Cosmic Rays

1. Cosmic Rays High Energy Astrophysics jlc@mssl.ucl.ac.uk http://www.mssl.ucl.ac.uk/

2. 5. Cosmic rays: Primary and secondary Cosmic Rays; Chemical composition; Energy spectrum; Isotropy; Origin of CR, Primary Gamma-rays [2]

3. 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

4. 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.

5. 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!

6. 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

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

8. Cosmic rays (cont.) • Features of interest are: - Chemical composition - Energy spectra - Isotropy - Origin

9. • 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

10. 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

11. 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.

12. 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

13. 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

14. 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

15. 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.

16. 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.

17. Isotropy Table Log E (eV)d(%) 12 ~0.05 14 ~0.1 16 ~0.6 18 ~2 19-20 ~20+

18. 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)

19. Direction of low-E Cosmic Rays = 1pc or << distance to Crab Nebula r = radius of curvature

20. 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.

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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-

31. 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

32. 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-

33. 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

34. 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

35. 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

36. 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

37. 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

38. COSMIC RAYS END OF TOPIC

39. 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