The Cherenkov Telescope Array – World’s Largest and Most Sensitive Gamma Ray Detector
A novel gamma ray telescope is under construction on Mount Hopkins, Arizona, as part of an international effort to develop and build the world’s largest, most sensitive gamma ray detector. The telescope is part of a large project known as the Cherenkov Telescope Array (CTA), which will be composed of hundreds of similar telescopes to be situated in the Canary Islands and Chile. The telescope on Mount Hopkins will be fitted with a prototype high-speed camera, assembled at the University of Wisconsin - Madison, and capable of taking pictures at a billion frames per second. (Image Credit: Deivid Ribeiro, Columbia University)
The Cherenkov Telescope Array – World’s Largest and Most Sensitive Gamma Ray Detector
In astrophysics, gamma rays are known to be produced by some of the most energetic objects in the universe: supernova explosions, pulsars, neutron stars, and the swirling environments around black holes. When the highly energetic gamma rays reach Earth, they interact with molecules high in the Earth’s atmosphere and create a fleeting pulse of Cherenkov light in an air shower. The burst of light particles or photons lasts less than the blink of an eye -- on the order of six nanoseconds.
Each pulse enables detection of a gamma ray a trillion times more energetic than can be observed with the human eye, according to Justin Vandenbroucke, a University of Wisconsin - Madison physics professor who has spent a decade working on the design, construction, and integration of the camera used in the new telescope.
Known formally as the prototype Schwarzchild-Couder Telescope, the new telescope is a working, high-end test bed for technologies to be used in the Cherenkov Telescope Array (CTA), a configuration of approximately 100 telescopes intended to give astrophysicists their best look ever at the transient effects of gamma rays interacting with particles high in the Earth’s atmosphere.
The heart of the new telescope is a novel, high-speed camera devised in part by scientists at the University of Wisconsin - Madison and in collaboration with researchers from other universities.
“It is a very high resolution, very fast camera that captures gamma rays in the atmosphere,” says Vandenbroucke, who is also affiliated with the Wisconsin IceCube Particle Astrophysics Center at UW - Madison. “It operates at a billion frames per second and captures the direct output of a gamma ray shattering a molecule of air.”
Combined with a complex dual-mirror optical system that promises much greater resolution for the telescope overall, the camera is expected to yield a trove of new data on the Cherenkov air showers. The telescope’s optics provide a wide field of view, necessary to capture the unpredictable bursts of light that occur when a gamma ray crashes into an air molecule.
The newly dedicated Schwarzchild-Couder Telescope is an important test bed for technologies to be included in CTA, the long-planned configuration of approximately 100 telescopes to be situated in the Canary Islands and in Chile. When completed, it will be the largest ground-based gamma ray detection observatory in the world. The international collaboration building CTA includes more than 1400 scientists from 32 countries.
The prototype high-speed camera at the core of the telescope is the size of a golf cart, weighs several hundred pounds, and is capable of taking pictures at a billion frames per second.
“This telescope pushes the technology to a very different regime,” explains Vladimir Vassiliev, a professor of physics and astronomy at the University of California Los Angeles (UCLA) and the lead scientist for the telescope under construction on Mount Hopkins in southern Arizona. The telescope, he notes, will have unparalleled mirror optics and the camera is designed to capture the fleeting pulses of blue Cherenkov light created when gamma rays crash into molecules of air in Earth’s atmosphere, creating a shower of diagnostic secondary particles.
“We’ll be able to make a movie at a billion frames per second of the particle shower developing in the atmosphere,” says Justin Vandenbroucke, the UW - Madison physics professor co-leading development of the prototype camera under the auspices of the Wisconsin IceCube Particle Astrophysics Center (WIPAC) with support from the National Science Foundation (NSF).
With its innovative optics and camera, the new telescope will be operated in concert with an existing array of four single-mirror telescopes that comprise VERITAS (Very Energetic Radiation Imaging Telescope Array System), located at the Fred Lawrence Whipple Observatory on Mount Hopkins.
Camera technology is critical, says Vandenbroucke, who has been working on the design and construction of the 800-pound, golf-cart sized prototype instrument since 2009 when he was a post-doctoral fellow at Stanford University.
The camera has finally come together in a basement laboratory in UW - Madison’s Chamberlin Hall, where Vandenbroucke’s group has been busy integrating and testing it.
The challenge for the camera, according to Vandenbroucke, is that the flashes of photons or particles of light that are of interest are incredibly fast. The Cherenkov pulse in an air shower may last only six nanoseconds, yet each pulse enables detection of a gamma ray a trillion times more energetic than can be seen with the human eye. The pulses occur at random, making telescopes and cameras with wide fields of view essential, says Vassiliev.
The combination of dual-mirror technology and the novel camera is intended to capture the Cherenkov air showers at unprecedented resolution. “This will be the first demonstration of this kind of optics for this kind of telescope,” says Vassiliev. “The payoff will be excellent imaging of Cherenkov air showers.”
What the Cherenkov Telescope Array Will See
Light is made up of waves of alternating electric and magnetic fields that can be measured by frequency (number of waves that pass by a point in one second) or wavelength (the distance from the peak of one wave to the next). The larger the frequency, the smaller the wavelength. The spectrum ranges from the very lowest frequencies of radio waves and microwaves; to the mid-range frequencies found in infrared, optical (visible) and ultraviolet light; to the very highest frequencies of X-rays and gamma rays. The frequency range of gamma rays is so vast that it doesn’t even have a well-defined upper limit. The gamma rays CTA will detect are about 10 trillion times more energetic than visible light.
The electromagnetic spectrum provides scientists with a variety of ways to view the Universe. As seen in the figures below, telescopes detecting different frequencies of light provide different perspectives of the Milky Way and the Crab Nebula, providing a more complete picture of the phenomena they are studying. With its ability to view the highest-energy processes in the Universe, CTA will be a vital asset in improving our understanding of these phenomena.
The gamma rays of interest to the CTA team span a wide range of energies. The telescope being built by Vassiliev’s team is designed to detect gamma rays in the central energy range. Objects powered by black holes, says Vandenbroucke, are among the likeliest sources of the gamma rays that will be parsed by the new CTA telescope.
Opening a new frontier in the detection and measurement of gamma rays, says Vandenbroucke, will help answer a raft of some of the most fundamental questions about the nature of matter and energy in the universe. “Gamma rays are the linchpin of multi-messenger astronomy,” says the Wisconsin scientist. “They have been essential to identifying the first gravitational wave signal from merging neutron stars and may play a similar role in the search for the sources of high-energy neutrinos.”
- Non-Thermal Emissions
Most of the light we are used to seeing is emitted by hot objects and is known as thermal radiation. The hotter the source of this radiation, the higher the frequency of the light produced. However, it is not possible for objects to get hot enough to produce gamma rays. These must be produced by a non-thermal mechanism. The mechanisms often rely on the presence of high-energy sub-atomic particles that are produced by some kind of cosmic particle accelerator.
Accelerated particles develop in special environments where a small fraction of the particles can take on an “un-fair” share (or fraction) of the energy available. In such a system, a small number of particles can be accelerated to very close to the speed of light and carry a significant fraction of the energy available. Since energy is no longer shared roughly equally among particles, as is the case in a “normal” hot environment, these processes are referred to as non-thermal processes.
These special environments are usually associated with violent events such as explosions -- outbursts or powerful jets of material produced close to the giant black holes at the center of galaxies. For this reason, gamma rays can be used to trace violent events in the universe.
- Cosmic Sources
CTA will be sensitive to the highest-energy gamma rays, making it possible to probe the physical processes at work in some of the most violent environments in the Universe. Although cosmic gamma rays cannot reach the Earth’s surface, CTA can detect them from the ground using the sub-atomic particle cascades that they produce in the atmosphere. Charged particles in these cascades travel at very close to the speed of light and emit visible (mostly blue) light known as Cherenkov light. The CTA’s large telescope mirrors and ultra-high speed cameras can then collect and record the nanosecond flash of light so that the incoming gamma ray can be traced back to its cosmic source.
CTA will be able to detect hundreds of objects in our galaxy. These galactic sources will include the remnants of supernova explosions, the rapidly spinning ultra-dense stars known as pulsars, and more normal stars in binary systems and large clusters. Beyond the Milky Way, CTA will detect star forming galaxies and galaxies with supermassive black holes at their centers (Active Galactic Nuclei or AGN) and, possibly, whole clusters of galaxies. The gamma rays detected with CTA may also provide a direct signature of dark matter and provide evidence for deviations from Einstein’s theory of relativity and help provide answers to the contents of cosmic voids -- the empty space that exists between galaxy filaments in the Universe.
- Cosmic Rays
What makes a ray a cosmic ray? Even though they are called “rays,” cosmic rays are really just normal atomic particles. Despite being “normal” matter, cosmic rays are special because they are accelerated to extraordinarily high energies, traveling very close to the speed of light. Primarily in the form of high-energy protons and atomic nuclei, cosmic rays constantly bombard the earth, but despite a century-long search, we know very little about their sources and the role they play in our own galaxy and beyond. Gamma rays are produced in the interactions of cosmic rays and provide the most sensitive means to study cosmic rays in and around their sources.
- Black Holes
Black holes are among the most mysterious objects in astronomy. They are thought to be very small regions in space-time with a gravitational pull so strong that nothing, not even light, can escape. And they are by no means “black.” They are some of the brightest sources of very-high energy (VHE) gamma rays.
It is believed that most black holes are the relics of massive stars following a supernova explosion. The core of the star collapses under its own gravity to form a black hole, which is typically only a few kilometers in radius but with a mass several times greater than our Sun. When black holes accrete material (that is, grow by gravitationally attracting more matter) from their surroundings, it is a violent, highly energetic process. Much of the material is devoured by the black hole and it grows in size, and the frictional forces within the material spiraling into the black hole make the object immensely luminous.
On a very different size-scale, supermassive black holes are a million to a billion times more massive than the Sun and are assumed to exist in the center of most galaxies, including our own. While the central black hole in the Milky Way is only detectable through the orbits of stars moving around it, about 10 percent of known galaxies, so-called “active galaxies,” harbor a supermassive black hole that is fueled by a huge accretion disk (a rotating disk of material or gas formed by the black hole’s accretion). The very hot disk can outshine all the stars in the galaxy itself and can produce powerful outflows called “jets,” -- in some cases longer than the diameter of the Milky Way.
The jets emitted from the centers of these active galaxies (Active Galactic Nuclei, or AGN), offer excellent conditions for particle acceleration to the highest energies and for the emission of gamma rays. AGNs account for one-third of all known Very-High Energy (VHE) gamma ray sources and are nearly the only objects we can detect at these energies that are not located in our own galaxy.
CTA aims to measure large samples of such active galaxies and galactic black holes to study particle acceleration and gamma ray emission processes. These observations will give us a picture of the conditions and physical processes occurring in and around some of nature’s most mysterious objects.
- Supernova Remnants
When certain stars reach the end of their natural lifetime they die in gigantic explosions called supernovas. The explosion causes a large part of the stellar material to be expelled at thousands of kilometers per second into the surrounding interstellar environment. The resulting shock fronts are called Super-Nova Remnants (SNRs), which emit radiation across the whole electromagnetic spectrum and play an important role in the evolution of galaxies.
It is now known that charged particles can be accelerated by SNRs to reach energies beyond those achievable with the most powerful man-made particle accelerator, the Large Hadron Collider (LHC) at CERN. SNRs may be the dominant source of the cosmic rays that bombard the Earth. Particles accelerated in SNRs are implicated in the growth of magnetic fields in the Universe and can influence star formation in galaxies.
CTA will be able to detect a much larger number of SNRs in gamma rays than is currently possible and measure the properties of these objects in much greater detail, helping us to understand the process of particle acceleration in SNRs and the propagation of these particles away from SNRs and their subsequent impact on the interstellar medium. Crucially, for the first time, CTA will be able to probe particle acceleration up to Peta-electron Volt (PeV, or 10^15 eV) energies in these objects and for any class of objects within our galaxy. We know from the locally measured cosmic rays that something in our galaxy accelerates particles to those energies, but the sources remain unknown. There is very recent evidence of particle acceleration in the Galactic Center, but it is not clear if it can provide the local cosmic rays.
When a massive star reaches the end of its life, it undergoes a supernova explosion, ejecting most of its outer layers. The remaining core of the star collapses and, depending on its mass, becomes a white dwarf, a neutron star, or a black hole. Neutron stars are formed from the collapse of ordinary stars roughly 8 to 20 times the size of our Sun and are incredibly dense -- the equivalent of the Earth’s mass condensed into a space the size of 1 to 2 football stadiums.
In the collapse process, as the radius of the star decreases, the magnetic field becomes stronger and the rate of rotation increases (often rotating many times per second). As it rotates, so does its magnetic field, creating an electric field on the surface that accelerates charged particles. The radiation produced by these particles during their acceleration leads to a beam of electromagnetic emission along the axis of the magnetic field. As the neutron star rotates, the jets may swing past the Earth’s direction, much as the light from a lighthouse passes over the sea, leading to the observation of pulsed objects or “pulsars.”
The pulsar’s rotation rate slows down over time, as it uses its rotational energy to accelerate particles to high energies. These particles, trapped by the magnetic field, rotate in sync with the pulsar out to large distances. At a distance called the light cylinder, the particles would have to travel at the speed of light to continue keeping up with the pulsar. Rather than break the laws of physics, the particles are able to escape from the immediate region around the pulsar at this location, streaming away and creating what is called a pulsar wind. When this ultra-fast wind plows into the surrounding material, it creates a shock wave where particles are accelerated, spreading out into a cloud called a Pulsar Wind Nebula (PWN).
Emissions from both pulsars and their wind nebulae have been detected at Tera-electron Volt (TeV) energies. Pulsar Wind Nebulae are the most populous class of galactic objects in this energy range. The most famous PWN is the Crab Nebula, which formed from the Crab supernova explosion in 1054 AD, as recorded by Chinese astronomers. The Crab is one of the brightest TeV sources and was the first TeV gamma ray source to be detected (in 1989). Pulsation from the Crab pulsar has been detected across the electromagnetic spectrum, from radio up to approximately TeV.
Observations with CTA will provide the first truly detailed gamma ray images of PWNs and make it possible to probe the motion and cooling of high-energy particles in PWNs. CTA also will greatly increase the number of known PWNs, helping to further the understanding of their evolution and the impact of their environment. Additionally, CTA will provide insights in the central engine of PWNs (the pulsar itself) and greatly expanding the number of known VHE pulsars as well as the precision with which they can be measured.
- Binary Systems
Binary systems are composed of two stars that closely orbit one other, exchanging matter and energy through accretion processes or via the periodic interaction of their respective winds. Stars much more massive than our Sun can have very powerful winds. In binary systems, these winds can collide and accelerate particles, producing gamma ray emissions. In binaries where one of the stars is a “compact object” like a black hole or a neutron star, the winds (or jets) can be travelling at close to the speed of light and particles can be accelerated to very high energies. As a binary system moves through its orbit, the physical conditions in the collision region change. For this reason, gamma ray binaries can be thought of as a laboratory for high energy astrophysics, allowing scientists to adjust the parameters of the system and see what happens.
A few hundred of these systems have been discovered in our galaxy thanks to the advent of X-ray astronomy in the 1960’s, but only a handful of binary systems emitting VHE gamma rays have been detected in our galaxy in recent years. The improved capabilities of current Cherenkov telescopes (MAGIC, VERITAS, and H.E.S.S.) have made these more recent discoveries possible. Their discovery has proven to be extremely useful to study high-energy processes, in particular particle acceleration, emission and radiation reprocessing, and the dynamics of the underlying magnetized flows. CTA will greatly expand the population of gamma ray binaries and allow us to precisely measure the behavior of many systems as a function of orbital phase and photon energy. These measurements are expected to cast light on the physics of particle acceleration, as well as the winds of pulsars and massive stars and the way they interact.
- Dark Matter
The nature of dark matter is one of the biggest outstanding questions in science. The material is known to exist due to its gravitational effects and in far larger quantities than normal matter, but close to nothing is known about what it is. Many hypotheses exist for dark matter, mostly postulating a new very weakly interacting particle (a Weakly Interacting Massive Particle, or WIMP). Some of the most promising theories predict WIMPs that can annihilate when they interact to produce more familiar particles. Such annihilations would inevitably produce gamma rays.
There is a strong idea as to how often these annihilations should happen in order to give the WIMPs the right density in the Universe today, as well as where to look for this signal, in places where the density of dark matter is very high (as in the center of our own galaxy). Up until now, there have not been instruments sensitive enough to see the predicted signal. It is hoped that the CTA will reach this critical sensitivity and complement other searches using the Fermi satellite, the Large Hadron Collider, and deep underground direct searches for WIMPs. Together these instruments have a very good chance to so.
- Cosmic Voids
Most of the Universe is very close to empty, with matter clustered into galaxy clusters, super-clusters, and filaments, separated by huge voids. How empty these voids are is a matter of great debate. In particular, are there any tiny magnetic fields in these regions that are a relic of the earliest moments of the Universe? CTA will be able to probe magnetic fields in the voids via observations of halos around active galaxies and also help to probe the proposed heating of low density regions in the Universe by TeV photon interactions. A known ingredient of the space between galaxy clusters is the extra-galactic background light, the integrated light of all processes over the history of the Universe in the infrared to ultraviolet range. CTA will be able to characterize these radiation fields via the absorption features that they leave in the spectra of the population of active galaxies seen by CTA.
American participation in CTA is supported by the National Science Foundation (NSF), but the overarching project is a huge international undertaking and, when completed, will be comprised of more than 100 telescopes sited in the Canary Islands and Chile. It will be the largest ground-based gamma ray detection observatory in the world. More than 1400 scientists from 32 countries are involved in the undertaking. The camera and telescope are being funded primarily by the NSF, with contributions from participating universities.
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