I am currently visiting Namibia, giving talks and meeting the astronomers here at the University of Namibia. My university, Cardiff University, has a formal university-wide collaboration with the University of Namibia through something called the Phoenix project. The level of collaboration between different departments goes from very strong to zero; for the Physics and Astronomy department it is non-zero but could be stronger. Hopefully, my visit here can help make it stronger.
Although the physics department at the University of Namibia is involved in many exciting areas of research, the one that interests me most is its involvement in a high-energy radiation system of telescopes called H.E.S.S. (High Energy Stereoscopic System). The name H.E.S.S., however, is no coincidence; it is also the name of Victor Hess, one of the founders of studies of cosmic rays. I blogged about him here.
H.E.S.S. (which I am now going to type as just HESS) is a system of telescopes which detect Cherenkov radiation produced when high energy particles (cosmic rays) or gamma rays from space strike atoms or molecules in the Earth’s atmosphere. This provides an indirect way of detecting this high energy radiation, as the radiation itself does not reach the ground; but the Cherenkov radiation it produces does reach the ground.
But, what kind of astronomical sources does HESS detect? What is Cherenkov radiation? And, why has HESS been sited in Namibia?
What kind of astronomical sources does HESS detect?
Radiation from astronomical sources falls broadly into four categories,
- thermal continuum radiation (also known as blackbody radiation)
- non-thermal continuum radiation
- non-thermal emission which is not a continuum
- line emission
I have blogged several times about blackbody radiation. Here I derived Planck’s radiation law using his original arguments of 1900, here I blogged about the fact that the Cosmic Microwave Background is a perfect blackbody (which is a very important fact in its interpretation as being due to radiation from the hot, early Universe), here I blogged about blackbody radiation and the ultraviolet catastrophe, and here I showed how we can use the fact that stars radiate as blackbodies to determine their sizes. And blackbody radiation has cropped up in several others of my blogposts.
Non-thermal continuum radiation (also known as synchrotron radiation) is something I have been planning to blog about for a while. I mentioned synchrotron radiation here when I derived the reason that accelerated electrons emit electromagnetic radiation. I will blog about the details of synchrotron radiation, as planned, in the near future; but for now I will just say that is produced when electrons spiral along magnetic field lines. As they spiral they accelerate (they are moving in a circle around the magnetic field line and moving along the field line at the same time) and, as I showed in that blogpost, accelerated electrons emit EM radiation.
Line emission is also something I have blogged about. For example, here in a blog entitled Emission Line Spectra, and here in my basic explanation of the three kinds of spectra we see in nature. It is the kind of emission given off by e.g. the Orion nebula, Messier 42.
The HESS telescope is looking for radiation which falls into the fourth category, non-thermal radiation which is not a continuum. In particular, it is looking for the high-energy end of this radiation, which is going to come from cosmic rays or gamma rays. As explained in my blogpost here, the source of cosmic rays is still hotly debated. They are not rays as such, put rather high energy charged particles. As I discussed in my blogpost here, some 89% of cosmic rays are high-energy protons (hydrogen nuclei), some 10% are high-energy helium nuclei (alpha particles), and the remaining 1% are high-energy electrons (beta particles).
We know what the cosmic rays are, but where they come from in terms of what kind of astronomical sources emit them is still a mystery. Also, we know that cosmic rays do not come from thermal sources; the energies are just too high. There is some kind of acceleration mechanism (often called cosmic accelerators) which are accelerating these charged particles to nearly the speed of light. Importantly for HESS’s work, gamma rays almost always accompany the cosmic accelerators, so we can use gamma rays to learn more about these mysterious phenomena.
Detecting gamma rays rather than cosmic rays has a distinct advantage; gamma rays travel in straight lines whereas cosmic rays, being charged particles, are bent by any magnetic fields. This has always been the main problem in determining the source of cosmic rays, as they do not travel in straight lines identifying their origin has been nigh-on impossible. I discussed that problem in more detail here.
A list of the sources detected by HESS since it saw first light in 2002 is given here. As you can see from this list, some are associated with supernovae remnants (such as the Crab nebula and the Vela nebula), some are from so-called active galactic nuclei (such as NGC 253), some are associated with Quasi-Stellar Objects (QSOs), and some have yet to be identified. The image below, taken from the HESS website, shows very high energy (VHE) gamma-ray emission from RCW 86, another supernova remnant.
What is Cherenkov radiation?
I will blog in more detail about Cherenkov radiation, showing the derivation of the formulae involved. But, for now, let me give a brief non-technical explanation.
Cherenkov radiation is produced when a high-energy charged particle (usually an electron) travels faster than the speed of light in that medium. You may think that you have heard that nothing can travel faster than the speed of light. This is true, in the sense that nothing can travel faster than the speed of light in a vacuum. But, it is possible for something to travel faster than the speed of light in a non-vacuum, where the speed of light is reduced by the medium through which the light is travelling.
So, for example, Cherenkov radiation is the preferred way of detecting neutrinos; the neutrinos strike a sub-atomic particle in a liquid, often heavy water (a rare event, but it does happen), and the accelerated sub-atomic particle may produce Cherenkov radiation if it is charged (so, if it is either an electron or a proton) and if that charged particle travels faster than the speed of light in that liquid.
When high-energy cosmic or gamma rays enter the Earth’s atmosphere, Cherenkov radiation may be produced when this radiation (a gamma ray or a cosmic ray) strikes a sub-atomic particle in the atmosphere. This collision may produce an electron-positron pair, and the Cherenkov radiation occurs if this pair travel faster than the speed of light in the atmosphere.
The HESS telescopes detect this Cherenkov radiation, and so are able to pin-point the place in the atmosphere where the collision took place. Through this, we can trace back to from where in space the cosmic or gamma rays entered the Earth’s atmosphere; and hence indirectly ‘see’ the cosmic or gamma rays. In the case of gamma rays, which are not bent by magnetic fields, it allows us to construct a gamma-ray image of the astronomical source.
Why is HESS in Namibia?
Parts of Namibia are ideal to site a telescope which is looking for Cherenkov radiation from the atmosphere. The country has some of the clearest and driest skies in the World, and the driest climate of any country in sub-Saharan Africa. The HESS telescopes are located about 100km to the south-west of the capital Windhoek, near the Gamsberg mountain in the Khomas Highland, which is a plateau at an elevation of nearly 2km above sea level.
The HESS telescopes saw first light in September 2002, with more telescopes being added to the system in 2004. In 2012 a much larger telescope, HESS II, went into operation which allows detection of lower energy cosmic and gamma rays.
HESS and HESS II are a collaboration between scientists from 32 scientific institutions in 12 countries, including the UK. It is an extremely exciting project, and one in which I hope my department in Cardiff can be more involved.