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Cosmic Rays. High Energy Astrophysics jlc@mssl.ucl.ac.uk http://www.mssl.ucl.ac.uk/. 5. Cosmic rays: Primary and secondary Cosmic Rays; Chemical composition; Energy spectrum; Isotropy; Origin of CR, Primary Gamma-rays [2].

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cosmic rays

Cosmic Rays

High Energy Astrophysics

jlc@mssl.ucl.ac.uk

http://www.mssl.ucl.ac.uk/

slide2
5. Cosmic rays: Primary and secondary Cosmic

Rays; Chemical composition; Energy spectrum;

Isotropy; Origin of CR, Primary Gamma-rays [2]

cosmic radiation
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
astrophysical significance of cosmic radiation
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 Earth’s atmosphere.
primary and secondary cr
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!
slide6

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
detecting cosmic rays
Detecting Cosmic Rays
  • Scintillation counters
  • Cerenkov detectors
  • Spark chambers
  • Large detector arrays are constructed on the ground to detect extensive air showers.
cosmic rays cont
Cosmic rays (cont.)
  • Features of interest are:

- Chemical composition

- Energy spectra

- Isotropy

- Origin

chemical composition

• 70 < E < 280 Mev/Nuc

° 1 < E < 2 GeV/Nuc

◊ Solar system

Solar System

CR

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

light element abundance
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
cosmic ray lifetime in galaxy
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.

escape from the milky way
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,

=> 3000 years for particles to escape at ~ c

  • BUT the magnetic field would trap them
energy spectra of particles
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-xdE
  • sometimes use integralspectrum N(>E) = kE-x

a

M

L

-18

Log Energy (eV per nucleon)

6 9 12

integral spectrum of primary cr
Integral spectrum of primary CR

Log N(>E)

Integral spectrum:

N(>E) is number of particles with energy > E.

m-2 s-1 ster-1

0

-4

-8

-12

-16

??

Log E (eV)

12

14

16

18

20

cosmic ray isotropy
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.

isotropy cont
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.
isotropy table
Isotropy Table

Log E (eV)d(%)

12 ~0.05

14 ~0.1

16 ~0.6

18 ~2

19-20 ~20+

isotropy and magnetic fields
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)

direction of low e cosmic rays
Direction of low-E Cosmic Rays

= 1pc or << distance to Crab Nebula

r = radius of curvature

slide20
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.

the origin of cosmic rays
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

28

32

32

32

origin of galactic cosmic rays
Origin of Galactic Cosmic Rays
  • Energy output required: assume Galaxy is sphere of radius 30kpc,

= m, => 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 => continuous acceleration

3

-13

6

-3

-3

50

17

10

-1

32

cosmic rays from stars
Cosmic Rays from stars

10

11

17

28

  • Ordinary starsToo 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 starsOptimistic!!!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

11

32

supernovae
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,

=> 3x10 J/s (also, SN produce heavy elements)

42

B

Particles

32

and from novae
And from Novae
  • Novaealso 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

extragalactic cosmic rays
Extragalactic Cosmic Rays
  • eV protons (r~1Mpc) cannot be contained

in the Galaxy long enough to remove original

direction

=> travel in straight lines from outside Galaxy

What conditions/geometry required to

produce energy density of cosmic rays

observed at these energies?

20

slide27

53

55

6

  • ‘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 V~10 m3

75

slide28

4

3

55

62

- the radio galaxies must be replaced

10,000 times

  • 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

-13

-3

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

3

-3

6

-3

pulsars as cosmic ray sources
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

=> E = 5 x 10 J = 3 x 10 eV

18

36

2

m

-8

SNR

-6

13

e-

power radiated per electron
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

12

2

2

e-

12

-11

-16

-15

-10

-2

-1

-1

19

-10

2

-10

2

38

30

slide32

30

-15

44

-13

-2

-1

syn

  • 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

44

45

-6

e-

slide33

40

10

  • Total energy is thus 4 x 10 J Assume 1 SN every 100 years for 10 years => total energy due to pulsars : 4 x 10 x 10 J = 4 x 10 J in a volume of ~10 m (ie. the Galaxy)
  • =>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

40

8

48

-3

63

63

48

-3

-15

-3

-15

-19

-3

4

-3

slide34

40

10

40

48

8

  • Total energy is thus 4 x 10 J Assume 1 SN every 100 years for 10 years => 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)

  • =>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

63

-3

48

63

-3

-15

-3

-15

-19

-3

4

-3

resolved image of a tev gamma ray source southern hemisphere snr rxj 1713 7 3946
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 > c/n in a medium of
  • refractive index n
  • Wave angled to the
  • particle direction such that
  • cos q = c/nv
image and spectrum of rxj 1713 7 3946 0 8 10 0 tev
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-2.19±0.2 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
cosmic ray problems to be further studied
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
cosmic rays38
COSMIC RAYS

END OF TOPIC

energy spectra of particles39

Log Particle flux

m2 s-1 ster-1 eV-1

L: 3 ≤ Z ≤ 5,M: 6 ≤ Z ≤ 9

H: Z ≥ 10

-6

H

-12

P

a

M

L

-18

Log Energy (eV per nucleon)

6 9 12

Energy spectra of particles