Back in 1912, Victor Francis Hess took thee electrometers
into the atmosphere by balloon, 5km up. He discovered that ionization (the
process by which atoms gain or lose an electromagnetic charge) occurred four
times faster than at ground level. And because he did this during a near total
eclipse, he ruled out the sun at a potential source. “The results of my
observation are best explained by the assumption that a radiation of very great
penetrating power enters our atmosphere from above," he said. Hess had discovered
cosmic rays, and received a Nobel Prize for his discovery in 1936.
Cosmic rays, as it was later discovered, were high-energy
particles that originated outside of our Solar System. These cosmic rays are of
great interest due to the damage they inflict not only on living organisms
outside the atmosphere, but also on electronics in satellites, other
spacecraft, and even high altitude flights. They consist mostly of single
subatomic particles, mainly protons, that move at such speeds that the highest
energy particles detected have the equivalent energy of a 90km/h cricket ball.
Most, of course, are not imbued with such energy, with the
majority having the energy of about 0.3 Giga electronvolts (GeV), or about one
three thousandth the energy of a flying mosquito. The scientific community
speculated for years as to the source of cosmic rays, with the general
consensus being that they originated from supernovae, the immense explosion
that occurs during the final stage of a supermassive stars’ collapse.
But cosmic rays had
been detected with energies of at least a few PeV (Peta electronvolts, or about
a thousand times the energy of a flying mosquito), and that implied our galaxy
had to have a source capable of generating them. And none of the existing sources
showed indications that they were the source. But in the Khomas Highland, near
the Gamsberg, a system of Imaging Atmospheric Cherenkov Telescopes was watching
the skies. The High Energy Stereoscopic System (or HESS, named after Victor
Hess) was designed to investigate cosmic gamma rays.
Normal cosmic rays, as I’ve noted above, is generally
subatomic particles, like protons and electrons. But protons and electrons are
charged particles, meaning their path can be deflected by electromagnetic
fields. This makes it all but useless to try and determine their origin without
knowing the location of all electromagnetic fields in its path – a nigh
impossible task.
But space is filled with vast gas clouds in certain regions,
left over from stellar formation, and when high energy cosmic rays hit these
gas clouds, it can result in a high energy gamma ray being released. And gamma
rays are different. A gamma ray is nothing other than a high-energy photon, the
same particle that normal light consists of. Like light, gamma rays are not
affected by electromagnetic fields, only by gravitational fields, and since
this is a high energy gamma ray, it can also penetrate matter to a much greater
degree.
Cosmic gamma rays can thus be traced to their source much
easier. And due to their ability to penetrate matter, they are not as obscured
by the aforementioned gas clouds in space as visible light is. But this also
poses a problem when trying to detect cosmic gamma rays, as you’d need a large
collecting area to detect them.
That is where the Imaging Atmospheric Cherenkov Telescope
comes in. A photon, being a particle of light, naturally moves at the speed of
light, having no mass. But Einstein’s discovery of the lightspeed constant
refers to the speed of light in a vacuum – and our atmosphere is most
empathically not a vacuum. And when a high-energy photon passes near an atom’s
nucleus, something strange happens. The photon’s energy is converted into
matter via Einstein’s equation E=mc^2, and an electron and a positron is formed.
This is known as pair production.
Given the high energy of the photon, these two new particles
are moving at a significant fraction of the speed of light. And since they’re
now charged particles, they get deflected by other particles, slowing them, and
producing Bremsstrahlung, or braking radiation – which produces another photon,
also with high energy! As you might imagine, this sets off quite a chain
reaction in the atmosphere of charged particles moving at high speed – this
shower of charged particles is known as an Extensive Air Shower.
These high energy particles, as I’ve mentioned, move at a
significant fraction of the speed of light. And as I’ve pointed out before, the
lightspeed constant is the speed of light in a vacuum. In other media, the
speed of light is slower than in a vacuum. It is thus possible for particles to
move faster than the local speed of light – and that is where the final piece
of the puzzle lies.
When particles move faster than the local speed of light,
they emit what is called Cherenkov radiation. Underwater nuclear reactors
frequently emit the faint blue glow of Cherenkov radiation, since the speed of
light in water is only three-quarters of its speed in a vacuum. Thus, when a
particle shower occurs in the atmosphere due to a high energy gamma ray, a
flash of Cherenkov radiation is produced, for about 5 to 20 billionths of a
second.
And that is what the five telescopes at HESS is looking for.
It’s four 12m mirrors and one 28m mirror collecting the light from the flashes
of Cherenkov radiation, indicating that a high-energy gamma ray has been
detected. Focused on Sagittarius A*, the supermassive black hole at the centre
of our Milky Way galaxy, they detected high energy cosmic gamma rays being
emitted from its surrounding gas clouds. Based on their energy signatures, they
concluded that Saggitarius A* was a source of PeV cosmic rays.
And thus, more than a hundred years later, a team on more
than 170 scientists from 32 scientific institutions and 12 different countries
could finally provide at least part of the answer that Victor Hess was
undoubtedly also searching for, using instruments named for him, half a world
away. The universe is an open book, if you just know where to look.
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