In order to get a clear picture of the Universe, it is important to study the light from objects across the entire electromagnetic spectrum. But until recently, getting a clear picture of the gamma-ray sky has proven difficult.
But the latest evolution of space-based and ground-based gamma-ray telescopes have given scientists a view of our high energy Universe that we have never had before. The new Fermi Space Telescope is providing the best data yet of the gamma-ray band up to 300 GeV. But that's not the whole story.
In order to probe the dynamic objects that our Universe has to offer in even higher energies a new technique needs to be adopted. The latest generation of ground-based gamma-ray telescopes allow us to see the gamma-ray universe at energies from 50 GeV to 30 TeV. One of the latest instruments of this type is the Very Energetic Radiation Imaging Telescope Array System (VERITAS).
History of VERITAS
The journey of VERITAS really began in the late 1960s with the development of the Whipple 10-meter reflector on Mount Hopkins in Amado, Arizona. The Whipple represented a new class of gamma-ray detector. Completed in 1968, the design of the telescope enables it to collect the faint blue light of the Cherenkov radiation produced after very high energy gamma-ray rays interact in our atmosphere. Using this collected light, the instrument can trace back the origin of the original gamma-ray.
At least that was how it was supposed to work, in theory. However, because of the lack of sophistication in the initial camera design, the system was not able to isolate the gamma-ray signals from the background. (It turns out that cosmic-rays will also produce Cherenkov radiation as they enter our atmosphere.) But as researchers began refining the technique, designing better cameras, and better understanding the background radiation the ability to use ground-based detectors to study gamma-rays became viable.
Then in 1989 Whipple detected the first gamma-rays from the Crab Nebula, marking a major achievement in astronomy. Since the gamma-rays from the nebula come at such a steady flux the Crab became the standard by which all other gamma-ray sources are measured.
But the Whipple reflector had its limitations. Specifically, the instrument had a difficult time detecting weak sources as well as determining the arrival direction of the initial gamma-rays. So once the Crab Nebula was detected, and the proof of concept was complete, researchers began envisioning the next generation of instrument. Then, at the dawn of the new century, work began on a new system; an array of four Cherenkov-type detectors based on the Whipple design. They would be larger, have more sophisticated hardware and ultimately work together to better achieve the goals set forth by its forbearer.
Design of VERITAS
VERITAS is an array of four telescopes, each with a 12-meter optical reflector comprised of 350 hexagonal mirrors. The Cherenkov radiation is reflected into a 499 pixel camera mounted on each telescope. Each pixel is a photo-multiplier tube (PMT) that is designed to amplify the signal from faint light.
The data is then fed into a computer and combined with the output from each of the other telescopes. Based on values such as differences in arrival times of the photons to reconstructed direction, the software can eliminate much of the background radiation, isolating the gamma-ray data. (There are still background photons in the data, but those will be removed during the final analysis performed by individual scientists as needed.)
Each telescope is spaced about 80 meters apart in a square-ish pattern. Originally, the VERITAS array was to be placed at a different location than at the base of Mount Hopkins at the Fred Lawrence Whipple Observatory, but due to political problems, the array has remained at this location. As a consequence, the array is not ideally arranged due to space constraints on-site.
The telescopes each have about a 3.5 degree field of view, allowing them to possibly observe multiple objects at once.
What Does VERITAS Study?
There are many types of objects that emit gamma-rays. The main types studied by VERITAS are supernova remnants, emission from black hole centers of galaxies, pulsars and quasars. All of these types of objects release massive amounts of energy due to explosions (in the case of supernovae) or the accretion of material from galaxies or companion stars. These violent processes allow for very high energy gamma-rays to be created.
As we study these objects in the very high energy gamma-ray regime, we learn a lot about the physics of what drives the emission. The hope is that we can one day completely understand the nature of how these objects are created, and how they live out their existence in our Universe.
