Astrophysics with H.E.S.S.
The basic physics goal of the H.E.S.S. experiment is to explore the production and propagation of high-energy particles in the Universe—i.e. to explore the Non-thermal Universe.
The Non-thermal Universe
Much of the radiation propagating in the Cosmos, and incident on the Earth,is thermal radiation generated in hot objects such as stars. Under extreme conditions, thermal radiation can reach into the keV energy range and beyond. It is well-known, however,that certain particle populations in the Cosmos cannot result from thermal processes, and must instead be produced by collective mechanisms, focusing the energy outflow from a source onto a relatively small number of particles. The best-known example of a non-thermal particle population are the cosmic rays. Their power-law spectrum shows no indication of a characteristic (temperature) scale, and their energies —up to 1020 eV and above—are well beyond the capabilities of any conceivable thermal emission mechanism.
So far, the Non-thermal Universe is still pretty much Terra Incognita, and while the sources and collective acceleration mechanisms for particles of TeV energies and beyond are subject of much speculation and theoretical work, the experimental identification of sources and of acceleration mechanisms is challenging. Before the advent of advanced instruments such as H.E.S.S., only few—and possible atypical—objects had been detected, and even less had been studied in any detail.
At the same time, however, the Non-thermal Universe is of significant importance for our understanding of the Universe, its objects, and their evolution. In our Galaxy, e.g., the energy density of cosmic rays is comparable to the energy density of starlight, of interstellar magnetic fields, and of the kinetic energy density of interstellar gas. In interplay between cosmic rays and magnetic fields influences the evolution of galaxies. Nonlinear amplification mechanisms transform a significant fraction of the kinetic energy released in supernova explosions into energies of highly relativistic particles.
The primary goal of the H.E.S.S. experiment is to provide the experimental basis for an improved understanding to the acceleration, propagation and interactions of such non-thermal populations of particles.
Using high-energy gamma-rays to locate cosmic accelerators
Cosmic particle accelerators are believed to accelerate primarily charged particles, such as electrons and ions, by acting on these particles with electric fields or magnetic fields. Acceleration can be a one-shot process, where particles are accelerated in huge electric fields generated, e.g., by rotating neutron stars. Other modes of acceleration result in a slow, but continuous increase in particle energy. In shock waves generated by supernova explosions, e.g., particles bounce between magnetic fields, gaining little bits of energy, and take 10000 years or more until they escape with high-energy from the acceleration zone.
High-energy gamma rays are almost always secondary products of the cosmic accelerators. Gamma rays are produced, e.g., when a proton accelerated in the supernova blast wave interacts with nuclei of the ambient medium, generating new particles in the collision, among them π0-mesons, which decay into two gamma rays. If the primary accelerator generates a beam of high-energy electrons, these electrons may undergo bremsstrahlung in the ambient medium, may suffer synchrotron radiation losses in local magnetic fields,or may, via the inverse Compton scattering process, transfer a significant part of their energy to an ambient photon, which then emerges as a high-energy gamma ray.
Compared to the charged particles, which are the primary products of cosmic accelerators, gamma-rays have the substantial advantage that they propagate on straight lines through the universe. The charged particles are deflected by galactic and intergalactic magnetic fields. Gamma-rays detected on earth therefore point back towards their sources and can be used to locate and study the sources. With charged particles, deflections are so large that over most of the energy regime, the pointing information is completely lost; only at the very highest energies, in the domain of the AUGER experiment, can the directional information possibly be exploited.
The flux of gamma rays from a source region is governed by the density of their charged parent particles, multiplied by the density of the target used to generate gamma rays—the ambient medium, the energy density in magnetic fields, or the energy density in low-energy target photons for the inverse Compton process. The energy spectrum of gamma rays is closely related to the spectrum of the parent particles.
A nice example of how photons can be used to trace high-energy cosmic rays and their sources is provided by the EGRET sky surveys. There, the Milky Way shows up as a continuous band of gamma rays, generated in interactions of the cosmic rays pervading the Milky Way with its interstellar gas. Superimposed on the continuum are point sources, reflecting acceleration sites or unusually concentrations of interstellar matter. The energy regime of the EGRET observations—a few 100 MeV to a few GeV, is three orders of magnitude below the energy range addressed by H.E.S.S., and well below the energies relevant for high-energy cosmic rays, but the example nicely illustrates the basic principles of gamma-ray astronomy.
Sources of high-energy particles in the Cosmos
A large variety of sources has been proposed to feed non-thermal particle populations in the universe. Many of them belong to the most extreme spots in the universe, regions where energy densities are huge and where the laws of physics are probed under unprecedented conditions. H.E.S.S. has already detected a number of new sources, see links given below, as well at the H.E.S.S. Source of the Month, the H.E.S.S. publications, and the H.E.S.S. source catalog.
Acceleration sites include:
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Supernovae. The shock wave launched into the circumstellar medium after the collapse of a star, that has burnt its nuclear fuel, can very efficiently accelerate particles. Models predict that 10% or more of the kinetic energy of the explosion is transferred to high-energy particles. Supernovae might be responsible for the bulk of the cosmic rays in the Galaxy, at least up to energies of 1015 eV. For example, H.E.S.S. has detected high-energy gamma rays from the Supernova remnants RX J1713.7-3946 and RX J0852.0-4622, and has resolved their shell structure.
Cas-A supernova remnant in X-rays ( details and references) - Pulsars and pulsar nebulae.
Pulsars—rapidly rotating neutron stars left over, e.g., after a supernova explosion— exhibit large electric and magnetic fields and act like dynamos accelerating particles. The pulsar-generated outflow—the pulsar wind—interacts with the ambient medium, generating a shock region where particles are accelerated. Such objects will therefore exhibit a pulsed component of radiation—from the immediate vicinity of the pulsar—and an unpulsed component from the shock region and beyond. The Crab Nebula is one of the few known TeV emitters of this type, and the best-studied object. H.E.S.S. has added a number of objects of this type to the source catalogs, including the supernova remnant G0.9+0.1, the binary pulsar PSR B1259-63, and the extended nebula MSH 15-52.
The Crab pulsar nebula ( details and references) - Binary stars.
Binary systems of stars, where one object accretes matter from the other, may serve as smaller-scale models for active galactic nuclei with the accretion disk surrounding the central massive black hole. In binary systems, the accreting objects could either be a neutron star, or a black hole of few solar masses. Galactic binary systems share many of their characteristics with their big brothers, and are expected to emit high-energy gamma rays, their lower luminosity being compensated by the much smaller distance from the Earth.
Illustration of a binary system ( details and references) - Giant molecular clouds.
Contrary to the active sources mentioned so far, molecular clouds represent an interesting class of passive sources of gamma-rays. Cosmic rays from external sources interact with the relatively dense material of clouds, generating localized sources of gamma rays. The flux of gamma rays may be further enhanced if cosmic rays are trapped in magnetic fields of the cloud. Particularly interesting would be a source of cosmic rays illuminating clouds at different distances, and allowing to study cosmic-ray propagation.
Molecular cloud, obscuring background stars ( details and references) - Starburst galaxies.
Starburst galaxies are characterized by high star-formation rates, resulting both in a high frequency of supernova explosions and in a plasma wind merging from the galaxy. The termination shock of this wind should be an acceleration site. With contributions both from the numerous supernovae and from the termination shock, starburst galaxies are predicted to be powerful gamma-ray sources.
The Starburst Galaxy NGC 253 ( details and references) - Clusters of galaxies.
Large clusters of galaxies are characterized by a relatively dense inter-cluster medium. Cosmic rays generated since the formation of the cluster turn out to be trapped inside the cluster, and cannot escape. Gamma-rays result from cosmic-ray interactions with the intra-cluster gas and allow to study the evolution of the cluster and the history of cosmic-ray acceleration.
The Perseus cluster of galaxies ( details and references) - Black holes in the centers of active galaxies.
Active galaxies are currently, together with the Crab Nebula, the best-explored sources of TeV gamma-rays, mainly due to the two objects Markarian 421 and 501, which show a large and highly time-variable gamma-ray flux. Particle acceleration is likely to take place in small regions of the relativistic jets emerging from the central black hole with a mass of about 108 solar masses. Correlations between TeV gamma-rays and non-thermal X-rays point to electrons as the primary non-thermal population. However, the acceleration mechanisms as well as the causes of the fast variability are still un resolved. Active galaxies studied with H.E.S.S. include PKS 2155-304 and the newly discovered PKS 2005-489.
The active galaxy Centaurus A ( details and references) - Relics of the Big Bang. An entirely different source of gamma-rays could be various types of heavy relics from the Big Bang, such as monopoles of cosmic strings. Such particles are predicted to have been generated in the Big Bang, and depending on their characteristics, some of them might have survived until the current époque. Decays of such objects have been proposed as the sources of the highest energy cosmic rays, and would also generate a steady flow of gamma-rays.
- Extra-galactic pair halos. If the gamma-ray emission from powerful active galactic nuclei stems to a significant extent from photons with energies > 10 TeV, then these gamma-rays will be absorbed in interactions with diffuse extra-galactic photons. The resulting electron-position pairs are deflected and isotropized in magnetic fields, and may interact with other background photons, scattering these to high energies, ultimately resulting in a cascade process leading to a halo of pairs around the source. The spectrum and spatial extent of high-energy photons emerging from this halo yields information both about the source distance and the photon fields near the source.
Goal of instruments such as H.E.S.S. is to detect a sufficient number of sources of each type to allow a meaningful taxonomy of sources and a classification of the acceleration mechanisms. For this purpose, the instrument emphasizes the ability to spatially resolve extended sources—most of the source types discussed above are extended objects—as well as good spectral resolution.
See also: H.E.S.S.physics working groups
