VERITAS


VERITAS is a major ground-based gamma-ray observatory with an array of four 12 meter optical reflectors for gamma-ray astronomy in the GeV – TeV photon energy range. VERITAS uses the Imaging Atmospheric Cherenkov Telescope technique to observe gamma rays that cause particle showers in Earth's atmosphere that are known as extensive air showers. The VERITAS array is located at the Fred Lawrence Whipple Observatory, in southern Arizona, USA. The VERITAS reflector design is similar to the earlier Whipple 10-meter gamma-ray telescope, located at the same site, but is larger in size and has a longer focal length for better control of optical aberrations. VERITAS consists of an array of imaging telescopes deployed to view atmospheric Cherenkov showers from multiple locations to give the highest sensitivity in the 100 GeV – 10 TeV band. This very high energy observatory, completed in 2007, effectively complements the Large Area Telescope of the Fermi Gamma-ray Space Telescope due to its larger collection area as well as coverage in a higher energy band.

Specifications and design

VERITAS is constructed of four 12 m diameter Imaging Atmospheric Cherenkov Telescopes with an approximate separation of 100 meters between each adjacent telescope. Each telescope comprises a large, steerable optical reflector and a high-speed photomultiplier tube camera. Multiple telescopes in an array are needed for stereoscopic observations of the Cherenkov light produced in extensive air showers. These stereoscopic observations allow precise reconstruction of the particle shower geometry, thus giving greatly improved angular and energy resolution compared to a single telescope. The angular direction of the incoming shower is determined by finding the central axis of the spread of the shower on each telescope and tracing those axes until they cross. The intersection of these axes determines the incoming direction of the primary particle that initiated the air shower in the upper atmosphere. It also determines the shower core position, i.e. the extrapolated position of the primary particle on the ground had it not interacted. The energy of the primary particle is determined from the total amount of Cherenkov light measured in each telescope, along with the distance of that telescope from the shower core.
Each of the individual telescopes has a 12 m diameter aperture and a 3.5 degree field of view. The telescopes are built on a Davies-Cotton optical design, which uses a spherical reflector and is straight forward to construct and align. This design does cause some time spread in the arrival of Cherenkov photons at the camera, but this spread is small. The reflector consists of 350 individual mirror facets, hexagonal in shape, mounted on a rigid optical support structure. The camera on each telescope has 499 individual pixels. VERITAS, like other IACTs, is sensitive to primary particles that produce sufficient atmospheric Cherenkov light to be detectable at the ground. Its full range of sensitivity is from 50 GeV to 50 TeV. The energy and angular resolution depend on the energy of the incident gamma ray but at 1 TeV the energy resolution is ~17%, and the angular resolution is 0.08 degrees. The entire array has a peak effective area of 100,000 square meters above 1 TeV. A very weak astrophysical source with a gamma-ray flux only 1% of the Crab Nebula can be detected by VERITAS in under 25 hours of observation. Stronger sources can be detected in significantly less time.
In order to distinguish between the background events or noise and the targeted data, VERITAS uses a three-level trigger system. Level one corresponds to a level crossing on each pixel using constant fraction discriminators. Level two is a pattern selection trigger, which selects photon-like showers, which have compact shapes, and eliminates most of the background showers, which produce more random shapes in each camera. Level three is the array trigger which looks for a coincidence in the arrival time of the shower at multiple telescopes.
The Cherenkov light that is produced by gamma rays in the upper atmosphere is very dim, so VERITAS observes best under clear, dark skies. Observations are not possible under cloudy or rainy skies, or when the Moon is very bright. However, observations are regularly made when the Moon is dim or moderate in brightness. The total yearly observation time is typically around 1,200 hours. The observatory does not generally collect data in July or August due to local monsoon conditions.

History

VERITAS was designed to explore the very high energy gamma-ray sky above 100 GeV, following up on the success of the Whipple 10 m gamma-ray telescope. The Whipple telescope pioneered the use of an imaging Cherenkov camera, coupled with a large 10 m diameter reflector, to make the first definitive detection of a VHE gamma-ray source, the Crab Nebula in 1989. Subsequently, the HEGRA telescope on La Palma demonstrated good sensitivity above 1 TeV using an array of imaging atmospheric Cherenkov telescopes. VERITAS combines the benefits of stereoscopic observations in an array with large reflectors for a low energy threshold. Compared to the Whipple telescope, VERITAS employs larger 12 m diameter reflectors, improved optics and light collection efficiency, and a finer pixelated camera. Both the recording and trigger electronics were significantly improved compared to earlier instruments. VERITAS was conceived in the 1990's, along with three other imaging atmospheric Cherenkov telescope arrays: CANGAROO-III, H.E.S.S. and MAGIC. VERITAS is currently the only IACT array operating in the western hemisphere.
The first proposal for VERITAS was submitted by Trevor Weekes to the Smithsonian Institution in 1995; this proposal described an array of nine 10 m diameter Cherenkov telescopes. In 1998, the first VERITAS collaboration meeting was held at the University of Chicago. In 2000, the concept of VERITAS as a seven telescope array was recommended by the 2000 Decadal Survey in Astronomy and Astrophysics as a moderate-sized project. Delays were incurred due to difficulties with two proposed sites in Arizona and due to a reduction in available funding. The proposal for a four telescope array was favorably reviewed in 2002 and construction of VERITAS started in 2003 at the Fred Lawrence Whipple Observatory. An initial prototype telescope was completed as Telescope #1 and saw first light in 2004. The construction of Telescope #2 was completed in 2005 and first stereo observations started that year. Telescopes #3 and #4 were completed by early 2007 and the first light celebration for the full for telescope array was on April 27–28, 2007. Regular science operations for VERITAS started in September 2007. The construction of VERITAS was largely funded in the U.S. by Department of Energy, the National Science Foundation, and the Smithsonian Institution. Additional construction funding was provided by Enterprise Ireland and the Particle Physics and Astronomy Research Council in the U.K.
Improvements and upgrades to VERITAS have been made periodically since 2007. Telescope #1 was moved in the summer of 2009 to a new location for better array geometry. Between 2009 and 2011 an upgrade program was carried out that improved the alignment of the VERITAS mirror facets and replaced the level 2 trigger system. Furthermore, in the summer of 2012 all of the camera photomultiplier tubes were upgraded to high-quantum-efficiency tubes, which again increased the sensitivity, especially near the low end of the gamma-ray energy range. Compared to its initial design sensitivity, the actual achieved sensitivity of VERITAS is significantly better with the time required to detect weak gamma-ray sources reduced by more than a factor of two.
In June 2017, a celebration was held at the Whipple Observatory to celebrate ten years of VERITAS science.

Science

VERITAS has a broad science program that combines key aspects of astronomy, exploring the universe in the new waveband of VHE gamma rays, and physics, searching for new particles of phenomena beyond the standard model of particle physics. The basic questions pursued include: understanding cosmic particle acceleration in our Galaxy and beyond our Galaxy, probing extreme environments near compact objects such as neutron stars and black holes, the nature of dark matter and the intergalactic magnetic field, and whether the speed of light is constant at these extreme gamma-ray energies. The VERITAS observational program includes Galactic sources such as supernova remnants, pulsars, pulsar wind nebulae, binary systems and the enigmatic gamma-ray source at the Galactic Center. Extragalactic sources include active galactic nuclei, starburst galaxies, and gamma-ray bursts. An important component of VERITAS observations is that associated with multi-wavelength and multi-messenger follow up, including fast radio burst, high energy neutrino, and gravitational wave events. VERITAS has an extensive dark matter program, in which indirect searches are conducted to find VHE gamma rays resulting from the annihilation of dark matter particles. Most of these searches target the Galactic Center and dwarf spheroidal galaxies. Starting in 2017, the VERITAS science program was expanded to include observations in the optical waveband through high-time-resolution measurements of asteroid occultations and stellar intensity interferometry.
As of 2019, VERITAS research had led to 58 Ph.D.'s and more than 100 peer-reviewed publications. Some of the scientific highlights include:
VERITAS researchers have also pioneered the use of a IACT to carry out Citizen Science. To improve the detection of muon events, the Muon Hunter project was created on the Zooniverse platform. The project showed images taken with VERITAS and citizen volunteers had to classify the images as muon or non-muon events. The researchers then trained a machine learned algorithm that performed better than the standard analysis. In Muon Hunter 2.0 the project will try to improve the result with a different machine learning approach.

Collaboration

The VERITAS collaboration was officially formed by the signing of a teaming agreement between nine institutions in the U.S.,
is supported by the United States Department of Energy, the National Science Foundation, the Smithsonian Institution, the Natural Sciences and Engineering Research Council in Canada, Science Foundation Ireland and the Particle Physics and Astronomy Research Council in the U.K.
The collaboration is composed of several members and other collaborating institutions.

Member institutions