The VERITAS Project:
Astronomy with Cherenkov Telescopes
1. What is VERITAS?
The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is a collection of four telescopes used to detect astrophysical sources of Very High Energy (VHE) gamma rays.
VERITAS is located at the Fred Lawrence Whipple Observatory south of Tucson, Arizona, U.S.A., and is operated by a collaboration of more than 100 scientists from about 20 different institutions in the United States, Ireland, England and Canada. VERITAS is funded by the U.S. Department of Energy, the National Science Foundation, the Smithsonian Institution, the Natural Sciences and Engineering Research Council of Canada, Enterprise Ireland, and the Particle Physics and Astronomy Research Council of the U.K.
2. The Imaging Air Cherenkov Telescope Technique
Unlike most telescopes, VERITAS does not directly collect light emitted by the astrophysical sources it is observing in the sky. Gamma rays cannot penetrate the Earth's atmosphere. Instead, some 10-20 km overhead, an incoming gamma ray collides with a proton or neutron inside an air molecule to produce a shower of secondary particles (mostly electrons and positrons) that move towards the ground. In this process, the energy carried by the original gamma ray is converted into the mass and kinetic energy of the secondary particles.
The secondary particles are also very energetic and move at nearly the speed of light in a vacuum, which is faster than the local speed of light in air. The result is an electromagnetic shock wave analogous to a sonic boom. In this case, the shock wave comes in the form of bluish light called Cherenkov radiation. The sum of all the Cherenkov radiation emitted by all the charged secondary particles reaches the ground and can be detected using large optical telescopes and fast cameras to create an image of the airshower. The airshower has the shape of a long, thin tube which points back to the origin of the incoming gamma ray.
Gamma rays are not the only particles that create airshowers. Cosmic ray particles (energetic protons and heavier atomic nuclei) undergo a similar process when they strike the Earth's atmosphere, but the resulting airshower is less uniform and is not as well constrained to a thin tube shape. secondary particles are created closer to the ground making cosmic-ray airshowers appear larger than those of gamma rays. Cosmic-ray airshowers are much more common than gamma-ray airshowers and are a source of background, or noise, to a gamma-ray telescope. To be successful, VERITAS must be able to correctly distiguish airshowers resulting from gamma rays from those resulting from cosmic rays. Gamma rays are identified by rejecting images that are too wide and not uniform enough to have resulted from a gamma ray.
Figure: Schematic comparison of gamma-ray and cosmic-ray airshowers.
The light emitted from an airshower is rather bright, but extremely short-lived. To capture an image of an airshower requires an extremely sensitive and fast camera. Each VERITAS telescope is equipped with a camera made up of sensitive light detectors called photomultiplier tubes. A single photomultiplier tube is capable of detecting an individual photon and, with help of sophisticated electronics, can determine the time at which the photon arrived to within a few billionths of a second. Each VERITAS camera contains 499 photomultiplier tubes arranged in a hexagonal configuration. Acting together as pixels to create an image, the camera can capture the development of an airshower as the secondary particles cascade towards the ground.
When a gamma-ray source is centered in the field of view of a single telescope, the images collected after observing many gamma-ray showers will all point back to the center of the camera, whereas the cosmic-ray showers will point in random directions.
In the image of a gamma-ray airshower from a single telescope, we can only say that the point of origin of the airshower lies somewhere on a line drawn along the length of the image. But if we have a collection of telescopes, each one will view the airshower from a slightly different perspective and the resulting images will have different orientations, as shown in the figure below. If we superimpose images of the same airshower as viewed by multiple telescopes, the gamma-ray source can be localized to the intersection of the lines drawn lengthwise through each image.
There are many other advantages to using multiple telescopes. Just as with optical telescopes, the sensitivity of a gamma-ray telescope depends on how much light it can collect. Each VERITAS telescope has a primary mirror, made up of 345 individual mirror facets, with a total diameter of 12 meters (39 feet). Using four telescopes instead of just one means VERITAS can collect four times as many photons and can detect weaker sources.
Another advantage of is improved energy resolution and sensitivity to gamma rays of lower energies. VERITAS scientists can determine the energy of an incoming gamma ray by measuring the size of the airshower and estimating the total amount of light it produces. The more energetic a gamma ray, the bigger and brighter its airshower will be. Having four telescopes instead of one means VERITAS will be able to detect very small airshowers, meaning it can detect gamma rays of lower energy than a single telescope. Also, with up to four independent measurements of the size of an airshower, the energy of the original incoming gamma ray can be more precisely determined.
Observing with VERITAS
Click to view a slide-based tour of the VERITAS site at the Fred Lawrence Whipple Observatory, where project collaborators use four giant gamma-ray telescopes to observe high-energy radiation from the most energetic objects in the Universe — such as black holes and exploding stars. VERITAS sits at the base of Mt. Hopkins about 50 miles south of Tucson, Arizona.