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Gamma-Ray Astronomy

Members

Dr. Mark Lang
Dr. Gary Gillanders
Dr. Pat Moriarty (adjunct)

Research Outline

What do we do?

The Gamma-Ray Astronomy Group is part of the VERITAS Gamma-Ray Collaboration (formerly the Whipple Gamma-Ray Collaboration) based at the Smithsonian Institution's Fred Lawrence Whipple Observatory near Tucson, Arizona. The group is involved in a search for point sources of TeV gamma rays. The Collaboration's early successes included the detection of TeV gamma rays from the Crab Nebula, the Active Galactic Nuclei Mrk421, Mrk501, H1426 428, 1ES2344 514, and 1ES1959 650, and TeV J2032 413, a source for which a counterpart has not yet been identified in other wavelength bands. Detailed scientific discussions of many VERITAS source detections are accessible via the recent results page of The Science of VERITAS section of the VERITAS Gamma-Ray Collaboration web site. There is a more general discussion, aimed at the general public, in the VERITAS Science section of the VERITAS Education site hosted by the Adler Planetarium, Chicago.

What are TeV gamma rays?

Radio waves, microwaves, infrared radiation, light, x-rays and gamma rays are examples of electromagnetic radiation. They differ from each other in energy with gamma rays having the highest energies. Photons (individual particles) of visible light have energies of two or three electron volts. Low-energy gamma rays from radioactive decays on earth have energies of about 1 MeV (one million electron volts). Gamma rays with these and higher energies are produced by a range of astrophysical objects. (See NASA's Astronomy Picture of the Day site for a picture of the gamma-ray sky along with nice links on gamma-ray astronomy.) Energies of the highest energy gamma rays are usually specified in units of GeV (thousand million electron volts) or units of TeV (million million electron volts).

Why search for TeV gamma-ray sources?

The search for the origin of the highest energy cosmic rays has been a major area of international astrophysical research for a few decades. Identification of point sources of cosmic rays is hampered by interstellar magnetic fields scrambling the arrival directions of charged particles that constitute 99.9% of high-energy cosmic rays. Thus, we depend on detection of the tiny neutral component of this radiation, namely gamma rays, in identification of compact cosmic ray sources.

Our view of the Universe in wavelength bands such as radio waves, visible light, and even the x-ray band, is dominated by objects emitting radiation via thermal processes. Due to their high energies, TeV gamma rays cannot be produced by thermal processes. They are always the product of exotic and extreme physical conditions such as high magnetic fields, high electric fields, shock waves, cataclysmic explosions, etc. As such they provide a unique probe of the high-energy relativistic universe of exploding stars (supernovae), pulsars, quasars, and black holes.

Gamma-ray observations can also be used to make indirect measurements of important astrophysical quantities. Observations of gamma rays from extragalactic sources, made by the Whipple Collaboration, have been used to determine the maximum density of infrared radiation in intergalactic space. This result is featured in a world wide web article at Physical Review Focus.

How do we detect TeV gamma rays?

Cosmic gamma rays do not reach the surface of the earth. They interact with gas nuclei high in the atmosphere. Thus they can only be detected directly by instruments carried above the atmosphere on satellites such as NASA's Compton Gamma-Ray Observatory (CGRO) ( taken out of service on 4 June 2000) and the Gamma-Ray Large Area Space Telescope (GLAST), which is scheduled for launch in 2007. However, these instruments are rather limited in size and so are not effective in searches for cosmic sources emitting gamma rays above energies of 10's of GeV where the flux of gamma rays (and thus the number of gamma rays you detect in any given time interval) is very small.

Fortunately, the atmosphere provides us with an indirect method of detecting TeV gamma rays. A cosmic ray interacting with a gas nucleus high in the atmosphere initiates a "shower" of charged particles and lower energy gamma rays down through the atmosphere. At energies below about 100 TeV, the shower is absorbed many kilometres up in the atmosphere. However, before being absorbed, particles in the shower are traveling through the atmosphere at a speed exceeding the local speed of light (but of course not exceeding the speed of light in a vacuum). This causes the emission of light, called Cherenkov light, beamed in the direction of motion of the particles. Cherenkov light from a shower arrives at the ground as a disc of about 150 metres in diameter but only 1 metre thick. The resulting pulse of light, seen at a Cherenkov telescope on the ground, lasts a few nanoseconds (thousand millionths of a second) and is recorded by very fast sensitive electronic cameras consisting of an arrays of photomultiplier tubes placed at the foci of 12 metre diameter dishes. (You can see pictures of the VERITAS telescopes and electronics, and the observatory site, in the multimedia section of the VERITAS Education site.) Typically, many hundreds of showers are recorded each minute. The shape of the Cherenkov light pool from showers initiated by gamma rays is different from that for showers initiated by charged cosmic rays. This allows us to use image processing techniques to select the small percentage of showers initiated by gamma rays thus enabling us to find cosmic sources of TeV gamma rays. (See the Air Cherenkov Detector page at NASA's High-Energy Astrophysics Learning Center site for more information.)

VERITAS - a state-of-the-art gamma-ray observatory.

New advances in astronomy and astrophysics often follow a major advance in telescope sensitivity. The current catalogue of TeV gamma-ray sources is a product of Cherenkov imaging gamma-ray telescopes, based on a technique developed by the Whipple Collaboration during the 1980's and early 1990's. The sensitivity of this technique can by improved greatly, thus allowing you to "see" fainter sources of TeV gamma rays, by operating an array of gamma-ray telescopes located at the same site in tandem.

VERITAS is a major ground-based gamma-ray observatory located at the Fred Lawrence Whipple Observatory in Southern Arizona. The first VERITAS telescope saw first light on February 1, 2005 with the detection of a signal from the Crab Nebula. The four-telescope array became fully operational in early 2007. The array has the capability of detecting a TeV gamma-ray source with a signal source as weak as 1% of the signal from the Crab Nebula.