High Energy Observational Astrophysics by yurtgc548

VIEWS: 7 PAGES: 17

									       Problems associated with
       Earth based observation
Optical band = stars and planets and nebulae.
Infrared band = low energy heat sources.
Radio band = dust shrouded environments.
What about the rest ????? !!!!!
      Problems associated with
      Earth based observation
How bad is the problem for X-rays and gamma rays?
       Problems associated with
       Earth based observation
What did astronomers do to get around this problem?




 Altitude by
 which half of the
 incoming
 radiation has
 been absorbed
       Problems associated with
       Earth based observation
What did astronomers do to get around this problem?




           Pic du Midi Observatory in the French Pyrenees
             Experiment          Total cost    Duration        Cost per hour
         Mountain observatory    £2,000,000     10 years       £50 per hour
               Aircraft          £240,000        1 day       £10,000 per hour
               Balloon           £300,000        1 day       £12,500 per hour
               Rocket            £500,000      10 minutes   £3,000,000 per hour
               Satellite        £200,000,000    5 years      £10,000 per hour
       Problems associated with
       Earth based observation
What did astronomers do to get around this problem?
Variation in counting rate as
function of galactic
longitude from rocket borne
proportional counter flown
in 1967. The hard line
represents the expected
distribution based on
known sources whilst the
circles represent the data
obtained in that flight.
              Experiment          Total cost    Duration        Cost per hour
          Mountain observatory    £2,000,000     10 years       £50 per hour
                Aircraft          £240,000        1 day       £10,000 per hour
                Balloon           £300,000        1 day       £12,500 per hour
                Rocket            £500,000      10 minutes   £3,000,000 per hour
                Satellite        £200,000,000    5 years      £10,000 per hour
       Problems associated with
       Earth based observation
What did astronomers do to get around this problem?




                                               NASA 1990s X-ray measurements
             Experiment          Total cost    Duration        Cost per hour
         Mountain observatory    £2,000,000     10 years       £50 per hour
               Aircraft          £240,000        1 day       £10,000 per hour
               Balloon           £300,000        1 day       £12,500 per hour
               Rocket            £500,000      10 minutes   £3,000,000 per hour
               Satellite        £200,000,000    5 years      £10,000 per hour
       Problems associated with
       Earth based observation
First artificial satellite, Sputnik 1, was launched by the Soviet Union in 1957.
       Problems associated with
       Earth based observation
Uhuru, launched in 1970 was the first earth-
orbiting mission dedicated entirely to
celestial X-ray astronomy and operated for
3 years.
It consisted of two proportional counters
and made the first comprehensive and
uniform all sky survey.




                                   Uhuru spun making one revolution every 12
                                   minutes whilst mapping out a scan of space
                                   either 0.5º or 5º wide between 2 - 20 keV.
       Problems associated with
       Earth based observation
The second NASA Satellite (SAS-2) launched in 1972 was dedicated to
gamma-ray astronomy in the energy range above 35 MeV using a wire
spark-chamber aligned with satellite spin axis. It provided the first detailed
look at the gamma-ray sky.
       Problems associated with
       Earth based observation
COS-B, launched in1975 by the ESA, measured high energy gamma data
(~30 MeV-5 GeV) using a Gamma-Ray Telescope comprising a spark
chamber and a proportional counter. It’s highly elliptical orbit enabled long
observation times enabling more detailed mapping.
       Problems associated with
       Earth based observation
Vela satellites operated by the U.S. Department of Defense in the 70s were
not intended primarily for astronomical studies but rather to search for
clandestine nuclear bomb tests. They did however provide much useful
astronomical data such as gamma-ray bursts (0.2 to 1.5 MeV) of 1 second
duration. Triangulation showed these were not confined to the galactic
plane and so must be extra-galactic in origin.
             What is up there now?
Chandra X-ray telescope satellite

                                                Launched in 1999
                                                Looks for:
                                                X-ray bursters
                                                X-ray pulsars



                                    Fermi Gamma ray space telescope

     Launched in 2008
     Looks for:
     Quasars
     AGNs
     Gamma ray bursters
 Techniques for detecting X-rays and gamma-rays
Photoelectric effect            Photon is absorbed and energy given to an electron
                                which is emitted. This is called a photoelectron.
 E kinetic energy  h f  E 0

Likelihood or probability
that interaction occurs is
called the cross section
(σ) and depends on
energy of the photon and
the Z (atomic number) of
the detector atom.

       E3.5

      Z          5
 Techniques for detecting X-rays and gamma-rays
Photoelectric effect

                         E kinetic energy  h f  E 0

  Imagine a ray of green light of wavelength λ = 530 nm incident on a
  detector with a work function of 1.1eV. What is the kinetic energy given to a
  photoelectron ejected from this target?




  What is the lowest wavelength of light that can release an electron from
  this target?
Techniques for detecting X-rays and gamma-rays
Compton effect
Einstein had proposed that despite all the evidence that light is a wave, it
also has particle-like properties (wave-particle duality).

                                    h
      Momentum of wave        p
                                    
Collision between X-ray and electron
Momentum of electron changes
Wavelength of photon changes

                h
     f  i       (1  cos  )
               me c
Techniques for detecting X-rays and gamma-rays
Compton effect
                                                           h
At what angle does maximum energy loss occur ?  f  i       (1  cos  )
                                                          me c
Figure shows energies of a 500 keV photon and electron after Compton
scattering.


Cross section for
Compton
scattering
increases slowly
with energy of the
incident photon.
Techniques for detecting X-rays and gamma-rays
 Compton effect
 Let’s imagine that we collide a gamma ray photon (λ = 3×10-14 m) with an
 electron. What is the momentum of the photon before the collision? What is
 the energy lost by the photon if following the collision its direction changes
 by 60 degrees?


               h
         p
               

            h
 f  i       (1  cos  )
           me c

								
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