The days when astronomers only studied the sky with simple optical telescopes are long gone. Today, unraveling the mysteries of the universe involves increasingly larger and more complex facilities that detect things like gravitational waves and various forms of electromagnetic radiation — the spectrum of energy that includes visible light and X-rays.
A particularly specialized branch of astronomy is gamma-ray astronomy. It does what it says on the tin, search gamma rays, the most energetic photons (particles of light) in the electromagnetic spectrum. In fact, they are millions of times more energetic than the light we can see.
In astronomy, gamma rays are produced by some of the hottest, most energetic events in the universe, such as stellar explosions and black holes violently “feeding” on surrounding matter. Although gamma rays are now linked to dozens of different kinds of sources, in many cases we still don’t know for sure what kind of energetic particles these rays create.
It’s exciting that gamma-ray astronomy is getting a huge head start with a new facility. Once the worldwide spread Cherenkov telescope array (CTA) is completed, it will view the gamma-ray sky with ten times more sensitivity than is currently possible.
With more than 60 telescopes, the CTA is expected to provide a deep understanding of the nature of dark matter – an invisible, hypothetical matter that makes up about 85% of the mass of the Universe. The array could also help solve one of astronomy’s longest-running mysteries: where cosmic ray particles (energetic nuclei and electrons in our galaxy and beyond) come from. Gamma rays are linked to these particles, making them detectable.
Read more: Why do astronomers believe in dark matter?
Flashes from space
Gamma-ray astronomy was born early sixties while space satellites were developed to search for energetic radiation from space.
NASA’s Fermi mission, launched in 2008 to low Earth orbit, has been cataloged so far several thousand sources of gamma rays. The Fermi spacecraft continues to provide live coverage of the sky 24 hours a day, measuring gamma rays with energies reaching several 1,000 giga-electron volts of energy. That’s about a trillion times the energy of visible light.
To study gamma rays at even higher energies, we need to use ground-based methods. Although Earth’s atmosphere shields us from radiation from space, we can still observe the secondary effects of this shielding on the ground.
That’s because when a gamma ray interacts with Earth’s atmosphere, it creates an electromagnetic cascade, or “air shower,” of more than a billion secondary particles. These particles are mostly electrons and their antimatter partners called positrons. These air showers contribute about 30-50% of the natural radiation we experience in our lives.
Making the invisible visible
While nothing can go faster than the speed of light in a vacuum, charged particles such as electrons and positrons (anti-electrons) can actually move faster than light when moving through air.
When this happens, a shock wave is created in the form of a flash of blue and ultraviolet light. This flash, called Cherenkov radiation, is named after the Soviet physicist Pavel Cherenkov who first discovered the phenomenon in 1934 (and the 1958 Nobel Prize in Physics together with two colleagues). The blue glow of Cherenkov radiation can be seen in water cooling ponds around nuclear reactors.
At ground level, telescopes with large mirrors and sensitive cameras can detect the Cherenkov light produced by a gamma ray striking our atmosphere. These cameras take about ten nanoseconds to capture a Cherenkov flash against the bright background of starlight and moonlight.
The first Cherenkov telescopes were developed in the 1960s. After many variants, it was the Whipple Telescope in the United States that came in 1989 discovered gamma ray photons from the Crab Nebula.
This was the first time gamma rays with energies above 1000 giga-electron volts (or 1 tera-electron volt, TeV) had been detected. Thus, tera-electron-volt gamma-ray astronomy was born.
Looking for the extremes
Today, all three of the world’s top TeV gamma-ray facilities – HESS in Namibia, MAGIC in La Palma, Spain and VERITAS in Arizona – have discovered more than 200 TeV sources of gamma rays. These powerful jets are linked to cosmic regions of particle acceleration, such as pulsars, supernova remnants, massive star clusters, and supermassive black holes in the Milky Way and other galaxies.
HESS has shown that our Milky Way galaxy is rich in TeV gamma-ray “light”, including at the center of the galaxy.
TeV gamma rays are also seen through mysterious gamma ray bursts and other fleeting, transient events. These now inform our understanding of the extreme conditions in which gamma rays are created.
The next-generation CTA will use the lessons learned from HESS, VERITAS and MAGIC by expanding the number of ground-based telescopes to more than 60 telescopes. CTA will also use a combination of three different telescope sizes optimized for three gamma-ray energy bands, offering unprecedented performance and “sharpness”.
It will have arrays at two ground locations: one in Paranal, Chile (51 telescopes) in the southern hemisphere, and one in La Palma (13 telescopes) in the northern hemisphere.
CTA has attracted membership from more than 1,000 scientists, including Australian scientists from seven universities. Things are progressing well, with the first northern telescope already detecting gamma rays from the Crab Nebula and several gamma-ray flares from active galaxies powered by supermassive black holes.
Within a few years, we expect that the first southern telescopes will also detect gamma rays, leading to many more discoveries. With CTA we have new insights into where extreme particle acceleration takes place in our galaxy.
Read more: New era of astronomy reveals clues about the cosmos