The High Energy Stereoscopic System (H.E.S.S.) near Windhoek, Namibia, is the world's leading gamma-ray observatory. It detects gamma-rays that carry more than a billion times the energy of visible light, and thereby probes the most violent places of the Universe, including black holes, neutron stars, and supernova remnants.
This array of five Cherenkov telescopes is an international project, led by German and French partners, with additional contributions from other European countries as well as the U.S.A., Australia, Armenia, South Africa and Namibia. Since 1999, North-West University is the principal South African partner H.E.S.S.
The Centre for Space Research at NWU is also involved in the next generation Air Cherenkov Telescope facility, the Cherenkov Telescope Array (CTA), which will be an array of about 100 Cherenkov telescopes, spread out over an area of about 10 km2. Together with partners at Wits University, the University of the Free State, and the University of Johannesburg, the CSR at NWU works closely together with Namibian partners on a bid by Namibia to bring the CTA to a site near Aus in southern Namibia. The South African CTA partners are organized in the South African Gamma-Ray Astronomy Programme (SA-GAMMA).
- Pulsars and pulsar wind nebulae:
Pulsars are extreme objects: incredibly compact stars rotating several hundred times per second, with plasma-filled magnetospheres threaded by intense magnetic and electric fields. They are the remnant cores of very massive stars that collapse at the end of their lives after having used up their thermonuclear fuel. Young pulsars drive powerful winds off their surface and into the surrounding shell of gas left over by the outer layers of the exploded star. The result is called a pulsar wind nebula (PWNe). PWNe are among the most abundant sources of very-high-energy gamma-rays, observable with H.E.S.S., within our own Milky Way. Old, very rapidly rotating (millisecond) pulsars (MSPs) occur abundantly in globular clusters (GCs). The advent of H.E.S.S. and the Fermi Large Area Telescope (Fermi-LAT) affords ample opportunity to study pulsar-related sources with much greater sensitivity, time and energy resolution, and energy range than previously possible.
New gamma-ray pulsar discoveries abound since the launch of Fermi in 2008, increasing their number by more than an order of magnitude to now over 100.
Due to their rapid rotation, pulsars produce very regularly pulsed emission from radio waves to gamma-rays. However, the light curves (pulse shapes) and other properties of their emission vary widely from one pulsar to the next. There is a variety of pulse profile shapes, peak separations, peak multiplicities, and radio-to-gamma phase lags. Phase-resolved spectroscopy is now feasible, allowing for deep scrutiny of current models.
Pulsed gamma-ray emission from pulsars has not only been detected by Fermi-LAT (at GeV energies), bu talso from ground-based gamma-ray telescopes like H.E.S.S., such as the Very Energetic Radiation Imaging Telescope Array System (VERITAS) and the Major Atmospheric Gamma-ray Imaging Telescope (MAGIC). This demonstrates the great potential of for TeV pulsar astronomy, and also challenges our understanding of very-high-energy (VHE) pulsar emission mechanisms.
Cherenkov telescopes have discovered around 160 VHE gamma-ray sources, and about 35 of these have been identified as PWNe. Fermi has detected five high-confidence PWNe, and 11 candidates.
Researchers in the CSR at NWU are conducting theoretical studies of the emission from pulsars and PWNe. This includes detailed simulations of the electromagnetic processes in pulsar magnetospheres to explain the pulse shapes and broadband spectra of pulsars, magnetohydrodynamic simulations of PWNe and their interaction with the surrounding supernova remnants.
- Supernova Remnants:
Researcher (s): Stefan Ferreira
Postdoc: Iurii Sushch
Student: Augusts van der Schyff
At the end of its life, very massive stars end their lifes in supernova explosions, in which the stellar core collapses to form a neutron star (which can sometimes be observed as a pulsar, see above). The outer layers of the doomed star explode outward to leave behind massive nebulae, called supernova remnants (SNRs). The picture on the left shows an example. SNRs are another abundant class of very-high-energy gamma-ray sources within our Milky way that are commonly detected both by Fermi-LAT, and ground-based Cherenkov telescopes, such as H.E.S.S. Researchers in the CSR at NWU employ different state of the art numerical models to simulate different astrophysical phenomena. These include Hydrodynamic (HD) and Magneto-hydrodynamic (MHD) modeling of stellar wind outflows, supernova remnant evolution and pulsar wind nebulae.
The shock waves of SNRs are known to accelerate elementary particles (electrons and protons to extremely high energies. These accelerated, relativistic particles are called Cosmic Rays (CRs).
A specialty to of our group is to include the contribution of cosmic rays accelerated inside these type of cavities to the local interstellar CR spectrum, taking advantage of the world-class research on CR transport calculations performed in the CSR.
Research in the CSR also involves direct participation in multiwavelength observations of SNRs, from radio observations all the way to VHE gamma-ray observations using H.E.S.S.
- X-ray / gamma-ray binaries and micro quasars:
Researcher (s): Markus Böttcher
Postdoc: Iurii Sushch
Neutron stars (NS) and black holes (BH) are often found in binary systems, where there are in a mutual orbit with a normal star. The compact object (i.e., the NS or BH) then pulls material over (a process called accretion) from the companion star. The accreted material forms an accretion disk, in which it heats up to millions of degrees. At these temperatures, the gas is a very efficient emitter of X-rays. Therefore, these binary system are usually referred to as X-ray binaries. In recent years, an increasing number of these X-ray binary systems has also been detected in gamma-rays by Fermi and ground-based Cherenkov Telescopes (such as H.E.S.S.). In most cases, the nature of the compact object is unknown. If the compact object is a NS, gamma-ray emission may emerge at the stellar-wind/pulsar-wind bow shock. Alternatively, if the compact object is a BH, these systems may contain a mildly relativistic jet and are then called micro-quasars, in analogy to their supermassive counterparts, quasars, which are powered by accretion onto supermassive black holes (millions of solar masses) in the centers of other galaxies (called active galactic nuclei, AGN). The sketch below illustrates these two gamma-ray binary scenarios. In the micro-quasar case, gamma-rays are most likely produced in the jet, analogous to AGN. Researchers in the CSR at NWU are involved boh in observational programs to study the time-variable emission, from radio waves all the way to gamma-rays, of these gamma-ray binaries, and in the theoretical simulation of particle acceleration and radiation mechanisms in gamma-ray binaries.
- Diffuse Galactic gamma-ray emission:
Researcher (s): Johan van der Walt
Postdoc (s): Sabrina Casanova
The question of the origin of cosmic rays.
Highly relativistic protons and nuclei, known as cosmic rays, pervade the Milky Way and strongly influence its dynamics. Diffusive shock acceleration in young supernova remnants is the most widely accepted scenario for the acceleration of cosmic rays up to at least 1015 eV (PeV), the so called knee, where the spectrum of cosmic-rays gradually steepens. Due to their capability of accelerating particles to PeV energies, the sources of such cosmic rays are often called Pevatrons.
There is, however, no general consensus on the maximum acceleration energy achieved in the process and, up to now, no observational evidence of the nature of Pevatrons.
The Fermi Large Area Telescope recently discovered in the gamma-ray spectra of two SNRs, IC 443 and W44, the characteristic pion-decay feature, which is produced by accelerated GeV protons interacting with ambient gas. This proves that SNRs are able to accelerate particles up to tens of GeVs, but does not prove that they are the long searched-for Pevatrons.
Cosmic ray transport in the Galaxy.
The direct observation of cosmic rays from the candidate injection sites is not possible, since CRs escape the acceleration sites and eventually propagate into the interstellar medium, where they interact with the Galactic magnetic field. Secondary CR data suggest that CR protons remain in the Galaxy for millions of years before eventually escaping from the Galaxy. During this time, particles accelerated by individual sources mix together and contribute to the bulk of Galactic cosmic rays known as the Cosmic-Ray Background.
Cosmic Rays are measured directly here on Earth. However, in the course of their transport from their production in SNRs and/or other sources, through interstellar and interplanetary space, their energy spectra are heavily modulated, so that the CR spectra measured here on Earth may not represent the CR spectra (and fluxes) produced at the source and/or in interstellar space. A promising way to measure the CR spectra in interstellar space indirectly is through the stud of the diffuse Galactic gamma-ray background.
While escaping their injection sources and diffusing in the Galaxy, CR protons and nuclei interact inelastically with the atoms and molecules of the interstellar gas. They thereby produce charged and neutral pions, which then produce gamma-rays through the decay of these pions. CR electrons emit gamma-rays through inverse Compton scattering off radiation fields in the Galaxy and through bremsstrahlung. The study of the angular and spectral features of the diffuse gamma-ray emission from the Galactic disk, produced by the diffusing CRs, can provide the density and energy spectra of these particles in different locations of the Galaxy.
Tracking the entire distribution of diffuse gamma-ray emission on the sky leads to information on the distribution and bulk properties of CR sources and on the diffusion and convection processes which the CRs undergo during their erratic life. Studies of individual Galactic gamma-ray sources, which are often extended as a consequence of the fact that CRs escape from their injection sources, offer a test of diffusive shock acceleration and lead to information on the location and and nature of specific CR sources, on their spectra and their injection rate, which gets lost once CRs leave their birth places and propagate in the Galaxy. Broad-band observations of diffuse gamma-ray sources are crucial in order to have a complete picture of the CR density and energy spectra in the Galaxy, from MeV to 100 TeV energies.
Fermi-LAT, with its wide field of view, is providing a breakthrough in the study of the diffuse emission from the Galaxy at GeV energies. The data recently released by the Fermi-LAT Collaboration show that the spectra of the Galactic diffuse emission can be explained by CR propagation models based on local observations of CR electron and nuclei spectra.
Researchers affiliated with the CSR at NWU are involved both in observational studies of the diffuse gamma-ray background and in simulations of CR transport through the Galaxy.
- High-energy emission from globular clusters:
Researcher (s): Christo Venter
"Millisecond pulsars (MSPs) hosted by globular clusters are expected to provide relativistic particles that that produce (synchrotron and inverse-Compton) radiation which can be seen in the radio, X-ray, and gamma-ray bands”.
Fermi has detected about a dozen globular clusters in GeV gamma-rays. Their pulsar-like spectra likely represent cumulative emission from the embedded MSPs. Globular clusters are promising sources for H.E.S.S. and the future Cherenkov Telescope Array (CTA).
Researchers in the CSR at NWU are engaged in the theoretical modeling of the radiation spectra from particles that escape from the MSP magnetospheres and upscatter ambient photons. Special attention is being paid to detailed particle transport and an accurate description of the stellar energy density profile. By modeling the spatial profile of the X-ray (synchrotron) emission from the MSPs in globular clusters, one can constrain particle propagation and general properties of the globular cluster and also predict the cumulative flux from a population of TeV-detected globular clusters.
- Active galactic nuclei:
The accretion of material onto BHs is often associated with the formation of highly collimated outflows (jets) of plasma, in which material streams out highly relativistically, i.e., with almost the speed of light. This is the case for Galactic microquasars (see section "X-Ray/Gamma-Ray Binaries and Micro-Quasars" above), but much more extreme examples are found in radio-loud active galactic nuclei (AGN), where these outflows typically move with speeds of 99 % of the speed of light (with bulk Lorentz factors of Gamma ~ 10), and gamma-ray bursts (GRBs, with Gamma > 100). AGN are the most numerous class of known sources of gamma-rays both in the Fermi-LAT (GeV) regime and in the VHE gamma-ray regime, observable, e.g., by H.E.S.S. Particularly abundant in the gamma-ray sky is a class of jetted AGN in which the jet happens to point almost along our line of sight to the AGN. These sources are called blazars. Researchers in the CSR at NWU are involved both in multiwavelength observations, from radio through gamma-rays of blazars and other gamma-ray loud AGN, and in theoretical investigations of particle acceleration and gamma-ray production in these objects.
- Gamma-ray bursts:
Researcher(s): Markus Böttcher
GRBs are the most powerful explosions in the universe, outshining the entire gamma-ray sky for a fraction of a second to tens of seconds and are thought to be the signature of black hole birth during the (supernova-like) death of an extremely massive star, followed by rapid accretion of surrounding material, accompanied by the formation of an ultra-relativistic jet. Some of the salient, open issues in our understanding of the physics of the relativistic jets in AGN and GRBs are:
- the composition of the matter in the jets, the knowledge of which would shed light on the jet launching mechanism
- the nature of the relativistic particles (protons or electrons/positrons) producing the observed gamma-rays;
- the dominant mode of acceleration of particles to ultra-relativistic energies (relativistic shocks / relativistic shear layers / magnetic reconnection).
Research in the CSR at NWU addresses these fundamental issues through a combination of numerical simulations of particle acceleration processes, leptonic and hadronic radiation modeling, and comparison to co-ordinated multi-wavelength and multi-messenger observations.
A particular focus of the research in this area is the construction of a complete simulation chain from first-principle plasma-physics and particle-acceleration simulations to time-dependent radiative output. This starts with particle-in-cell (PIC) simulations of magnetic-field generation, development of microphysical plasma turbulence, and acceleration of particles to supra-thermal energies at relativistic shocks and shear layers. The resulting magnetic turbulence characteristics and particle distributions can be fed as input into Monte-Carlo (MC) simulations of particle acceleration to ultrarelativistic energies. These, in turn, may then be coupled to leptonic and hadronic radiation modules to evaluate the radiation output from these systems. In the study of GRBs, particular emphasis will be placed on prospects of their detection with ground-based Cherenkov Telescope facilities, such as H.E.S.S.-II and CTA.