaging atmospheric Cherenkov telescopes…

The following sections are included:

• References
• Glossary