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Gamma Rays
• By Phil Gilley and Katie Gray
Where do Gamma Rays
Come From?
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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
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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
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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
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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
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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
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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” http;//[email protected]
“Gamma Rays” http://www.smgaels.org/physics/gamm_1.htm
“Gamma-Ray Burst Physics” http://www.astro.psu.edu
“How do scientists’see’ gamma rays” http://imagine.gsfc,nasa.gov
“Gamma-Rays,” http://astrosun.tn.cornell.edu
“Gamma-rays” http://imagers.gsfc.nasa.gov/ems/gamma.html
Moche,L. Dianh Ph. D. Astronomy Today , Random House, New York, New York, 1992
P.14-15
“Structure and Evolution of the Universe”
http://www.gsfc.nasa.gov/gsfc/spacesci/structure/cgro.htm
“A Mystery of Gamma-Ray Bursts”
http://csep10.phys.utk.edu/astr162/lect/cosmology/gammaray.html
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.
http://tigre.ucr.edu/halo/visual.html
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.
http://imagers.gsfc.nasa.gov/ems/gamma.html
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.
http://tifrc1.tifr.res.in/~pnbhat/vhe.html
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.
One of the 25
Cerenkov
Telescopes used
in PACT. High
Energy Gamma
Ray Observatory
at Pachmarhi,
Central India
http://tifrc1.tifr.res.in/~pnbhat/vhe.html
Who discovered Gamma Rays?
Paul Villard, a French physicist, is credited with
discovering gamma rays in 1900. 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.
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980209c.html
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.
http://cossc.gsfc.nasa.gov/epo/news/catalog.html
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
http://tigre.ucr.edu/halo/logdat_annot.html