Very high energy gamma-ray astronomy

## Contents

* 1 Introduction
* 1.1 The Mystery of Cosmic Rays
* 1.2 Gamma Rays
* 1.3 Production Mechanisms
* 1.3.1 Synchrotron Radiation and Curvature Radiation
* 1.3.2 Inverse Compton Scattering
* 1.3.3 Hadronic Interactions and Neutral Pion Decay
* 1.3.4 Exotic Phenomena
* 2 Detection Techniques
* 2.1 Space-based Techniques
* 2.2 Ground-based Observations
* 2.2.1 Atmospheric (Electromagnetic) Showers
* 2.2.2 Hadronic Cascades
* 2.2.3 Atmospheric Cherenkov Emission
* 2.3 Detection Techniques
* 2.3.1 Imaging Atmospheric Cherenkov Telescopes
* 2.3.2 Stereo Imaging Operation
* 2.3.3 Cherenkov Sampler Detectors
* 2.3.4 Particle-on-ground Detectors
* 3 A short history of Gamma-ray Astronomy
* 4 The Gamma-ray Sky
* 4.1 A new view on the Milky Way
* 4.2 The Distant Universe
* 5 Gamma-ray Astronomy, Cosmology and fundamental Physics

# [edit] Introduction

## [edit] The Mystery of Cosmic Rays

Earth is constantly bombarded by energetic particles known as cosmic rays, whose energy extends to macroscopic levels. The first indications of their existence date back to the late 19th century with the observation of spontaneous discharge of vacuum electroscopes, but it was Victor Hess who, through a series of balloon flights in 1912, established their extraterrestrial origin. One hundred years later, although our understanding of cosmic rays has greatly advanced, their origin remains partially mysterious.

We now know that cosmic radiation is mainly composed of charged particles, particularly protons (88%), and nuclei. It also contains electrons, as well as anti-particles in small quantities (antiprotons and positrons) and neutral particles, including gamma rays. These particles are believed to be produced in the most violent phenomena of the Universe, such as the environment around supermassive black holes (active galactic nuclei), explosions of massive stars (supernovae), neutron star mergers, or other phenomena.

The spectrum of cosmic rays (i.e., their flux as a function of energy), close to a power law over more than 10 orders of magnitude, indicates that they are not produced directly at high energy, but result from the progressive acceleration of particles through numerous interactions in astrophysical plasmas. In such mechanisms, only charged particles can be accelerated by electromagnetic fields, and neutral particles, mainly gamma rays and neutrinos, result from the interaction of the accelerated charged particles with their environment. These are then referred to as “secondary particles”.

While primary cosmic rays are likely charged particles, primarily protons, nuclei, and electrons, one of the main obstacles to directly identifying their sources lies precisely in the fact that they carry an electric charge: as they travel through the cosmos towards Earth, they are continually deflected by galactic and extragalactic magnetic fields and follow an essentially chaotic trajectory. Upon arrival on Earth, they appear to come from a random direction, no longer bearing any relation to the direction of their source.

Conversely, if secondary neutral cosmic rays are not directly produced at the sources, they have the major advantage of traveling in a straight line, thus allowing us to trace them back to their source.

## [edit] Gamma Rays

Gamma rays are high-energy photons, the particle counterpart of electromagnetic waves, located beyond X-rays in the electromagnetic spectrum. Conventionally, gamma rays are associated with nuclear interactions and the decay of radioactive nuclei, corresponding to energies on the order of MeV, or 106 times the energy of visible light. However, they can have much higher energies, on the order of hundreds of TeV (1012 eV). In fact, there is no upper limit to their energy; gamma photons conventionally correspond to all photons with energies higher than those of X-rays.

## [edit] Production Mechanisms

In general, astrophysical objects do not directly produce gamma rays. Various mechanisms associated with astrophysical plasmas and electromagnetic waves propagating within them accelerate charged particles to colossal energies. These particles escape from astrophysical sources and interact with the surrounding medium (matter and radiation) to produce very high-energy gamma photons.

### [edit] Synchrotron Radiation and Curvature Radiation

In the presence of magnetic fields, charged particles "spiral" around field lines and follow a helical trajectory. They then lose energy in the form of electromagnetic radiation, called synchrotron radiation, whose maximum energy is directly related to the particle's energy and the intensity of the magnetic field. This radiation covers a wide range of frequencies, from radio waves to X-rays. In rare cases, such as in the presence of a relativistic plasma jet directed towards Earth, synchrotron radiation can extend into the gamma-ray domain. When magnetic field lines are strongly curved, we also speak of "curvature radiation."

### [edit] Inverse Compton Scattering

Charged particles can also interact with ambient radiation, mainly composed of visible and infrared light. In the well-known phenomenon of Inverse Compton Scattering, a high-energy photon interacts with an electron in the ambient medium, extracting it from the electron cloud of an atom while transferring a significant portion of its energy to it. The "mirror" interaction occurs when a high-energy electron interacts with a low-energy photon and transfers most of its energy to it. This interaction, called "Inverse Compton Scattering," enables the production of high-energy gamma photons from accelerated electrons.

### [edit] Hadronic Interactions and Neutral Pion Decay

Other phenomena can lead to the formation of high-energy photons. The interaction of protons or nuclei with the interstellar medium can lead to the formation of numerous particles, including pions and other unstable mesons, composed of two quarks. Some of these, notably neutral pions (π0), preferentially decay into pairs of gamma photons.

### [edit] Exotic Phenomena

Finally, the emission of high-energy gamma photons could arise from the annihilation or decay of new hypothetical particles, possible constituents of enigmatic dark matter. Their detection represents an important challenge that would open a gateway to new physics.

# [edit] Detection Techniques

## [edit] Space-based Techniques

Similar to X-rays, the atmosphere is opaque to gamma rays, as they interact with the Coulomb field of nuclei to produce an electron-positron pair, which has long required spatial detection. Gamma rays are also too penetrating to be detected by a photographic plate or a semiconductor detector, such as a CCD. They are not reflected by dielectric surfaces like mirrors, so they cannot be focused. All these reasons explain the late advent of gamma-ray astronomy, which relies on techniques inherited from particle physics.

Figure 1: Operating principle of a pair production telescope.

At energies of a few MeV, gamma photons mainly undergo Compton scattering on the electron cloud of nuclei, knocking off a peripheral electron that can then be detected through the energy losses it undergoes in the detection medium by ionization or atomic excitation. There is a certain relationship between the direction of the incident photon and that of the ejected electron, but this relationship is not unique, so an event cannot be definitively associated with a direction in the sky. Only one space instrument, Comptel, aboard the CGRO satellite, successfully exploited this technique between 1991 and 2000.

At higher energies, pair production predominates over Compton scattering: the incident gamma photon interacts with the Coulomb field of a nucleus and annihilates with a virtual photon to produce an electron-positron pair. The trajectory of these particles can then be measured in a tracker to trace back to the direction of the incident gamma photon, as shown in Figure 1. The energy of the photon is obtained by measuring the energy of the two particles in the pair using an electromagnetic calorimeter. Several space instruments have observed the sky in gamma rays using this technique, up to energies of a few tens or even hundreds of GeV: OSO-3 (without a tracker), SAS-2 (1972), COS-B (1975), EGRET (1991), followed by AGILE (2007) and Fermi-LAT (2008, still in operation).

Two other space missions, although not specifically designed for gamma-ray astronomy, are capable of observing, to some extent, the conversion of a gamma photon into a pair. These are the AMS-02 (Alpha Magnetic Spectrometer) and DAMPE (DArk Matter Particle Explorer) experiments, primarily designed for the detection of charged particles.

## [edit] Ground-based Observations

### [edit] Atmospheric (Electromagnetic) Showers

As stated, gamma photons do not reach the ground. Instead, they interact in the upper atmosphere with the Coulomb field of an atom to create an electron-positron pair. Formally, this is the interaction of a real photon with a virtual photon, which can occur above the energy threshold given by:

\\[ \tag{1} E_\gamma \ge 2 m_e c^2 = 1.022 \, \mathrm{MeV} \\]

where ​\\( E_\gamma \\) is the energy of the gamma photon and \\( m_e \\) ​ is the mass of the electron. This condition reflects the equivalence between mass and energy, with the incident gamma photon producing particles of nonzero mass. The electron and positron thus produced then interact with the Coulomb field through braking radiation, or "bremstrahlung," to emit secondary photons of lower energy. Through successive interactions, a cascade of particles develops over kilometers in the atmosphere, with the energy of the incident gamma photon redistributed among an increasing number of particles. Below a certain energy, called the "critical energy" specific to the medium (approximately 84 MeV in air), energy losses due to ionization and atomic excitation become predominant over braking radiation, and the particle cascade rapidly extinguishes.

Figure 2: Development of cascades for different energies of primary particles in the atmosphere. The lower horizontal axis corresponds to the depth traversed from the top of the atmosphere, while the upper axis gives the equivalence in altitude. The average number of charged particles in the cascade is indicated by the vertical axis.

The development of cascades for different energies of primary particles is illustrated in Figure 2. It is observed that the higher the energy of the incident gamma photon, the more particles the cascades contain, and the deeper they penetrate into the atmosphere. Some orders of magnitude of characteristic parameters are indicated in Table 1.

This development of particle cascades is similar to what occurs in particle physics detectors designed to measure particle energy, known as "calorimeters", except that the density of the atmosphere varies with altitude. Thus, the atmosphere acts as an "in-homogeneous calorimeter" able to contain cascades produced by primary particles over a very wide energy range from a few GeV to several hundred TeV.

Figure 3: Illustration of the variability in the development of atmospheric cascades. Ten cascades of the same energy (300 GeV) were simulated numerically and exhibit different developments. In these simulations, neutral particles (photons) are indicated by green traces, while charged particles – electrons and positrons – are indicated in blue. The vertical axis corresponds to altitude relative to sea level, in kilometers.

The elementary processes governing the development of atmospheric cascades are inherently stochastic in nature. Consequently, for identical initial parameters, two atmospheric cascades will have different developments, which is an intrinsic limitation of the method and necessitates the use of highly sophisticated numerical simulations. This variability of cascades is illustrated in Figure 4, showing 10 cascades produced by 300 GeV gamma photons, simulated numerically. It can be observed that the different cascades are all distinct, and the altitude of the maximum development can vary from one cascade to another by several kilometers. However, the general properties (overall shape, number of particles, etc.) remain similar.

### [edit] Hadronic Cascades

Gamma photons represent only a very small portion (less than 1%) of the high-energy particles reaching the Earth. The majority consists of protons and nuclei (more than 99%), which interact via nuclear interactions with the oxygen and nitrogen nuclei present in the upper atmosphere, also leading to the development of atmospheric cascades. However, these cascades are quite different from those produced by photons, as they contain several components:

* Nuclear fragments resulting from collisions with nuclei
* Mesons (hadronic particles composed of two quarks instead of three for protons and neutrons), both neutral and charged, and isolated nucleons
* Muons (heavier cousins of electrons, about 207 times more massive), resulting from the decay of charged muons and capable of propagating for kilometers, or even tens of kilometers
* Neutrinos, neutral particles, very light and interacting only through the weak interaction, and therefore very difficult to detect, produced by the decay of mesons and charged muons
* Electromagnetic sub-cascades, generated by the decay of neutral mesons (mainly π0) into pairs of gamma photons

Moreover, hadronic interactions are more fluctuating than electromagnetic interactions. As a result, the cascades produced by protons and nuclei are significantly more fluctuating than those generated by gamma photons, as illustrated in Figure 4. The altitude of the first interaction fluctuates considerably, the shape of the cascades is much more irregular than that of cascades generated by gamma photons, they contain penetrating particles (mainly muons) that can reach ground level, and one can often observe the superposition of several electromagnetic cascades starting at different altitudes and with different orientations.

Figure 4: Simulations of 10 atmospheric cascades initiated by 300 GeV protons. As with Figure 4, neutral particles (photons) are indicated by green traces, while charged particles – electrons and positrons – are shown in blue. The highly penetrating muons are indicated in red.

### [edit] Atmospheric Cherenkov Emission

Figure 5: Cherenkov emission pattern of an electromagnetic shower.

Air is a dielectric medium (like water or glass) that polarizes when an electromagnetic wave passes through it, altering the propagation of the wave. One of the main consequences is a slightly slower propagation of waves – on the order of 0.01%, varying with altitude. Another consequence is the refraction of light rays, responsible, among other things, in the presence of significant temperature gradients, for the formation of "mirages." A lesser-known consequence, but crucial for gamma-ray astronomy, is Cherenkov emission, which occurs when charged particles propagate faster than light in the medium (but, of course, slower than light in a vacuum).

This emission, predicted in 1934 by the Soviet physicist Pavel Cherenkov [1] has the following characteristics (illustrated in Figure 5) :

* emission in the form of a cone directed forward, with an opening angle \\( \theta_c \\) expressed in terms of the dielectric index \\( n \\) of the medium and the relative speed \\( \beta = v/c < 1 \\) of the particle :

\\[ \cos \theta_c = \frac{1}{\beta n} \\]

* a photon spectrum peaking in the blue, corresponding to the number of photons emitted per unit track length \\( \mathrm{d} z \\) of the particle and per unit wavelength \\( \mathrm{d} \lambda \\):

\\[ \frac{\mathrm{d}^2 N_\mathrm{photons}}{\mathrm{d} z \, \mathrm{d} \lambda} = 2 \pi \alpha Z^2 \frac{\sin^2 \theta_c} {\lambda^2} \\]

where \\( \alpha \\) is the fine-structure constant and \\( Z \\) is the charge number of the incident particle (-1 for an electron).

* A very short duration and very low intensity, making it completely invisible to the naked eye (except in an environment where the emission of multiple particles overlaps, such as in a nuclear fuel cooling pool).

In the case of the atmosphere, the dielectric index is very close to 1 and depends on density and temperature. The resulting Cherenkov emission angle is on the order of degrees (compared to about 30° in water) and decreases steadily with the emission altitude. The variation in density with altitude changes two aspects of cascade development:

* As the cascade penetrates deeper into the atmosphere, the density increases, reducing the characteristic distances of pair creation and braking radiation, leading to a more compact development.

* At the same time, the dielectric index increases, resulting in an increase in the Cherenkov angle and an increase in the amount of light emitted per unit track length.

These two effects exactly compensate for each other (opposite dependence on density), resulting in the Cherenkov emission remaining constant per unit thickness of material traversed, allowing for a "calorimetric" measurement of the incident photon's energy. Furthermore, as illustrated in Figure 5, photons emitted at different altitudes reach the ground at approximately constant distances from the cascade axis (about 125 m at sea level), with the variation in the Cherenkov angle with altitude compensated by the variation in distance traveled. This results in the formation of a Cherenkov light ring on the ground, illuminating an area of approximately, enabling the detection of particle cascades even at relatively large distances from their main axis.

## [edit] Detection Techniques

### [edit] Imaging Atmospheric Cherenkov Telescopes

The operation of an Imaging Atmospheric Cherenkov Imaging Telescope (IACT) is illustrated in Figure 6, left: a telescope equipped with a large collecting area and a very fast camera (exposure time on the order of a few nanoseconds) "photographs" the Cherenkov light emitted by the cascade. This requires a sophisticated triggering system capable of analyzing in real-time what is captured by the camera to detect the passage of a cosmic ray and record its image. The image of the cascade in the camera is extended since it is an "object" located at a finite distance. The detailed analysis of the light distribution in the camera allows, statistically, to determine the type of particle arriving on Earth, its energy, and its direction of arrival.

### [edit] Stereo Imaging Operation

When multiple telescopes are utilized (Figure 6, right), stereoscopic reconstruction, using the different viewpoints of the telescopes, becomes feasible: By combining the images seen by the various telescopes into a virtual camera, the direction of the primary particle (red star) is directly obtained at the intersection of the principal axes (dashed line, corresponding to the plane containing the telescope and the cascade); This significantly improves the reconstruction accuracy as well as the angular resolution and energy resolution. Moreover, stereoscopic vision enables better differentiation between electromagnetic and hadronic cascades.

Figure 6: Left: Operation of an Imaging Atmospheric Cherenkov Telescope. Right: Network of 4 Telescopes Operating in Stereoscopic Mode. Each telescope in the network observes the cascade and records its image from a unique angle, which depends on the relative position of the cascade and the telescope. Combining the views from different telescopes enables easy reconstruction of the cascade's geometry. The principal axes of the images, represented by dashed lines, correspond to the plane containing the cascade trajectory and the telescope. The intersection of the principal axes of the different telescopes (on the right) provides the direction of the primary particle (red star).

### [edit] Cherenkov Sampler Detectors

Instead of focusing Cherenkov light onto a camera placed at the focus of a telescope, thereby creating an angular image, it is possible to sample the light over a large ground surface, creating a spatial distribution. Several concepts have been developed, either using multiple telescopes with single-pixel detectors distributed over a large area or using large steerable mirrors directing the light onto a secondary optics (solar farm concept). However, these systems have proven to be less efficient than imaging telescopes and have been abandoned.

### [edit] Particle-on-ground Detectors

By placing detectors at high altitudes, close to the cascade's maximum development, one can directly access the electrons and positrons produced in the cascade and measure their distribution. This can be done with detectors for charged particles, such as scintillators combined with light detectors or resistive plate detectors. Alternatively, water can be used, in which electrons and positrons emit Cherenkov light, which is then detected by photomultipliers. In comparison to imagers, a much larger surface area needs to be instrumented (on the order of 105 m2). Although less precise than imagers, these detectors have the advantage of being able to operate day and night and have a much wider field of view. Therefore, both techniques are complementary.

# [edit] A short history of Gamma-ray Astronomy

The development of gamma-ray astronomy has been a long journey filled with obstacles. It took nearly a century of experimental developments to reach modern instruments. The first experimental detection of Cherenkov radiation is credited to the American physicist Robert Wilson in 1934, who, however, did not explore the application of this discovery to gamma-ray astronomy. It wasn't until the 1950s that John G. Galbraith suggested using Cherenkov light to detect high-energy gamma rays, and the first detection experiments were conducted, notably by John G. Jelley.

In 1956, Jelley and Galbraith published a seminal paper titled "Detection of shower particles at ground level," describing an experiment to detect atmospheric showers by the Cherenkov light emitted by the charged particles present in them. Despite this proof of concept, the technologies of the time (especially the lack of fast digitization instruments) did not allow for the implementation of this idea on a scale sufficient to detect astrophysical sources.

It took another 30 years for the first Atmospheric Cherenkov Telescope (ACT) to be built (in the 1980s), notably with the Whipple Telescope at Kitt Peak, Arizona, which in 1989 discovered the very high-energy gamma-ray emission from the Crab Nebula, thus kicking off modern atmospheric Cherenkov gamma-ray astronomy.

In the following decades, the technique of Atmospheric Cherenkov Imaging improved and became increasingly sophisticated, with larger telescopes (such as Whipple), the advent of stereoscopic observations (HEGRA), and the use of advanced electronics (CAT) and more advanced detection technologies. International collaborations were formed to build networks of Cherenkov telescopes, such as the H.E.S.S. array in Namibia, the VERITAS array in Arizona, USA, and the MAGIC array in the Canary Islands, among others. These arrays allow for more sensitive detection and more precise localization of very high-energy gamma-ray sources in the cosmos.

The next generation of Atmospheric Cherenkov Telescopes (ACT), known as the Cherenkov Telescope Array (CTA), is currently under construction and will comprise around a hundred telescopes distributed over two sites (Canary Islands and Chile). It represents a global effort involving more than 80 countries with a budget of several hundred million euros.

Meanwhile, other teams have explored sampling techniques. Some concepts studied in the 1990s, such as the conversion of old solar power plants (CELESTE, STACEE), have since been abandoned, while others have shown promise. The Milagro Observatory, operational in New Mexico, USA, between 2000 and 2008, used a 60,000-ton water pool as a detection medium for very high-energy gamma rays. In this technique, called water Cherenkov, charged particles produced in the showers and reaching the ground are detected by a network of photomultipliers immersed in water. This concept was later refined by the High Altitude Water Cherenkov Observatory (HAWC), located in Mexico at an altitude of 4100 meters, and operational since 2015. Instead of a single tank, HAWC uses over 300 water Cherenkov detectors spread over an area of over 22,000 m². This segmentation improves measurement precision and allows for better distinction between cascades produced by photons and those produced by protons and nuclei, while increasing the dynamic range from a few hundred GeV to several tens of TeV.

The latest comer in the field is the Large High Altitude Air Shower Observatory (LHAASO): LHAASO is a Chinese observatory under construction in the Sichuan province, China, which combines various detection techniques on the same site. It includes a network of water Cherenkov detectors spread over an area of about 1 km2, atmospheric Cherenkov telescopes with a wide field of view, as well as detectors for charged particles (scintillators). Primarily designed for the study of charged cosmic rays, it can also detect cascades produced by gamma rays and study associated astrophysical sources.

Following the concept of HAWC, the Southern Wide-field Gamma-ray Observatory (SWGO) is a proposed water Cherenkov observatory to be built in Mexico. Its main feature is its very large effective area of about 100 km2, which would make it the largest gamma-ray astronomy observatory in the world.

Over the past few decades, atmospheric Cherenkov gamma-ray astronomy has opened a new window on the universe by providing unique insights into the astrophysical processes associated with very high-energy gamma-ray sources. It continues to be an active and dynamic area of research in modern astrophysics.

# [edit] The Gamma-ray Sky

The combined use of space-based and ground-based instruments has allowed the study of the sky in a new energy window, representing an incredible dynamic range of over 6 orders of magnitude in magnitude (from hundreds of MeV to hundreds of TeV), revealing an unsuspected menagerie. Unlike other energy domains, where so-called thermal emissions (blackbody spectrum emitted by an object at a given temperature) dominate (infrared emissions from dust, visible light from stars, X-rays from neutron stars and other compact objects, and even the cosmic microwave background), the domain of gamma-ray astronomy is one of non-thermal phenomena, linked to the acceleration of particles and their interaction with each other and with their environment. The phenomena associated with gamma-ray emission are often very brief and violent (supernova explosions, gamma-ray bursts), associated with compact objects (active galactic nuclei, pulsars), and variable on very short timescales (sometimes less than a minute). Gamma-ray astronomy paints a picture of a young, violent, and rapidly evolving universe. It is, in a way, the merging of the infinitely small (the physics of elementary constituents of matter) with the infinitely large (the large structures of the universe), which is why this domain is often called "physics of the two infinities."

## [edit] A new view on the Milky Way

The most intense source of gamma-ray radiation is the Milky Way itself as a whole (Figure 7), betraying the omnipresence of cosmic rays interacting with stellar matter, producing various particles including neutral pions that decay into pairs of gamma-ray photons. Studying these photons reveals, much like a tomograph, the concentrations of matter in the Galaxy and the distribution of cosmic rays.

Figure 7: All-sky map of the cosmos in high energy gamma-rays, using data collected by the Fermi Gamma-ray Space Telescope from 2008 to 2017. (Image credit: NASA/DOE/Fermi LAT Collaboration).

Superimposed on the large-scale diffuse emission, numerous sources have been identified in the Milky Way, corresponding to various types of objects (Figure 8). These include:

* Supernova remnants, shells left behind by the explosions of massive stars nearing the end of their lives. In these gigantic explosions, the outer layers of the star are ejected in the form of a shock wave sweeping through the interstellar medium, accelerating particles. These supernova remnants are probably responsible for the majority of cosmic rays, at least at energies below ~10¹⁷ eV, where their origin is galactic.
* Pulsars, magnetized neutron stars rotating rapidly, and ultimate remnants of star explosions. These act as celestial dynamos, gradually slowing down while emitting radiation in the form of periodic beams sweeping across the Universe like lighthouses. Some pulsars (such as the Crab and Vela pulsars) emit photons up to energies exceeding several TeV, implying efficient particle acceleration in the outer magnetosphere, relatively far from the neutron star. Others, particularly fast ones, have particularly short rotation periods, of the order of milliseconds, even though they are rather old and dim: these are pulsars harbored in binary systems, re-accelerated by mass transfer from their companion star. Others, associated with particularly high magnetic fields, are called magnetars and may host mysterious transient phenomena, such as fast radio bursts.
* Pulsar wind nebulae, regions near pulsars where electrons and positrons emitted by the central pulsar are trapped for tens of thousands of years, forming a bright nebula from X-rays to gamma rays, indicating synchrotron emission in the pulsar's magnetosphere.
* Binary systems, consisting of a compact object (black hole or neutron star) and a companion star orbiting each other and interacting. In the case where the compact object is a black hole, one can observe matter accretion and the formation of plasma jets. These are then called microquasars. In the case of recurrent novae, the compact object is a white dwarf, which accretes matter from its companion star until the accumulated matter triggers the restart of nuclear reactions, causing a thermonuclear explosion on the surface of the white dwarf. Some novae are known for their repetitive behavior on the scale of a few years or tens of years. Some binary systems formed by particularly massive stars, especially Wolf-Rayet stars, may also be the site of violent interaction between the stellar winds of the two stars, which can accelerate particles and emit gamma-ray radiation.
* The central black hole, Sgr A*, with a mass of about \\( 4 \times 10^6 M_\odot \\), which accretes matter and behaves as a powerful particle accelerator. Various clues suggest that this black hole was much more active in the past, perhaps behaving as an active galactic nucleus.

Figure 8: The Milky Way in Very High Energy Gamma-Rays as seen by the H.E.S.S. array of telescopes.

## [edit] The Distant Universe

Outside of the Milky Way, numerous gamma-ray sources have been discovered in recent decades. Most of them are associated with supermassive black holes at the center of almost all galaxies, but others are associated with stellar activity – the birth or death of stars.

Figure 9: The standard AGN unification scheme. Depending on the orientation, an observer will see different emitting regions and thus, an intrinsically same objects would appear to him as a different type of AGN (M. Polletta, adapted from Urry & Padovani, 1995)

* Active galactic nuclei consist of a supermassive black hole accreting matter (Figure 9). Some of them exhibit gigantic plasma jets, emerging orthogonal to the accretion disk and extending over distances ten times greater than the size of the host galaxy. In these jets, violent shocks occur between elements ejected at different speeds by the black hole's environment, causing particle acceleration to very high energies. Depending on the orientation of the jet relative to Earth, different forms of emission are observed. When the jets are observed more or less laterally, they are called radio galaxies because the radio emission from the jets appears particularly intense. Conversely, when the jet is directed towards Earth, the emission of very high energy dominates widely and even obscures the host galaxy. These objects are often characterized by very rapid variability in their gamma-ray emission, with sudden eruptions and changes in brightness on the scale of minutes, or even less, which are still largely not understood.

* Outside of the Milky Way, numerous gamma-ray sources have been discovered in recent decades. Most of them are associated with supermassive black holes at the center of almost all galaxies, but others are associated with stellar activity – the birth or death of stars.

* Active galactic nuclei consist of a supermassive black hole accreting matter. Some of them exhibit gigantic plasma jets, emerging orthogonal to the accretion disk and extending over distances ten times greater than the size of the host galaxy. In these jets, violent shocks occur between elements ejected at different speeds by the black hole's environment, causing particle acceleration to very high energies. Depending on the orientation of the jet relative to Earth, different forms of emission are observed. When the jets are observed more or less laterally, they are called radio galaxies because the radio emission from the jets appears particularly intense. Conversely, when the jet is directed towards Earth, the emission of very high energy dominates widely and even obscures the host galaxy. These objects are often characterized by very rapid variability in their gamma-ray emission, with sudden eruptions and changes in brightness on the scale of minutes, or even less, which are still largely not understood.

# [edit] Gamma-ray Astronomy, Cosmology and fundamental Physics

In addition to studying astrophysical sources and particle acceleration mechanisms, gamma-ray astronomy contributes to the study of the Universe as a whole and its large-scale structures. It contributes to the search for mysterious dark matter through the investigation of signatures of massive particle annihilation, or the possible presence of new interactions, such as those between photons and hypothetical axions. By studying the emission spectra of objects located at great distances, it allows for an integrated measurement of the amount of light emitted in the Universe, thereby providing constraints on the formation of large structures. It also enables the testing of fundamental symmetries of Nature, such as Lorentz Invariance, at energies or precisions never reached before.