Study Themes

Addressing questions in and beyond astrophysics

Further Reading

White Paper on CTAO Science Case (October 2017)

Download from arXiv

Study Themes


With an unprecedented performance at energies up to 300 TeV, the CTAO will provide a more detailed view of the gamma-ray sky than ever seen before. The CTAO will transform our understanding of the high-energy Universe by seeking to address a wide range of questions in and beyond astrophysics. These questions fall under three major study themes:



Theme 1: Understanding the Origin and Role of Relativistic Cosmic Particles


  • What are the sites of high-energy particle acceleration in the Universe?
  • What are the mechanisms for cosmic particle accelerations?
  • What role do accelerated particles play in feedback on star formation and galaxy evolution?



Theme 2: Probing Extreme Environments


  • What physical processes are at work close to neutron stars and black holes?
  • What are the characteristics of relativistic jets, winds and explosions?
  • How intense are radiation fields and magnetic fields in cosmic voids, and how do these evolve over cosmic time?



Theme 3: Exploring Frontiers in Physics


  • What is the nature of dark matter? How is it distributed?
  • Are there quantum gravitational effects on photon propagation?
  • Do axion-like particles exist?


To learn more about how the CTAO will address these themes, click on the theme links above or continue reading below.

Top Background Image Credit: NASA/ESA

Relativistic Cosmic Particles

Image Credit: Chandra X-Ray Observatory

»With an unprecedented performance at energies up to 300 TeV, the CTAO will provide a more detailed view of the gamma-ray sky than ever seen before.«

Relativistic particles appear to play a major role in a very wide range of astrophysical systems, from supernova explosions to active galactic nuclei (AGN) with outbursts reaching tens of millions of light years in size. Within the interstellar medium of our own Galaxy, these cosmic rays (CRs) are close to pressure equilibrium with turbulent motions of gas and magnetic fields, yet the relationship between these three components, and the overall impact on the star-formation process and the evolution of galaxies, is very poorly understood. By observing different astrophysical systems at different scales, the CTAO will probe the cosmic ray protons and nuclei contribution to the observed non-thermal emission over the energetically sub-dominant electrons responsible for the observed radio and X-ray emission. The precise energy and morphology measurements done with the CTAO will provide insights into the processes of acceleration, transport and the CR-mode feedback mechanisms in these systems.


Cosmic Accelerators

One of the major goals of gamma-ray astrophysics has been to establish the nature of the cosmic sources that are accelerating charged particles, specifically, those that contribute to the locallymeasured cosmic rays, which are 99 percent protons and nuclei.


While a great deal of progress has been made, key questions remain unanswered:


  • To what extent do supernova remnants contribute to Galactic cosmic rays? 
  • Where in our Galaxy are particles accelerated up to PeV energies? 
  • What are the sources of high-energy cosmic electrons? 
  • What are the sources of the ultra-high energy cosmic rays (UHECRs)? 


The CTAO will follow two main approaches to address all these questions and analyze the mechanism(s) for particle acceleration at work in cosmic sources:


  • A census of the sources entailing particle acceleration in the Universe by means of Galactic and extragalactic surveys and deep observations of key nearby galaxies and galaxy clusters 
  • Precision spectral, morphological and timing measurements of archetypal bright and nearby sources that will provide a deeper physical understanding of the processes at work in cosmic accelerators 

Propagation and Influence of Accelerated Particles

Beyond the question of how and where particles are accelerated in the Universe, is the question of what role these particles play in the evolution of their host objects and how they are transported up to larger scales.

Above, simulated CTAO images of the TeV-bright supernova remnant RX J1713 3946 for different emission scenarios. Source: Nakamori T. & et. al (2015). Prospects for CTA Observations of RX J1713.73946. Astroparticle Physics.


The CTAO will map extended emission around many gamma-ray sources and look for energy dependent morphology associated with diffusion (in the case of hadrons) or cooling (in the case of electrons). As energy dependence is expected to be opposite in the two cases, such mapping provides another means to separate emission from these two populations. In the gamma-ray domain, the CTAO’s unprecedented angular resolution, energy resolution and background rejection power will make this possible.

Extreme Environments

Image Credit: NASA/JPL-Caltech

Particle acceleration to very high energies (VHE) is typically associated with extreme environments – areas close to compact objects such as neutron stars and black holes or in relativistic outflows or explosions. Therefore, VHE emission from accelerated particles is a probe of these environments, providing access to time and distance scales inaccessible in other wavebands. The following extreme environments have been identified as key areas for which CTAO data may have a transformational impact:

Active galactic nuclei (AGNs) are thought to harbour supermassive black holes (SMBH), which accrete material and from whose vicinity collimated relativistic outflows are launched. Similarly, accreting stellar mass black holes are known to produce jets, and particle acceleration seems to be universally associated with black hole-powered jets. Acceleration may occur in many places in these systems: from extremely close to the SMBH to where the largest AGN jets finally terminate. Active galaxies are also thought to accelerate the ultra-high energy cosmic rays with energies up to 1020eV, but, so far, there is no strong evidence for hadronic acceleration in AGN jets. 


CTAO data will play a key role by establishing the presence of VHE particles, identifying the presence of hadrons and probing the bulk ultra-relativistic motions of the inner jet. The CTAO will be able to measure minute timescale variability and work in synergy with observatories in other wavelengths to study the simultaneous broadband variable jet emission in both Galactic and extragalactic systems.

Graph of CTA simulation of active galaxy jet

The figure above, a simulation of a CTAO light curve for the 2006 flare of PKS 2155 304, illustrates how the CTAO will probe the ultra-fast variability in the inner jet of an active galaxy. Source: Sol H., Zech A., Boisson C. et al. (2013). Active Galactic Nuclei under the scrutiny of CTA. Astroparticle Physics, 43, 215 

The CTAO will probe the environment around neutron stars via pulsed gamma-ray emission from the magnetosphere of pulsars and the ultra-relativistic outflows of these systems via mapping and spectral measurements of the associated synchrotron/IC nebula and (possibly and uniquely) the un-shocked pulsar wind. Because young and energetic pulsars cluster tightly along the galactic plane, most objects will be covered by the CTAO’s galactic plane survey.


Binary systems including a pulsar provide a unique opportunity to study a relativistic outflow under changing physical conditions as the orbit progresses via energy-dependent light-curve measurements and including data from other wavebandssuch as radio and X-ray.


Merging neutron stars and other compact object mergers are among the counterparts of short gamma-ray bursts (GRBs). Such transients have been associated with detectable gravitational waves (GWs). The CTAO will rapidly respond to triggers provided by GW interferometers and GRB monitoring instruments, thus probing the highest-energy processes associated with such events.

Much of the Universe consists of extremely under-dense regions known as cosmic voids. VHE photons interact within these voids and allow us to probe the radiation fields and magnetic fields that they contain. The extragalactic background light (EBL) is the integrated emission from all stars and galaxies. The EBL carries the signature of the evolution history of the Universe, and, as such, it represents a valuable tool for cosmology. However, it is extremely difficult to measure directly due to very strong foregrounds from the solar system and the Milky Way. Yet, the EBL leaves an imprint on the measured spectra of gamma-ray sources via the process of gamma-gamma pair production. The wide-band, high-quality spectra that the CTAO will collect from many extragalactic objects will bear the signature of the EBL absorption, thus leading to a precise measurement of the EBL spectrum from the optical to the far infrared and of its evolution through cosmic time. 


The pair-production of TeV photons in voids also offers the prospect of measuring the extremely weak magnetic fields, which are reminiscent of the initial magnetic fields that existed in the early Universe. These intergalactic magnetic fields lead to a deviation in the direction of the lowerenergy secondary gamma rays produced by the interaction with the EBL. A gamma-ray halo is formed around point-like sources, possessing specific energy and time signatures that can be revealed by the CTAO, itself, or in conjunction with other high-energy observatories.

Physics Frontiers

Image Credit: NASA/ESA

»An energy resolution of 10 percent will improve CTAO’s ability to look for spectral features and lines associated with the annihilation of dark matter particles.«

The CTAO has considerable potential for discovery in the area of fundamental physics. CTAO observations can be used to reveal the nature and properties of dark matter, in the form of interactions of supersymmetric particles or asaxion-like particles, and to probe the validity or violation of the Lorentz Invariance in gravitational interactions. Each of these effects would represent an incredibly significant discovery. The major boost in sensitivity and energy coverage of the CTAO makes the manifestation of these effects a possibility and will undoubtedly lead to further discoveries in fundamental physics.

Dark matter is thought to account for a large part of the total mass of the Universe, but its nature remains one of the greatest mysteries in science. There are numerous lines of evidence for the presence of an unknown form of gravitating matter that cannot be accounted for by the Standard Model of particle physics. The observation of the acoustic oscillations imprinted into the cosmic microwave background quantifies this dark component as making up about 27 percent of the total Universe energy budget. 


The CTAO will reach the expected thermal relic cross-section for self-annihilating dark matter for a wide range of dark matter masses, including those inaccessible to the Large Hadron Collider. The long travel times of gamma rays from extragalactic sources combined with their short wavelength make them a sensitive probe for energy-dependent variation of the speed of light due to quantum-gravity induced fluctuations of the metric. The CTAO will be sensitive to such effects on their expected characteristic scale: the Planck scale.

Above, CTAO sensitivity to a WIMP (weakly interacting massive particle) annihilation signature as a function of WIMP mass, for nominal parameters and for multiple CTAO observations. The dashed horizontal line indicates the cross-section for a thermal relic WIMP.

The CTAO’s detection of photons of energy extending up to hundreds of TeV from distant cosmic sources will make it a powerful tool to search for new physics beyond the Standard Model. Apart from the search for annihilation/decay signals from dark matter, there is the exciting possibility of detecting axion-like particles (ALPs) and finding evidence of Lorentz Invariance Violation associated with quantum gravity effects on space-time at the Planck scale. Blazars and gamma-ray bursts have been identified as the most promising (bright and distant) target classes for both these searches. 


On their long journey, gamma rays may couple to other light particles such as axions, under the influence of intergalactic magnetic fields. Such photon-axion oscillations effectively make the Universe more transparent to gamma rays and, akin to neutrino oscillations, introduce a spectral modulation. ALPs are expected to convert into photons (and vice versa) as they traverse cosmic magnetic fields. In the case of a very distant active galactic nuclei, the ALP/photon coupling can result in a detectable enhancement of the TeV photon flux (which competes with the absorption on the EBL), depending on the ALP mass. 


It has been suggested that quantum gravity effects may induce time delays between photons with different energies traveling over large distances, corresponding to a non-trivial refractive index of the vacuum. Measurements of the gamma rays from bright gammaray bursts and blazars over a wide energy range will allow the CTAO to probe such effects with an unprecedented statistical significance.

Learn about the cosmic sources the CTAO will target in order to address these themes on the Key Targets page.