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.

Much of the Non-thermal Universe remains to be explored, 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 remains 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 was and 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 100s to 1000s of years 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 Fermi 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 unusual concentrations of target interstellar matter. Except for very strong sources, space instruments such as Fermi run out of gamma rays at energies between a few GeV to a few 10s of GeV, due to the limited detection area. This is where ground-based instruments such as H.E.S.S. take over, exploring the gamma ray sky in the very high energy (VHE) domain. With the new H.E.S.S. II telescope with its larger dish and lower energy threshold, a seamless connection to the Fermi range will be possible.

Sources of high-energy particles in the Cosmos

Sources vs time
Discoveries of very high energy gamma ray sources with time (click to enlarge)

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 large number of new sources - in fact, the majority of all known sources - see side panel. The images below show the sky with the H.E.S.S-detected gamma ray sources, as provided by TeVCat (as of July 2012), and the H.E.S.S. Galactic Plane Survey revealing a large number of Galactic sources. More information is given in the H.E.S.S. Source of the Month, the H.E.S.S. publications, and the H.E.S.S. source catalog.

TeVCat sky map of H.E.S.S.-discovered gamma ray sources, as of July 2012. The colors indicates the likely nature of a source.
H.E.S.S. Galactic Plane Survey, with source identifiers indicated (click to enlarge).

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 below are extended objects—as well as good spectral resolution.

Demonstrated or suspected sites of particle acceleration and particle interaction in the Cosmos include:

-- Supernovae

cas A
Cas-A supernova remnant in X-rays (details and references)
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, RX J0852.0-4622, RCW 86 or G356.6-0.7 and has resolved their shell structure.

-- Pulsars and pulsar nebulae

The Crab pulsar nebula ( details and references)
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 significant number of objects of this type to the source catalogs, including the supernova remnant G0.9+0.1, the binary pulsar PSR B1259-63, or the extended nebulae MSH 15-52, HESS J1825-137, the Kookaburra sources, HESS J1718-385, Vela-X, and many more. The pulsar wind nebula N157B in the Large Magellanic Cloud is only extragalactic stellar source, and the most distant stellar source of very high energy gamma rays detected up to 2012. H.E.S.S. results have revealed pulsar wind nebulae as the most abundant type of sources of very high energy gamma rays.

-- Binary stars

accretion disk
Illustration of a binary system
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. H.E.S.S. has detected variable emission from the binaries PSR B1259-63 and LS 5039, and has discovered the new gamma ray binary HESS J0632+057.

-- Giant molecular clouds

molecular cloud
Molecular cloud, obscuring background stars ( details and references)
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. The diffuse very high energy gamma ray emission from the Galactic Center region is most likely tracing high-energy cosmic rays propagating in the gas clouds. Particularly interesting would be a source of cosmic rays illuminating clouds at different distances, and allowing to study cosmic-ray propagation. The supernova remnant W28 with its nearby gamma-ray emitting clouds is generally considered a good candidate for such a scenario.

-- Star clusters

The young stellar cluster Westerlund 1 (details and references)
Stellar clusters have been speculated to offer an environment suitable for particle acceleration, with high-velocity winds of massive stars colliding and forming termination shocks. H.E.S.S. has detected very high energy gamma rays from clusters such as Westerlund 1 and Westerlund 2. However, given that the clusters may also contain binary systems, neutron stars, and supernova remnants, pinpointing the exact origin of the gamma rays proves difficult. A similar challenge is the interpretation of a gamma ray signal from the old stellar (globular) cluster Terzan 5.

-- "Dark" sources

For some sources of very high energy gamma rays, such as HESS J1507-622, no plausible counterpart at other wavelenghts is known. This may only partly be explained by insufficient exposure by radio and X-ray instruments; potentially these objects represent a new type of source - some authors speculate e.g. that some could be GRB remnants. On the other hand, experience has shown that many of the past unidentified sources have meanwhile found conventional explanations, see e.g. HESS J1813-178 or HESS J1857+026.

-- Starburst galaxies

ngc 253
The Starburst Galaxy NGC 253 (details and references)
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 were predicted to be powerful gamma-ray sources. Indeed, H.E.S.S. succeeded in detecting the starburst galaxy NGC 253.

-- Black holes in the centers of active galaxies

cen A
The active galaxy Centaurus A (details and references)
Active galaxies are currently among the best-explored sources of TeV gamma-rays, due to objects such as Markarian 421 and 501 or PKS 2155-304, 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 the remarkable PKS 2155-304 with its minute-scale outbursts, closeby radio galaxis such as Centaurus A or M 87, or distant blazars such as PKS 1510-089

-- Clusters of galaxies

The Perseus cluster of galaxies (details and references)
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. So far, no cluster has been detected in very high energy 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. So far, no pair halo has been detected in VHE gamma rays.

Beyond probing cosmic particle accelerators, very high energy gamma ray astronomy addresses a range of topics from astrophysics, cosmology and fundamental physics, such as:

-- Measuring the extragalactic background light

Over extragalactic distances, gamma rays suffer absorption by interactions with infrared and visible extragalactic background light (EBL), resulting in electron-positron pair creation. Since gamma rays of a given energy preferentially interact with photons of a reciprocal energy, the spectrum of EBL photons is imprinted on the spectrum of gamma rays arriving on Earth. The EBL represents the light emitted from all stars since the beginning of the Universe and its level reflects the star formation history in the Universe. However, due to the strong foreground light from within the solar system and the from the Galaxy, the level of EBL is difficult to measure directly. Gamma ray spectra from AGN offer a means to probe this cosmologically interesting quantity, see discussion of the blazars 1ES 1101-232 or 1ES 0229+200.

-- The spectrum of cosmic-ray electrons

Contrary to nucleonic cosmic rays, which propagate long distances through the Galaxy, cosmic ray electrons rapidly lose their energy; their sources should be relatively nearby and specific sources such as pulsar wind nebulae may produce specific spectral features such as bumps in the spectrum. The good particle identification capability of Cherenkov telescopes allowed H.E.S.S. to extract the electron yield among the cosmic ray background, for the first time revealing a break or cutoff in the electron spectrum around one TeV.

-- Probing fundamental physics with very high energy gamma rays

Lorentz invariance states that the speed of light is identical in every reference frame, and for all photon energies. However, models of quantum gravity predict violations of Lorentz invariance. In quantum gravity, space-time aquires a structure on length scales of the Planck scale (10-35 m), sometime referred to as a space-time foam. Directly probing these scales requires energies of the Planck energy scale, about 1028 eV, far beyond all energies of particles produced in accelerators or observed in cosmic rays. However, even at much lower energies, such a hypothetical space-time foam may influence propagation of particles such as photons, modifying their speed of propagation. Particles will be more susceptible to effects of a space-time foam the shorter their wavelength is, and hence the higher their energy is. For example, studies of the energy dependence of flares in PKS 3155-304 show constant speed of light across the TeV energy range and provide constraints on Lorentz-violating mass scales, approaching the Planck scale.

-- Search for dark matter particles and other relics of the Big Bang

An entirely different source of gamma-rays could be various types of heavy relics from the Big Bang, ranging from supersymmetric dark matter particles to monopoles or 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 or - in case of dark matter particles - pair annihilation of such objects would generate a steady flow of gamma-rays, with distinct spectral signatures. H.E.S.S. limits on dark matter annihilation at the core of the Milky Way are the currently most stringent limits at TeV energies.

W. Hofmann, July 2012