For most of human history, our view of the Universe and the marvels it contains has been limited to what we can see in the visible spectrum of light. However, due to technological advances over the progress of time, astronomers and laypeople alike have been able to peek at the Universe in nearly all wavelengths of the electromagnetic spectrum, from radio waves to the infrared to gamma rays, learning new science in each new window.
Very-high-energy (VHE) gamma ray astronomy refers to the observation and study of gamma rays in the VHE regime (here, defined as energies above about 100 GeV).
Many different astrophysical objects—from pulsars to supernovae to black holes and more—are known to emit high-energy gamma ray photons (particles of light). When these high-energy gamma rays are incident on the Earth’s atmosphere, they interact with and excite atmospheric nuclei, triggering a radiation and charged particle cascade known as an extensive air shower (EAS). Then, the relativistic charged particles cascading through the atmosphere produce Cherenkov radiation emitted along the direction of the EAS.
Cherenkov radiation is electromagnetic radiation produced when a charged particle passes through a dielectric medium at a velocity greater than the phase velocity of light in that same medium, in this case, the atmosphere. The common analogy used to describe Cherenkov radiation is a sonic boom. The sound waves generated by a supersonic body can only propagate at the speed of sound, so the sound waves travel slower than that speeding body and cannot propagate forward from the body, thus forming a shock front. Similarly, a charged particle can generate light “shock waves” as it travels through a dielectric medium.
These flashes of Cherenkov radiation, herein called Cherenkov pulses, last for about 5 to 10 ns and, at ground level, peak in the ultraviolet with wavelengths of about 350 nm. These gamma ray – induced Cherenkov pulses are so faint that they can only be detected from the ground with large optical light collectors equipped with photomultiplier tubes.
Imaging Atmospheric Cherenkov Telescopes (IACTs)
IACTs function by imaging the very short pulses of Cherenkov radiation generated by gamma rays, as discussed above. If one can detect the aforementioned gamma ray – induced Cherenkov pulses, one can localize the source of the EAS on the sky and thus discover the source of the high energy gamma rays that caused them.
VERITAS uses four IACTs in its array. Each IACT is composed of a large segmented reflecting aperture 12 m in diameter, consisting of 350 individual mirror facets. These mirrors work together to reflect and focus the faint Cherenkov light onto the camera, which is composed of 499 pixels (where each pixel is a photomultiplier tube combined with a preamplifier). These pixels are then coupled to fast electronics which serve to amplify, digitize, process, and record the Cherenkov pulse signals.