VERITAS at UMass

Very

Energetic

Radiation

Imaging

Telescope

Array

System

Very high energy gamma rays can be used to probe some of the most powerful objects in the universe, such as
active galactic nuclei, supernova remnants and pulsar-powered nebulae. VHE gamma rays can also be used to study primordial black holes, quantum gravity and dark matter.

The VERITAS telescope array will image the Cherenkov light from gamma ray induced showers in the upper atmosphere. Showers in the energy range from 50 GeV to 50 TeV will be accessible to this technique. By studying the orientation of the shower images, one can determine the origin of the primary gamma rays.

The VERITAS gamma ray experiment

The VERITAS experiment is an extension of the original Whipple telescope design to a multitelescope array, planned to be installed at Kitt Peak in Arizona. The initial phase of VERITAS will include four telescopes, each 10 meters in diameter and covering a 3.5 degree field of view. This phase is expected to be completed in 2006. An upgraded version of the experiment with an additional three telescopes will be pursued later. The first telescope for VERITAS has been installed at Mount Hopkins in Arizona and saw first light in September 2003. Use of that telescope for science objectives should begin in Oct. 2004. The detailed VERITAS proposal is available at http://veritas.sao.arizona.edu/.

The VERITAS array will be a unique astronomical tool in the northern hemisphere. It complements similar telescope arrays being constructed in the southern hemisphere (CANGAROOIII in Australia and HESS in Namibia), as well as space-based gamma ray telescopes (GLAST) that operate at lower energy.

The figure at the right shows an aerial view of Kitt Peak. For a live view from the peak see http://www.noao.edu/kpno/kpcam/index.shtml

Detection of VHE Gamma Rays

The Atmospheric Air Cherenkov technique for VHE gamma rays is based on detecting Cherenkov radiation from gamma-ray-initiated showers in the upper atmosphere. When a gamma ray enters the atmosphere, it interacts with an air molecule to produce an electromagnetic air shower several kilometers long. Electrons and positrons in the shower are travelling faster than the speed of light in air and emit Cherenkov radiation, mostly in the forward direction.
A large optical telescope (the VERITAS telescopes are 10m in diameter) fitted with an array of a few hundred phototubes (assembly shown at right) collects the Cherenkov light and forms an image of the shower. By measuring the size, shape and orientation of the image in the telescope, one can discriminate against backgrounds induced by cosmic rays, determine the direction of the parent gamma ray and estimate its energy. This technique was pioneered in the 1980's by the very successful Whipple collaboration and is now in use at several facilities around the world.

The Whipple telescope in Arizona is representative of the first generation of such experiments, which include the CANGAROO telescope in Australia, the HEGRA telescopes in the Canary Islands, and the CAT telescope in France, among others. In these experiments, cosmic ray backgrounds are typically a hundred times larger than gamma ray signals. With very tight selection on the characteristics of the image, a signal can usually be revealed, at the cost of some efficiency. After selection, much of the remaining background comes from low altitude muons passing near the telescope.

The diagram at the left shows a schematic of a gamma ray air shower and the corresponding elliptical telescope image. If the shower occurs exactly along the axis of the telescope, the image should be circular and located at the center of the field of view. If the shower is parallel to the axis but offset, as in the diagram, the image should be elliptical and pointing back to the center of the field of view.

light area

The air shower usually starts at 10 to 20km altitude and extends for many kilometers. Each charged particle in the shower emits a cone of Cherenkov light which sweeps out a broad area on the ground. The blue region in the diagram at the right shows the area illuminated by a single track aimed straight at the ground; compare this to telescope in green. If this area covers the telescope and if the particle is within the field of view of the telescope camera (3.5 degrees diameter for VERITAS), light will reach the camera. Only a small fraction of light from each particle is collected by the telescope, but the thousands (up to hundreds of thousands) of particles in a typical shower will contribute to a measurable signal in the telescope.

For the VERITAS array, the range of sensitivity includes gamma ray energies from about 50 GeV to a about 50 TeV. Smaller showers shine too little light in the telescopes to be discriminated effectively from cosmic rays and night sky background, while higher energy showers are too large to be contained within the field of view of the telescopes. The energy estimate of the primary gamma ray should be accurate to about 10%, depending on the energy.

shower images

Images of some typical triggers of the telescope are shown in the figure at the right from the Whipple experiment. The upper left image shows a probable gamma ray shower: compact, fairly elliptical, and pointing back to the center of the field of view where the source should be located. Showers from charged cosmic rays of comparable energy (upper right image) tend to be more diffuse and irregular, due primarily to the weakly decaying particles in the shower. A single charged particle such as muon (lower left) forms a ring in the focal plane. Night sky background (lower right?) often due to city light reflection off of clouds or haze, tends to trigger a very small cluster of phototubes.

Evidence for a gamma ray source is obtained from a population of shower images. Gamma rays shower images from the source point preferentially back to the center of the field of view. The histogram below shows the measured angles of the major axis of each image, relative to the center of the field of view. The excess of showers at zero angle (yellow histogram) is evidence for a gamma ray source at the center of the field of view. The red hashed histogram shows similar data with no gamma ray source in the field of view.

The VHE Gamma Ray Sky

The first VHE gamma ray source, the Crab Nebula, was unambiguously identified in 1989. It remains one of the brightest and most dependable sources of VHE gamma rays in the sky. Since then, scientists have discovered 17 other sources, including other pulsar nebulae, active galactic nuclei of the blazar class, supernova remnants, an Xray binary, and one unidentified source. The image at the right shows the VHE gamma ray sky as of January 2003. Several additions have been made since then.

With the next generation of gamma ray telescope arrays (including VERITAS), researchers are hoping to add hundreds of new objects to the list of VHE gamma ray sources.

The image at the left is an Xray-optical composite of the Crab Nebula.

VHE Gamma Ray Science

Very High Energy Gamma Ray physics combines many of the scientific interests of astrophysics and particle physics.

On the astrophysics side, VHE gamma ray science drives the search for cosmic sources of gamma rays and strives to understand the mechanism of gamma ray production in those sources. To date, high-energy gamma rays have been observed from a variety of sources (active galactic nuclei, pulsar-powered nebulae, supernova remnants and an X-ray binary system), offering a wealth of scientific information. Because gamma rays are not as strongly attenuated in the galaxy as longer wavelength radiation, they offer a clear and unique view through the galactic plane. In addition, because of their interaction in infrared photon fields and in strong magnetic fields via pair production, they can be used to probe intergalactic radiation fields.

On the particle physics side, the highest energy gamma rays, which are beyond any energies that can be produced on Earth, provide a unique probe of several important topics in fundamental physics.

Two studies in this category have especially captured our interest at UMass: measurement of gamma ray dispersion (relevant to theories of Quantum Gravity) and search for neutralino annhilation into gamma rays (an important study in the quest for dark matter). For the particle physicist, the cosmos contains the highest energy accelerators available, and may ultimately prove to be the final frontier in fundamental physics.

Gamma Ray Dispersion

In 1997, scientists proposed that gamma-ray bursts could be used to detect dispersion of electromagnetic waves in vacuum. Such dispersion is predicted to be a consequence of many quantum gravity theories. In these scenarios, the granularity of spacetime at the Planck scale affects the propagation of high energy photons, and alters their speed somewhat as if they were traveling through a physical medium. The highest energy gamma rays are most affected due to their shorter wavelength, which is sensitive to shorter length scales. An absence of detectable dispersion could be used to set lower limits on the energy scale of quantum gravity effects.

Mrk421 flare

Subsequently, members of the Whipple collaboration (Biller et al., PRL 83, p. 2108, 1999) applied this idea to a 1996 flare of the active galactic nucleus (AGN) Markarian 421. By comparing the arrival times of low and high energy gamma rays from the rapidly varying source, they were able to set a lower limit on the energy scale of quantum gravity. The figure at the right shows the 1996 flare, which at the peak shows a doubling time of less than 15 minutes.

The VERITAS telescope array will greatly extend this study to other distant AGN's. The larger effective area of VERITAS and the wider energy range gives it a great advantage over the current generation of gamma-ray telescopes. To date, this is the only experimental subject we know of that is sensitive to physics approaching the Planck scale.

Neutralino Dark Matter

It is now commonly accepted that most of the gravitationally attractive mass of the universe is made up of non-baryonic (exotic) material. One of the prime candidates for dark matter is the lightest superpartner of supersymmetric theories, which is often taken to be the neutralino. Although searches at accelerator experiments have seen no sign of these particles, there is a good possibility they could be seen in the cosmos. If neutralinos contribute to the dark matter halo of the galaxy they should be concentrated at the center of the galaxy. In this region of high neutralino density there may be a significant cross section for neutralino-neutralino annihilation into two gamma rays or into a gamma-Z boson pair. These decays might be seen in a gamma ray telescope as a monochromatic gamma ray signal (Bergström, Ullio and Buckley Astropart. Phys. 9, p. 137, 1998). For neutralino masses in the range of sensitivity of gamma ray telescopes (50 GeV to 50 TeV for VERITAS) this may be the most promising method to detect supersymmetry in the near future.

galactic center

Two advantages of the VERITAS array will have a significant impact on this search. First of all, the increased range of energy sensitivity will allow a search over a broader neutralino mass range. Secondly, and most importantly, the energy resolution will be much better than for a single telescope. This comes about primarily because parallax in the different telescopes can be used produce an estimate of the altitude of the shower. After correction for the distance to the shower, the correspondence between the amount of light detected in the telescopes and the total of energy in the shower is greatly improved (from about 30-40% energy resolution for a single telescope to better than 10% resolution for the VERITAS array). This resolution is critical when searching for a monochromatic signal.

The image at the right shows the first detection of a VHE gamma ray signal from the galactic center, taken from astro-ph/0403422

UMass Role

People

Students

Amaresh Datta
Chris Jones
Jennifer Joyce
Laura Sparks

Faculty

Guy Blaylock

Science Interests and Analysis Techniques

So far, our science interests on VERITAS have concentrated on topics close to fundamental physics, focussing especially on the dispersive effects of quantum gravity and on the annihilation of neutralinos. We have also studied shower selection techniques using neural net algorithms to better discriminate between gamma ray showers and cosmic ray showers. In the best of cases, we can achieve a factor of 2.5 reduction in background over the standard cut-based selection technique. We are also currently exploring the use of vertexing techniques in High Energy Particle Physics for 2D gamma ray source location.

Support Work

The UMass group provides support for the experiment in several areas of software development and maintenance. In particular, UMass has taken responsibility for two critical software projects: a database system and software management system.

Database

The VERITAS database is used to manage information for run control, calibrations, settings and monitor data, star catalogs, and data file information (location, processing level, etc.). In short, it stores everything except the data events themselves. The system UMass has developed is based on the open source system by MySQL.

The primary database server is located at the experiment for use by runtime processes. Mirrored systems are located elsewhere for use by analysts. Backups will be made from one of the mirrored sites to long term media (tape or CD) on a daily basis. Diagnostic tests (and repairs if necessary) should also be performed regularly and frequently by at least one database expert. Both the backup and diagnostic tasks require developing code for automation as well as ongoing human attention.

The interface to runtime and analysis code is provided in a c++ class library, which consists of two layers of code. The lowest layer provides a general interface for database commands while providing mediation for multiple users, error recovery, and data conversion from c++ to database variable types. The second (top) layer of code provides the database methods specific to each c++ program requirement, shielding the user both from database syntax and from the internal structure of the database. Designing and writing this layer of code requires close collaboration with many VERITAS groups, and requires long-term attention as designs change slightly over the years.

Tutorials and a detailed description of the full system are available at:
http://www-unix.oit.umass.edu/~blaylock/Database/

Software Repository and Software Management

A software repository provides centralized storage for project software (and documents) as well as
a system for managing software versions and releases that maintains a record of the complete evolution of the software. The system UMass has developed for VERITAS is based on the open source software from Concurrent Versions Systems (CVS).

In the repository system, new software versions are tagged and released for general use as the software evolves. The state of the repository is preserved after each change is made. In the UMass system, daily backups are also made to LTO tape. Any previous version may be retrieved at later times.

An individual can query the repository to check out any version of any piece of software or to add or update new software. Remote access is available through the cvs command line interface via ssh or through the graphical interface provided by ViewCVS. A (password-protected) web servlet interface to the documents portion of the database has also been developed by Jennifer Joyce. Tutorials and a detailed description of the full system are available at:
http://www-unix.oit.umass.edu/~blaylock/CVSRepository/

Over the long term, one or more system administrators are needed to manage the submissions to the repository and oversee the large-scale development of code. These administrators would coordinate, for example, the change from a development release to a production release. As part of the project, the UMass team will also provide basic quality control and testing. We will verify that submitted code complies with VERITAS software standards and gives consistent results on standard data samples. These important tasks require a substantial long-term responsibility.

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Last updated $Date: 2004/11/18 20:40:51 $ UTC