Cosmic Rays

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Cosmic Rays

• High Energy Astrophysics
• jlc_at_mssl.ucl.ac.uk
• http//www.mssl.ucl.ac.uk/

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• 5. Cosmic rays Primary and secondary Cosmic
  
• Rays Chemical composition Energy
  spectrum
• Isotropy Origin of CR, Primary Gamma-rays
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Cosmic Radiation

• Includes
• Particles (2 electrons, 98 protons and atomic
  nuclei)
• Photons
• High energies (
  )
• Gamma-ray photons from high energy particle
  collisions
• Surprisingly there are many unanswered questions

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Astrophysical Significance of Cosmic Radiation

• Where do CR particles come from?
• What produces them and how?
• What can they tell us about conditions along the
  flight path?
• Primary CR can only be detected above the
  Earths atmosphere.

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Primary and Secondary CR

• Magnetic fields of Earth and Sun deflect primary
  cosmic rays (especially at low energies).
• Only secondary particles reach the ground - and
  they can spread over a wide area of km2
• Extensive air showers can deposit up to
  particles/km2 - good because high energy primary
  particles are rare!

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Development of Cosmic Ray Extensive Air Showers

• Incoming primary cosmic ray particle, proton or
• heavier nucleus, interacts with an atmospheric
  nucleus
• Disintegration products are
• ? Neutrons and protons that cause a
• nucleonic cascade at the core
• ? p mesons that cause an outer electro-
• magnetic cascade
• Primary gamma-rays undergo pair production
• to cause an electromagnetic cascade only
• Secondary particles spread over a wide area
• with 1010 particles/km2
• Largest array, the Pierre Auger system in
• Argentina, will have 1600 Cerenkov detectors
• on an area of 3000 km2

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Detecting Cosmic Rays

• Scintillation counters
• Cerenkov detectors
• Spark chambers
• Large detector arrays are constructed on the
  ground to detect extensive air showers.

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Cosmic rays (cont.)

• Features of interest are

• - Chemical composition
• - Energy spectra
• - Isotropy
• - Origin

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Chemical Composition

• Cosmic abundances of the elements in the CR and
  the local
• values plotted against nuclear charge number

a) Relative to Si at 100
b) Relative to H at 1012
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Light element abundance

• Overabundance of Li, Be and B due to spallation -
  medium (C, N, O) nuclei fragment in nuclear
  collisions remains are almost always Li, Be or
  B.
• Quantitative analysis is complicated requires
  collision X-sections for various processes and
  relative abundances seem to change with energy.
• However

Abundance weighted
formation probability (mbarn)
Measured CR abundance (Si 100)
Li 24 136 Be
16.4
67 B 35
233

• while mean path that medium elements must pass
  through
• to create observed (Li, Be, B) abundances is
  48 kg/m2
• which is similar to the galactic mean free path

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Cosmic Ray lifetime in Galaxy

• CR mean free path through galaxy is
  
• - however all high-mass particles break
  up.
• Assuming particles of v c traverse a path of
• 
  
• in disc.

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Escape from the Milky Way

• Lifetime could be 10 or 100x larger in the
• Galactic halo where the density is lower.
• Note - galactic disk thickness 1kpc,
• gt 3000 years for particles to escape at
  c
• BUT the magnetic field would trap them

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Energy spectra of particles
Log Particle flux m s ster eV
-2 -1 -1 -1
L M H
-6
H
-12
P

• this is a differential spectrum N(E) dE kE-x
  dE
• sometimes use integral spectrum N(gtE) kE-x

a
M
L
-18
Log Energy (eV per
nucleon)
6 9 12
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Integral spectrum of primary CR

• Integral spectrum

Log N(gtE)
N(gtE) is number of particles with energy gt E.
m-2 s-1 ster-1
0
-4
-8
-12
-16
??
Log E (eV)
12
14
16
18
20
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Cosmic Ray Isotropy

• Anisotropies are often quoted in terms of the
• parameter d
• where and are the minimum and
• maximum intensities measured in all directions.

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Isotropy (cont.)

• So far, experimental results indicate only small
  amounts of anisotropy at low energies, with d
  increasing with E.
• Below E eV, solar modulation hides the
  original directions.
• For higher energies, direction of maximum excess
  is close to that of the Local Supercluster of
  Galaxies.

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Isotropy Table

• Log E (eV) d()
• 12
  0.05
• 14
  0.1
• 16
  0.6
• 18
  2
• 19-20
  20

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Isotropy and magnetic fields

• At low energies, magnetic fields smear original
• directions of particles, e.g. eV protons
  in an
• interstellar magnetic field of Tesla
• and
• (r radius of curvature)

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Direction of low-E Cosmic Rays

• 1pc or ltlt distance to Crab Nebula

r radius of curvature
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• Thus information about the original
• direction would be totally lost.
• At higher energies, particles should retain
• more of their original direction (r increases
• with E), but their (number) fluxes are lower so
• no discrete source has been observed yet.
• At eV, r 1Mpc
• - these particles cannot be confined to the
  Galaxy,
• hence they must be extragalactic.

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The Origin of Cosmic Rays

• Galactic
• Ordinary stars (produce 10 J/s)
• Magnetic stars (produce up to 10 J/s)
• Supernovae (produce 3x10 J/s)
• Novae (produce 3x10 J/s)
  
• Extragalactic

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Origin of Galactic Cosmic Rays

• Energy output required
  assume Galaxy is sphere of radius 30kpc,
  
• m, gt volume m
• Energy density CR 10 J m (10 eV m ) Thus
  total energy of CR in Galaxy 10 J.
• Age of Galaxy 10 years, 3x10 sec
  hence average CR production rate 3x10 J s
• Possible sources must match this figure
• Particles shortlived gt continuous acceleration

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-13
6
-3
-3
50
17
10
-1
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Cosmic Rays from stars

• Ordinary stars Too
  low!!!
• Sun emits CR during flares but these have
  low-E (up to 10 -10 eV) rate only 10 J/s,
  total 10 J/s (10 stars in Galaxy)
• Magnetic stars
  Optimistic!!! Magnetic field about a million
  times higher than the Sun so output a million
  times higher, but only 1 magnetic (and low-E)
  10 J/s

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Supernovae

• Supernovae - a likely source
• Synchrotron radiation observed from SN so we
  know high energy particles are involved.
• Total particle energy estimated at 10 J per SN
  (taking B from synchrotron formula and arguing
  that
• U U though this is uncertain
  due to magnetic field and volume estimates).
• Taking 1 SN every 100 years,
  
• gt 3x10 J/s (also, SN produce heavy
  elements)

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B
Particles
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And from Novae

• Novae also
  promising Assuming 10 J per nova and a rate
  of about 100 per year, we obtain a CR production
  rate of 3x10 J/s.

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Extragalactic Cosmic Rays

• eV protons (r1Mpc) cannot be contained
• in the Galaxy long enough to remove original
• direction
• gt travel in straight lines from outside Galaxy
• What conditions/geometry required to
• produce energy density of cosmic rays
• observed at these energies?

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• Limited extragalactic region, r 300Mpc
  estimate 1000 radio galaxies in that region,
  emitting 10 -10 J in their lifetime, 10 yrs.
• Volume of region the local supercluster, is
  V10 m3

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75
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• Total energy release over life of Universe
  10 x 10 x 10 J
  10 J (1000 radio galaxies)
• Energy density 10 J m this is the order of
  the energy density required for the Local Group
  volume if the value measured at Earth is
  universal
• Quasars are another possible source of CR

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3
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- the radio galaxies must be replaced 10,000 times
-13
-3
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Electron sources of Cosmic Rays

• Electron mass small compared to protons and heavy
  nuclei, gt lose energy more rapidly
• Lifetimes are short, gt electron sources are
  Galactic.
• Observed energy density 4x10 eV m (total
  for cosmic rays 10 eV m )

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-3
6
-3
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Pulsars as cosmic ray sources

• Assuming Crab pulsar-like sources
• can Galactic pulsars source CR electrons?
• Need first to calculate how many electrons
  produced by the Crab nebula.
• Observed synchrotron X-rays from SNR,
• n 10 Hz 4 x 10 E B Hz
• assume B 10 Tesla
  
• gt E 5 x 10 J 3 x 10 eV

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2
m
-8
SNR
-6
13
e-
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Power radiated per electron

• P 2.4 x 10 E B J/s
  2.4 x 10 x 2.5 x 10 x
  10 J/s 6 x 10 J/s
• Observed flux 1.6 x 10 J m sec keV
• Distance 1kpc 3 x 10 m
• Total luminosity, L 1.6 x 10 x 4pd J/s
  1.6 x 10 x 10 x 10 J/s
  1.6 x 10 J/s

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2
2
e-
12
-11
-16
-15
-10
-2
-1
-1
19
-10
2
-10
2
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• Number of electrons luminosity/power per e-
  1.6 x 10 / 6 x 10 2.6 x 10
• Synchrotron lifetime, t 5 x 10 B E s
  
• 30 years
  Thus in 900yrs since SN
  explosion, must be 30 replenishments of electrons
  and these must be produced by the pulsar.
• Total no. electrons 2.6 x 10 x 30
  
• 8 x 10
  
• each with E 5 x 10 J

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-15
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-13
-2
-1
syn
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-6
e-
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• Total energy is thus 4 x 10 J
  Assume 1 SN every 100 years for 10 years gt
  total energy due to pulsars
  4 x 10 x 10 J 4 x 10 J
  in a volume of 10 m (ie. the Galaxy)
• gt energy density of electrons produced by
  pulsars 4 x 10 / 10 J m
  4 x 10 J m
  4 x 10 / 1.6 x
  10 eV m 2.5 x 10
  eV m
• Observed e- energy density is 4 x 103 eV

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10
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48
-3
63
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48
-3
-15
-3
-15
-19
-3
4
-3
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• Total energy is thus 4 x 10 J
  Assume 1 SN every 100 years for 10 years gt
  total energy due to pulsars
• 4 x 10 x 10 J 4 x 10 J
  
• in a volume of 10 m (i.e. the Galaxy)
• gt energy density of electrons produced by
  pulsars 4 x 10 / 10 J m
  4 x 10 J m
  
• 4 x 10 / 1.6 x 10 eV m
  
• 2.5 x 10 eV m
• and observed e- energy density is 4 x 103 ev/m3

10
40
48
8
63
-3
48
63
-3
-3
-15
-15
-19
-3
4
-3
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Resolved Image of a TeV Gamma-ray Source
-Southern Hemisphere SNR RXJ 1713.7 - 3946

• An array of Cerenkov telescopes located in
  Namibia, imaged the SNR
• in the range 0.8 10.0 TeV
• Each telescope has a 13m segmented parabolic
  collector that reflects
• light onto a 960-photomultiplier focal-plane
  array
• Incoming gamma-ray photons creates a shower of
  electrons and
• positrons by pair production particles are
  highly relativistic

• Cerenkov radiation, like a
• sonic shock wave, occurs
• when a particle travels at
• v gt c/n in a medium of
• refractive index n
• Wave angled to the
• particle direction such that
• cos q c/nv

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Image and Spectrum of RXJ 1713.7 3946 (0.8
10.0 TeV)

• SNR image shows that TeV gamma-rays originate
  from the outer shell
• i.e. from the shock as do the keV X-rays, and
  not from centre!
• Spectrum for both gammas and X-rays indicates
  non-thermal emission
• for X-rays almost certainly by synchrotron
  process
• Gamma-ray spectrum dNn/dE k E-2.190.2 photons
  m-2 s-1 TeV-1
• Gamma-ray production by
• - Inverse Compton scattering by relativistic
  electrons or
• - Decay of neutral pions following collision of
  TeV protons with
• nuclei in an interstellar cloud

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Cosmic Ray Problems to be Further Studied

• Summary of problems from Longair, Vol 1, p 296
• - Acceleration of particles to very high energy,
  E 1020 eV
• - Nature of acceleration processes that generate
  power-law particle energy spectra
  particularly in SNR
• - Origin of high light element abundances (Li,
  Be, B) and (Sc, Ti, V) in CR as compared to
  Solar System values
• - Overall preservation of universal element
  abundances throughout the periodic table
• - Origin of anisotropies in the distribution of
  CR
• - Astrophysical sources of the CR and their
  propagation

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COSMIC RAYS

• END OF TOPIC

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Energy spectra of particles