Title: Cosmic Rays
1Cosmic Rays
- High Energy Astrophysics
- jlc_at_mssl.ucl.ac.uk
- http//www.mssl.ucl.ac.uk/
2Cosmic Radiation
- Includes
- Particles (2 electrons, 98 protons and atomic
nuclei) - Photons
- High energies (
) -
- Gamma-ray photons produced in collisions of high
energy particles
3Astrophysical Significance of Cosmic Radiation
- Where do CR particles come from?
- What produces them and how?
- What can they tell is about conditions along the
flight path? - Primary CR can only be detected above the
Earths atmosphere.
4Primary 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 1km2 - Extensive air showers can deposit up to
particles/km2 - good because high energy primary
particles are rare!
5Detecting Cosmic Rays
- Geiger counters
- Scintillation counters
- Cerenkov detectors
- Spark chambers
- Large detector arrays are constructed on the
ground to detect extensive air showers.
6Cosmic rays (cont.)
- - Chemical composition
- - Energy spectra
- - Isotropy
- - Origin
7Chemical Composition (Cont.)
- 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
8Light 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
9Cosmic Ray lifetime in Galaxy
- CR mean free path through galaxy is
- - however all high-mass particles break
up. - Assuming
-
- in disc.
10Escape 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,
- gt3000 years for particles to escape at c.
- BUT the magnetic field would trap them
11Energy spectra of particles
Log Particle flux m s ster eV
-2 -1 -1 -1
L M H
-6
H
-12
P
NB this is a differential spectrum N(E).
Sometimes integral spectrum N(gtE).
a
M
L
-18
Log (eV per nucleon)
6 9 12
12Integral spectrum of primary CR
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
13Cosmic Ray Isotropy
- Anisotropies are often quoted in terms of the
- parameter d
- where and are the minimum and
- maximum intensities measured in all directions.
14Isotropy (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.
15 Isotropy Table
- Log E (eV) d()
- 12
0.05 - 14
0.1 - 16
0.6 - 18
2 - 19-20
20
16Isotropy and magnetic fields
- At low energies, magnetic fields smear original
directions of particles, - eg. eV protons in interstellar magnetic
field of Tesla - and
- (rradius of
curvature)
17Direction of low-E Cosmic Rays
- 1pc, ltlt distance to Crab Nebula
rradius of curvature
18- 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 may be
- extragalactic.
19The 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
28
32
32
32
20Origin of Galactic Cosmic Rays
- Energy output required
assume Galaxy is sphere 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
Particles shortlived gt continuous acceleration.
3
-13
6
-3
-3
50
17
10
32
-1
21Cosmic Rays from stars
- Ordinary stars Too
low!!! Our Sun emits CR during flares but
these have low-E (up to 10 -10 ) rate only
10 J/s, total 10 J/s (10 stars in
Galaxy) - Magnetic stars
Optimistic!!! Mag 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
10
11
17
11
28
32
22Supernovae
- 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 ). - Taking 1 SN every 100 years,
gt 3x10 J/s. (also, SN produce heavies)
42
B
part
32
23And 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.
38
32
24Extragalactic Cosmic Rays
- 10 eV protons (r1Mpc) cannot be contained in
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?
20
25- Limited extragalactic region, r 300Mpc
estimate 1000 radio galaxies in that region,
emitting 10 -10 J in their lifetime, 10 yrs. - Volume of region, V10 m3
53
55
6
75
26- Total energy release over life of Universe
10 x 10 x 10 J
10 J (1000 radio galaxies) - Energy density 10 J m
...the order of the energy density required IF
the value measured at Earth is universal - Quasars are another possible source of CR
4
3
55
62
- the radio galaxies must be replaced 10,000 times
-13
-3
27Electron 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 )
3
-3
6
-3
28Pulsars 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
18
36
2
m
-8
SNR
-6
13
e-
29Power 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
12
2
2
e-
12
-11
-16
-15
-10
-2
-1
-1
19
-10
2
-10
2
38
30
30- 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
30
-15
44
-13
-2
-1
syn
44
45
-6
e-
31- 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
40
10
40
8
48
-3
63
63
48
-3
-15
-3
-15
-19
-3
4
-3
32Cosmic 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 - - 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 -
33COSMIC RAYS