The CTAO’s Study Themes
Ground-based gamma-ray astronomy is just a few decades old but has enormous scientific potential, as demonstrated by the current generation of instruments that have detected ~200 gamma-ray emitters. With its superior performance, the prospects for the CTAO combine the in-depth understanding of known objects with the anticipated detection of new classes of gamma-ray emitters and a great potential for fundamentally new discoveries, expanding the number of known gamma-ray objects by up to 1,000 new sources.
The CTAO will transform our understanding of the high-energy Universe by seeking to address a wide range of questions in astrophysics and fundamental physics.
- Understanding the Origin and Role of Relativistic Cosmic Particles
- Probing Extreme Environments
- Exploring Frontiers in Physics
Theme I: Understanding the Origin and Role of Relativistic Particles
Even though they are called “rays,” cosmic rays are just standard atomic particles. They are 99% protons and helium nuclei, with the remaining 1% comprising heavier nuclei, electrons, muons and other particles. Despite being “normal” particles, cosmic rays are special because they are accelerated to extraordinarily high energies in the Cosmos, traveling very close to the speed of light. Cosmic rays constantly bombard the Earth, but despite a century-long search, we know very little about their origin sources and the role they play in our own Galaxy and beyond. This is because cosmic rays are electrically charged, and thus, their paths are scrambled in the magnetic fields between their sources and the Earth, making it impossible to trace them back to their origin.
What we do know is that they are the parent of the gamma rays the CTAO observes. Gamma rays are produced in the interactions of cosmic rays and provide the most sensitive means to study cosmic rays in and around their sources. Gamma rays are not electrically charged and so, their direct path towards Earth allows the gamma rays to transport information about their sources and the relativistic particles that created them. The CTAO’s broad energy coverage and unprecedented angular resolution will enable us to look for the possible sources of cosmic rays within our own Galaxy and beyond and map the role they play in the feedback processes at work as stars form and galaxies evolve.
Target Sources
Supernova Remnant (SNR)
Produced by the interaction of the matter expelled when a massive star dies in a supernova explosion with the matter of the surrounding medium, SNRs are expected to be “PeVatrons,” sources that are capable to accelerate particles up to Petaelectronvolts or PeV energies (a trillion times more energetic than visible light).
Pulsar Wind Nebula (PWN)
These sources originate when the ultra-fast wind of particles expelled by pulsars, remnants of the explosion of massive stars, plows into the surrounding medium. This interaction creates a shock wave where particles are accelerated, spreading out and forming the PWN. Such particles give rise to radiation that can reach gamma-ray energies. The Crab Nebula is one of the most famous PWN, serving as a standard candle for gamma-ray astronomy.
Star-Forming Regions
Cosmic rays may play a major role in the regulation of the star-formation process, so it is important to understand where they are accelerated, how they propagate and where they interact in the interstellar medium. The interaction of cosmic rays in the star-forming environments yields to the emission of gamma rays. Hence, CTAO observations of star-forming systems are expected to give insight into the relationship between high-energy particles and the star-formation process.
Theme II: Probing Extreme Environments
The gamma rays the CTAO will detect are at energies well beyond those of X-rays or even gamma rays detected by space instruments. As such, they encode information about the physical processes at work in some of the most energetic environments in the Universe. Resulting from the explosion of massive stars that reach the end of their lives, black holes, neutron stars and white dwarfs, collectively known as compact objects for their high density and strong gravitational pull, are of particular interest. Strong magnetic fields, accretion disks, jets of light and relativistic particles, powerful winds around these objects, turn them into key targets for gamma-ray astronomy. The capabilities of the CTAO will enable us to address the physical mechanisms therein with an unprecedented level of accuracy.
Target Sources
Active Galactic Nucleus (AGN)
Some galaxies harbor supermassive black holes at their centres, which can be tens of billions of times more massive than our Sun, exhibiting an intense gravitational pull that causes surrounding material to form an accretion disk and become very luminous. They are known as Active Galactic Nuclei (AGNs). Some of these AGNs feature relativistic jets of particles where light is emitted at very high energies.
Pulsar
In the collapse process of some massive stars, as the radius of the star decreases, the magnetic field becomes stronger and the rotation speed of the remnant core increases, often rotating many times per second. Thus, a neutron star is created. There is a type of neutron star, the pulsar, that is particularly interesting to the CTAO. Extremely dense and rapidly rotating, pulsars emit light in the shape of two beams that become visible to us as they sweep across our field of view, turning them into “cosmic beacons.”
Binary Systems
Binary systems are composed of two objects that closely orbit one other. Stars much more massive than the Sun can have very powerful winds, which in star binary systems, collide and can accelerate particles, producing gamma-ray emissions. When one of the companions of the binary system is a compact object, such as a black hole or a neutron star, they pull in material from the stellar companion through an accretion disk, emitting jets where particles are accelerated and gamma rays are produced. In some cases, the compact object is a white dwarf that gathers material from the companion star. Due to the accumulation of material on the white dwarf’s surface, this process can cause a sudden and dramatic increase in brightness known as a nova that is visible in the gamma-ray regime, too. Unlike the explosions of supernovae that mark the end of a star’s life, novae are recurring events.
Theme III: Exploring Frontiers in Physics
A major step forward in sensitivity and energy coverage brings discoveries in fundamental physics, or how the Universe behaves at its most basic level, well within the CTAO’s reach. Specifically, the CTAO will seek to discover the nature and properties of dark matter, probe the existence of axion-like particles and test possible deviations from Einstein’s theory of Special Relativity. Any of these discoveries would mean a revolution for particle physics and cosmology.
Target Sources
Dark Matter
Dark matter is thought to account for a far larger part of the total mass of the Universe than normal matter and manifests itself by its gravitational effects, but its nature remains one of the greatest mysteries in science. The CTAO will attempt to find dark matter by looking for the gamma rays emitted as a consequence of dark matter particles (believed to be weakly interacting massive particles or WIMPs) annihilating one another when they interact. Current instruments are not sensitive enough to detect the signal predicted by models, but the CTAO will reach this critical sensitivity.
Quantum Gravity
It has been suggested that quantum gravity effects may induce time delays between photons with different energies traveling over large distances, in other words, photons could be dispersed in space in different ways depending on their energy, which would imply a deviation from Einstein’s theory of Special Relativity, known as Lorentz Invariance Violation. Measurements of the gamma rays from very bright sources will allow the CTAO to probe such effects with unprecedented statistical significance.
Cosmic Voids
Most of the Universe is very close to empty, with matter grouped into galaxy clusters, super-clusters and filaments, separated by huge voids. How empty these voids are is a matter of great debate, but it is believed they could contain relics of the earliest moments of the Universe. To probe these voids, the CTAO will be looking for a known ingredient in the space between galaxy clusters – the extragalactic background light (EBL). EBL represents the light emitted by all galaxies since the birth of the Universe and includes clues to the history of star formation. When gamma rays collide with photons of the EBL, they generate a specific spectral signature, or “fingerprint,” that can be measured by the CTAO to gain insight into how the Universe was formed.
Multi-Wavelength and Multi-Messenger Astronomy
Multi-wavelength astronomy refers to the scientific approach of observing the light of the Universe at various energies, from radio waves to gamma rays. Moreover, the Cosmos provides information through other messengers besides light: we can study it by directly observing cosmic rays, neutrinos or gravitational waves. The combination of data from photons and other messengers is known as multi-messenger astronomy. As different wavelengths and messengers are linked to diverse physical processes, by integrating observations from different instruments and observatories, scientists can piece together a more complete picture of the astrophysical phenomena.
The CTAO will play a key role in both multi-wavelength and multi-messenger fields in the next decades thanks to its enhanced performance that will allow it to provide fundamental gamma-ray information to probe the most extreme scenarios.
Target Sources
Transients
The Universe hosts a diverse population of astrophysical objects that explode, flare up or intensify their activity in dramatic and unpredictable fashion across the entire electromagnetic spectrum and over a broad range of timescales, spanning milliseconds to years. Collectively designated as “transients,” many are known to be prominent emitters of high-energy gamma rays and are also likely sources of non-photonic, multi-messenger signals such as cosmic rays, neutrinos or gravitational waves. They are of great scientific interest, being associated with catastrophic events involving compact objects such as neutron stars and black holes that thrive in the most extreme physical conditions of our Universe. With its exceptional sensitivity to very high-energy gamma rays at very short timescales, the CTAO has the potential to break new ground in explaining the physics of cosmic transients and discovering entirely new classes of transient sources.
Gamma-Ray Burst (GRB)
GRBs are extremely energetic explosions and the brightest events in our Universe. These transient phenomena can be classified as “short,” if their initial prompt phase lasts less than a couple of seconds, or “long,” if it lasts more, up to hundreds or even thousands of seconds. The short bursts are produced by colliding neutron stars, and most of the long bursts are thought to be created by particularly energetic supernova explosions, sometimes called “hypernovae.” GRBs are extremely interesting sources for multi-wavelength and multi-messenger observations as they emit light across the full electromagnetic spectrum, and can produce neutrinos, cosmic rays and gravitational waves.