Observing the highest-energy processes in the Universe

Even though they are called “rays,” cosmic rays are really just standard atomic particles. Despite being “normal” matter, cosmic rays are special because they are accelerated to extraordinarily high energies, traveling very close to the speed of light. Primarily in the form of high-energy protons and atomic nuclei, 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. 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.

Black holes are among the most mysterious objects in astronomy. They are thought to be very small regions in space-time with a gravitational pull so strong that nothing, not even light, can escape. But don’t be fooled by their hallmark “black” – their surroundings are are some of the brightest sources of very-high energy (VHE) gamma rays.

It is believed that most black holes are the relics of massive stars following a supernova explosion. The core of the star collapses under its own gravity to form a black hole, which are typically only a few kilometers in radius but with a mass several times greater than the Sun. When black holes accrete (grow by gravitationally attracting more matter) material from their surroundings, it is a violent, highly energetic process. Much of the material is devoured by the black hole and it grows in size, and the frictional forces within the material spiraling into the black hole make the object immensely luminous.

On a very different size-scale, supermassive black holes are a million to a billion times more massive than the Sun and are assumed to exist in the centre of most galaxies, including our own. While the central black hole in the Milky Way is only detectable through the orbits of stars moving around it, about 10 percent of known galaxies, so-called “active galaxies,” harbour a supermassive black hole that is fueled by a huge accretion disk (a rotating disk of material or gas formed by the black hole’s accretion). The very hot disk can outshine all the stars in the galaxy itself and can produce powerful outflows called “jets,” in some cases longer than the diameter of the Milky Way.

The jets emitted from the centres of these active galaxies, called active galactic nuclei (AGNs), offer excellent conditions for particle acceleration to the highest energies and for the emission of gamma rays. AGN account for one-third of all known very high-energy (VHE) gamma-ray sources and are nearly the only objects we can detect at these energies that are not located in our own Galaxy.

The CTAO aims to measure large samples of such active galaxies, and galactic black holes, to study particle acceleration and gamma-ray emission processes. These observations will give us a picture of the conditions and physical processes occurring in and around some of nature’s most mysterious objects.

When massive stars reach the end of their natural lifetime, they die in a gigantic explosion called a supernova. The explosion causes a large part of the stellar material to be expelled at thousands of kilometres per second into the surrounding interstellar environment. The resulting shock fronts are called supernova remnants (SNRs), which emit radiation across the whole electromagnetic spectrum and play an important role in the evolution of galaxies.

It is now known that charged particles can be accelerated by SNRs to reach energies beyond those achievable with the most powerful man-made particle accelerator, the Large Hadron Collider at CERN. SNRs may be the dominant source of the cosmic rays that bombard the Earth. Particles accelerated in SNRs are implicated in the growth of magnetic fields in the Universe and can influence star-formation in galaxies.

The CTAO will be able to detect a much larger number of SNRs in gamma rays than is currently possible and measure the properties of these objects in much greater detail, helping us to understand the process of particle acceleration in SNRs and the propagation of these particles away from SNRs and their subsequent impact on the interstellar medium. Crucially, for the first time, the CTAO will be able to probe particle acceleration up to Petaelectronvolt (PeV or 10^15 eV) energies in these objects and for any class of objects within our Galaxy. We know from the locally measured cosmic rays that something in our Galaxy accelerates particles to those energies, but the sources remain unknown. There is very recent evidence of particle acceleration in the Galactic Centre, but it is not clear if it can provide the local cosmic rays.

When a massive star reaches the end of its life, it undergoes a supernova explosion, ejecting most of its outer layers. The remaining core of the star collapses and, depending on its mass, becomes a white dwarf, a neutron star or a black hole. Neutron stars are formed from the collapse of ordinary stars roughly 8-20 times the size of the Sun and are incredibly dense – the equivalent of the Earth’s mass condensed into a space the size of 1-2 football stadiums.

In the collapse process, as the radius of the star decreases, the magnetic field becomes stronger and the rate of rotation increases (often rotating many times per second). As it rotates, so does its magnetic field, creating an electric field on the surface that accelerates charged particles. The radiation produced by these particles during their acceleration leads to a beam of electromagnetic emission along the axis of the magnetic field. As the neutron star rotates, the jets may swing past the Earth’s direction, much as the light from a lighthouse passes over the sea, leading to the observation of pulsed objects or “pulsars.”

The pulsar’s rotation rate slows down over time, as it uses its rotational energy to accelerate particles to high energies. These particles, trapped by the magnetic field, rotate in sync with the pulsar out to large distances. At a distance called the light cylinder, the particles would have to travel at the speed of light to continue keeping up with the pulsar. Rather than break the laws of physics, the particles are able to escape from the immediate region around the pulsar at this location, streaming away and creating what is called a pulsar wind. When this ultra-fast wind plows into the surrounding material, it creates a shock wave where particles are accelerated, spreading out into a cloud called a pulsar wind nebula (PWN).

Emissions from both pulsars and their wind nebulae have been detected at TeV energies. Pulsar wind nebulae (PWNe) are the most populous class of galactic objects in this energy range. The most famous PWN is the Crab Nebula, which formed from the Crab supernova explosion in 1054 AD, as recorded by Chinese astronomers. The Crab is one of the brightest TeV sources and was the first TeV gamma-ray source to be detected (in 1989). Pulsation from the Crab pulsar has been detected across the electromagnetic spectrum, from radio up to approximately TeV.

Observations with the CTAO will provide the first truly detailed gamma-ray images of PWNe and make it possible to probe the motion and cooling of high-energy particles in PWNe. The CTAO will also greatly increase the number of known PWNe, helping to further the understanding of their evolution and the impact of their environment. Additionally, the CTAO will provide insights in the central engine of PWNe, the pulsar itself, greatly expanding the number of known VHE pulsars and the precision with which they can be measured.

Binary systems are composed of two objects that closely orbit one other, exchanging matter and energy through accretion processes or via the periodic interaction of their respective winds. 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 emission. In some cases, one of the companions of the binary system is a “compact object,” such as a black hole or neutron star, which pulls in material from the companion star through an accretion disk. From the vicinity of such compact objects, jets of particles that travel close to the speed of light can be emitted, giving rise to high-energy electromagnetic radiation. As a binary system moves through its orbit, the physical conditions in the collision region change. For this reason, binaries can be thought of as a laboratory for high–energy astrophysics, allowing scientists to adjust the parameters of the system and see what happens. 

A few hundred of these systems have been discovered in our Galaxy thanks to the advent of X-ray astronomy in the 1960’s, but only a handful of binary systems emitting VHE gamma rays have been detected in our Galaxy in recent years. The improved capabilities of Cherenkov telescopes (H.E.S.S., MAGIC and VERITAS) have made these more recent discoveries possible. Their discovery has proven to be extremely useful to study high-energy processes, in particular particle acceleration, emission and radiation reprocessing, and the dynamics of the underlying magnetized flows. The CTAO will greatly expand the population of gamma-ray binaries and allow us to precisely measure the behaviour of many systems as a function of orbital phase and photon energy. These measurements are expected to cast light on the physics of particle acceleration, as well as the winds of pulsars and massive stars and the way they interact. 

The nature of dark matter is one of the biggest outstanding questions in science. Dark matter is known to exist due to its gravitational effects and in far larger quantities than normal matter, but close to nothing is known about what it is. Many hypotheses exist for dark matter, mostly postulating a new weakly interacting massive particle, or WIMP). Some of the most promising theories predict that WIMPs can annihilate when they interact to produce more familiar particles. Such annihilations inevitably produce gamma rays. 

There is a strong idea as to how often these annihilations should happen in order to give the WIMPs the right density in the Universe today, as well as where to look for this signal – in places where the density of dark matter is very high (for example, the centre of our own Galaxy). Up until now, there have not been instruments sensitive enough to see the predicted signal. The CTAO will reach this critical sensitivity and complement other searches using the Fermi satellite, the Large Hadron Collider (LHC) and deep underground direct searches for WIMPs. Together these instruments have a very good chance to solve the mystery of dark matter within this decade. More about how the CTAO will study dark matter.

Most of the Universe is very close to empty, with matter clustered into galaxy clusters, super-clusters and filaments, separated by huge voids. How empty these voids are is a matter of great debate. In particular, are there any tiny magnetic fields in these regions that are a relic of the earliest moments of the Universe? The CTAO will be able to probe magnetic fields in the voids via observations of halos around active galaxies and also help to probe the proposed heating of low-density regions in the Universe by TeV photon interactions. A known ingredient of the space between galaxy clusters is the extragalactic background light, the integrated light of all processes over the history of the Universe in the infrared to ultraviolet range. The CTAO will be able to characterize these radiation fields via the absorption features that they leave in the spectra of the population of active galaxies seen by the CTAO.