Gamma Rays By Phil Gilley and Katie Gray

Where do Gamma Rays Come From? Black Holes may be a source of mysterious gamma rays in our universe. The first picture is the event horizon of a black hole. Gases are violently colliding with each other, which in turn releases gamma rays. The second picture is an acceleration disc, the black hole in the middle of the acceleration disc is throwing out jets of gas. Gamma rays are created through the collisions of particles in the gas jets. The third picture is a jet, where the bright gamma rays are being emitted. One theory is the universe contains black holes with jets pointed at us which are emitting gamma rays.

Stellar Wind and Neutron Star Gamma Rays A computer image showing how winds from massive stars could be the source of mysterious gamma rays near the galactic plane. A star ten to twenty times larger than our sun throws a stream of electrically charged gas at high speeds. Neutron stars are created from supernovas. At the right are the whirling magnetic fields produced by its rapid rotation. The acceleration produces gamma rays. The second picture shows the gamma ray beam rotating with the star, resulting in a bright burst of gamma rays.

Gamma Ray Bursts One of the greatest mysteries of modern astronomy Gamma ray bursts are powered by collapse events. The energy density in a burst is so large that a fireball is expected to form. Brief flashes of intense radiation appear once a day at unpredictable times in unpredictable directions. Their origin is unknown.

Gamma Ray Bursts: Continued A gamma ray burst lights up the sky at least once a day with a spectacular explosion. The image at the right is taken from the BATSE instrument on NASA’s Compton Gamma Ray Observatory. Spot size=peak flux during gamma ray bursts. Spot color= average energy, blue is of highest energy. It is clear that they don’t come from our galaxy, so in order for us to see them they must be a 100 times more powerful than the energy from a supernova.

What do Gamma Rays Show Us If one could see the gamma ray night sky, it would look strange and unfamiliar. At the right is the moon as seen with gamma rays. In high energy gamma rays the moon is brighter than the sun. Seeing with gamma rays can let one peer into solar flares, supernovae, neutron stars, black holes and active galaxies.

Gamma Rays for Medicine: Not Just for Seeing the Universe Gamma rays are used in medicine to kill and treat certain types of tumors and cancers. They can detect brain and cardiovascular abnormalities. Sometimes a gamma camera is used to view the body. A gamma source is attached to a chemical which is carried throughout the body. The camera detects the emission of gamma rays to give a picture of the inside of the body.

Bibliography “Curiouser and Curiouser” “Gamma Rays” “Gamma-Ray Burst Physics” “How do scientists’see’ gamma rays” “Gamma-Rays,” “Gamma-rays” Moche,L. Dianh Ph. D. Astronomy Today, Random House, New York, New York, 1992 P “Structure and Evolution of the Universe” “A Mystery of Gamma-Ray Bursts”

Gamma Ray Radiation Computer-generated image illustrates the excess high-energy gamma-ray emission surrounding the Milky Way, found by scientists at the University of California, Riverside, Clemson University and the University of Chicago. Violet and blue portions represent the halo of gamma rays far off the galactic plane that cannot be accounted for by known celestial sources. The horizontal band of light colors represents the plane of the Milky Way, where stars and other celestial bodies, including known gamma-ray sources, are concentrated. Bright localized spots are gamma-ray sources such as active galaxies -- the galaxy 3C 279 is shown in the upper right -- or pulsars, such as the Geminga pulsar to the far right slightly above the galactic plane. Gamma rays represented in the image exclude those that are predicted from the ordinary interaction of cosmic rays with light and matter in the Milky Way galaxy as well as those from a uniform gamma-ray "glow" which is thought to originate outside the Milky Way.

What are Gamma Rays? Gamma rays are the highest energy and the shortest wavelength (less than 0.03 nm) of the electromagnetic spectrum. Gamma rays are powerful enough to kill living cells but they are absorbed by the atmosphere so they can only be detected by high altitude balloons and satellites like the Compton Observatory.

How do we detect gamma rays? Gamma rays are hard to see or detect because: -there are not many of them compared to photons that are similar in terms of frequency -they have so much energy that they are hard to “capture” and tend to pass through most objects without leaving a trace. Gamma rays cannot be focused with traditional telescope techniques (i.e. lenses and mirrors). To “see” gamma rays scientists use spark chambers and scintillation detectors. COMPTEL utilizes a process called Compton scattering, where a gamma ray strikes an electron and loses energy, similar to a cue ball striking an eight ball. COMPTEL has two sets of detectors that scatter gamma rays -- that is, the detectors act like billiard balls. The detectors are aligned one below the other. The gamma ray passes through, striking an electron in one detector and then another electron in the second detector. By combining measurements of the loss of energy and the change of trajectory, the COMPTEL scientists can construct a likelihood map of the probable gamma-ray source location.

One of the 25 Cerenkov Telescopes used in PACT. High Energy Gamma Ray Observatory at Pachmarhi, Central India A Gamma Ray Telescope Cerenkov radiation is similar to Compton scattering in that a gamma ray strikes a material exciting it’s electron which cause light to be emitted or vibrations which are then recorded. This is the method of detection used by the telescope below.

Who discovered Gamma Rays? Paul Villard, a French physicist, is credited with discovering gamma rays in Villard recognized them as different from X-rays (discovered in 1896 by Roentgen) because the gamma rays had a much greater penetrating depth. It wasn't until 1914 that Rutherford showed that they were a form of light with a much shorter wavelength than X-rays.

The Compton Gamma Ray Observatory Gamma rays occupy the highest energy range in the electromagnetic spectrum, well beyond visible light, ultraviolet and X-rays. They are produced by extreme forces of energy and by atomic decay. The COMPTEL catalog comprises 63 gamma-ray sources. Thirty-two of these are steady sources, such as neutron stars and black hole candidates; the remaining 31 are mysterious gamma-ray bursts, which outshine the entire universe before fading within a few seconds. A major priority for high-energy astrophysicists has been to isolate and understand gamma-ray bursts, which appear without warning somewhere in the observable universe about three times a day. The COMPTEL team at UNH has since written software to allow COMPTEL to zero-in on the bursts within the few seconds they are going off. The software connects COMPTEL to the Gamma-Ray Burst Coordinates Network, a NASA-operated network that notifies dozens of telescopes about bursts in real-time.

EGRET data The bright horizontal band along the center is the gamma-ray emission from the Milky Way galaxy, and is caused by high-energy cosmic rays interacting with interstellar matter and light