Jyväskylä, 25-29, August 2008

Physics of high-energy cosmic raysLecture 1: Origin of cosmic raysLecture 2: Propagation and interactions of cosmic raysLecture 3: Experiments and results

Tiina SuomijärviInstitut de Physique Nucléaire

Université Paris XI-Orsay, IN2P3/CNRSFrance

tiina@ipno.in2p3.fr

References: High Energy Astrophysics, vol. 1 and 2, Malcolm S. Longhair, Cambridge University Press, 1997Cosmic Rays and Particle Physics, Thomas K. Gaisser, Cambridge University Press, 1990

Jyväskylä, 25-29, August 2008

Physics of high-energy cosmic raysLecture 1: Origin of cosmic rays

Tiina SuomijärviInstitut de Physique Nucléaire

Université Paris XI-Orsay, IN2P3/CNRSFrance

ContentsWhy to study cosmic rays?

Dimensions and time-scales

Historical perspective

Some characteristics of cosmic rays

Contents of the Universe

Origin of cosmic rays: Cosmic accelerators?

Origin of cosmic rays: Something exotic?

Conclusions

Why to study cosmic rays ?

Cosmic rays span over an enormous range of energies, up to 1020eVThey are abundant and serve an important role in the energybalance of galaxy. Their energy density 1 eVcm-3 is comparable tothat contained in the galactic magnetic field or in the cosmicmicrowave background.They can be evidence of powerful astrophysical accelerators(supernovae, active galactic nuclei…) and can be used to studythese accelerators

Why to study cosmic rays…They propagate through universe and can give information onproperties of cosmic environment (magnetic fields, matterdensities…)Their chemical composition, modulated by propagation,reflects the nucleosynthetic processes occurring at their originand can also be used to measure age of astrophysical objectsThey can be used to study the validity of physical laws inextreme conditions (violation of Lorentz invariance?)They can be messengers of « new physics » or yet unknownparticles

Dimensions and time scale

Formation of galaxies

Electroweak transition

15 milliards yearsT=3K

1 seconde1 MeV

10-6 s1GeV

Today

Quark-hadron transition

Grand Unification

Nucleosynthesis

10-11 s103GeV

10-35 s1015 GeV

10-4 pcSolar system

1.3 pcClosest star

25k pcDiameter ofour galaxy

700k pcClosest galaxy

1M pcSize ofLocal cluster

50M pcSize of local super cluster

The unit of distance in astronomy iscalled the parallax-second, or parsec.It is defined to be the distance atwhich the mean radius of the Earth’sorbit about the sun subtends an angleof one second of arc.1 pc = 3.08 1016 m = 3.26 light years1 erg = 10-7 J

Historical perspective

-

Great triumphs of 19thcentury

H.R. Hertz

J.C. Maxwell

Unification of electricity and magnetismMaxwell 1864

20 years later experiments of Hertzconfirmed that the light is a form ofelectromagnetic radiation

Experiments in electricity and magnetismConduction of electricity through gases:Good vacuum tubes and high voltages between the positive and negativeelectrodes

1879 : Crookes tube

1897 : Thomson measuredthe charge to mass ratio ofcathode rays by deflectionthe radiation by crossedelectric and magnetic fields:Discovery of the first sub-atomic particle, electron

Crookes

Thomson

Discovery of cathode rays

1895 Röntgen discovered X-rays: Photographic plates leftclose to Crookes tubes were darkened

Search for other sources of X-ray emission

1896 Becquerel discovered natural radioactivity bystudying uranium samples

1898-1900 Rutherford, P. et M. Curie and Villardunderstood that there were several types of radioactivity:α, β, γ

New discoveries

Discovery of Cosmic Rays (CR)Early experiments in radioactivity used electroscope

When electroscope is charged, the leaves (A) are pushed apartThe ionisation of the gas inside discharge the electroscope andthe leaves move towards each otherThe rate at which the leaves came together measured the amountof ionisation

Spontaneous discharge of the electroscope !

1901 Wilson observes that the discharge isidentical on the ground and in a tunnelRutherford shows that this is due to thenatural radioactivity

Étienne Parizot Les Rayons Cosmiques Ultra-Énergétiques 7 janvier 2003Ultra-

La tour Eiffel et la

science...

80 mètres : flux/2

300 meters : flux/15??Eiffel Tower and the science

1910 Father ThéodoreWulf (jesuite, amatoryphysicists) conductedexperiments on top of theEiffel tower

Radiation intensitydecreases with height butless than expected

--

1912 and 1913 : Hess and Kolhörster mademanned balloon flights to measure theionisation of the atmosphere with increasingaltitudeAverage ionisation increasewith increasing altitude

Up to the sky

Victor Hess, 1912Victor Hess, 191217,000 feet17,000 feet

« The result of theseobservations seems to beexplained in theeasiest way by assuming thatan extremely penetratingradiationenters the atmosphere fromabove » (V. Hess)

1929 Geiger-Müller detector was inventedFast response time allow to count individual cosmic rays and alsoto determine precisely their arrival time.

1929 Bothe and Kolhörster performed the first coincidence (to about0.01 s !) experiment by using two counters one placed above the otherand introducing thick absorber between the two.

Coincident events indicated that the cosmic rays were charged, verypenetrating particle (gamma rays were stopped in the absorber).

Counting improves

Extensive air showersMeasurements in the Alps1938: Pierre Auger and P. Ehrenfest, Jr

Pierre Auger, CERN 1990

Two particle detectors positioned highin the Alps signaled arrival ofparticles at exactly the same timeExplanation: particles are generated inair-showersE > 1015 eV

Cosmic rays and the discovery ofelementary particles

C.T.R. Wilson

Invention of cloud chamber

Fast expansion of gas in order todecrease temperature and oversaturate the gasCondensation of drops on theions produced by particles

Ionisation tracks

Positon ⇒ antimatter!

Muon

Pions : π 0, π +, π -

Kaons (K)

Lambda (Λ)

Xi (Ξ)

Sigma (Σ)

1932

1936

1947

1949

1949

1952

1953

The beginning of Particle Physics !

Some characteristics ofcosmic rays

Cosmic ray spectrum

Very steep spectra!

Low energy CR

Solar modulation

1 particle per m2-second

Knee(1 per m2-year)

γ≈ 2.7 - 3.0

Ankle(1 per km2-year)

All-particleCR energyspectrumdirect measurements galactic CRs

air shower measurementsdiffusion losses of galactic CRs ?

air shower measurementsextragalactic CRs ?

• Composition (at ~GeV): 85% H (p) 12% He (α) 1% heavier nuclei 2% e± (≥90% e- ) 10-5-10-4 antiprotons.

Cosmic ray composition

Angular distributionIsotropique!

Contents of the Universe

Large scale distribution of matterand radiation in the Universe

•Measurements of the cosmicmicrowave background(CMB): evidence for theoverall isotropy of theUniverse•Discovered by Penzias andWilson 1965•CMB is the cooled remnantof the early phase of theUniverse

Distribution of visible matter

Sky distribution of approximately 30000galaxies from CfA Catalog.

The distribution of galaxies is highlyirregular, with huge holes, filamentsand clusters occurring in the localUniverse.

The Wilkinson Microwave Anisotropy Probe (WMAP)team has made the first detailed full-sky map ofthe oldest light in the universe.

•The most striking features about the CMB is itsuniformity.•Only with very sensitive instruments can detectfluctuations.•By studying these fluctuations, one can learnabout the origin of galaxies and large scalestructures and measure the basic parameters ofthe Big Bang theory.

The Virgo cluster•The Virgo Cluster with its some 2000member galaxies dominates ourintergalactic neighborhood.•It represents the physical center of ourLocal Supercluster and influences all thegalaxies and galaxy groups by thegravitational attraction of its enormousmass.•The center of the Virgo cluster is about15-20 Mpc from our galaxy.

The Virgo Cluster of Galaxies, and is centered onthe giant elliptical galaxy M87. The two brightgalaxies on the right (west) are (right-to-left)M84 and M86; starting from these two, a chain ofgalaxies ("Markarian's chain") stretches well tothe upper (northern) middle of our image (andbeyond, well to M88 which is slightly outsideabove the sky area photographed our image).

Hubble lawHubble 1929: the Universe of galaxies is in a state of uniformexpansion.All galaxies are receding from our galaxy, the further away agalaxy is from us, the greater its velocity of recession v:

v=H0r,r is the distance of the galaxyH0 is the Hubble constant

The velocity is measured from the shift of the spectral lines in thegalaxy’s spectrum to longer wavelengths => redshift

The current value of the Hubble constant is still debated, values nearthe high and low ends of 50 and 100 km s-1/Mpc.

The galaxiesGalaxies are the basic building blocks of the Universe. Basic distinction is between spiral and elliptical galaxies.

Spiral galaxy: The Milky Way is the galaxy whichis the home of our Solar System together with atleast 200 billion other stars (more recentestimates have given numbers around 400billion) and their planets, and thousands ofclusters and nebulae

Elliptical galaxy: The giant elliptical galaxy M87, alsocalled Virgo A, is one of the most remarkableobjects in the sky. It is perhaps the dominant galaxyin the Virgo Cluster of galaxies.M87's diameter corresponds to a linear extension of120,000 light years, more than the diameter of ourMilky Way's disk. It fills a much larger volume, andthus contains much more stars (and mass) than ourgalaxy, certainly several trillion (1012) solar masses.

M51

M87

The Milky Way

Funny galaxies

The The ‘‘AntennaeAntennae’’The The ‘‘CartwheelCartwheel’’

Galaxies with active nucleiThe first class of galaxies with active nuclei was discovered by Seyfert(1940): Seyfert galaxies.

Spiral galaxies but posess star like nuclei

Strong and broad emission lines

The next class of galaxies with active nuclei discovered was the radiogalaxies.

Sources of vast fluxes of high energy particles and magnetic fields

The first quasars were discovered early 1960Look like star but has a luminosity much greater than galaxies.

Radio quiet quasars, blazars, were discovered in 1965

BL Lacertae er BL-Lac objects are the most extreme examples of activegalactic nuclei.

Similar to quasars but luminosity vary rapidly (days): compact objects.

Optical spectra featureless and radiation strongly polarized.

Model for generating energy in AGNs

When the jet is directedtowards us the luminosityincreases.

If the matter fell directly into the blackIf the matter fell directly into the blackhole on a purely radial orbit, then no energyhole on a purely radial orbit, then no energywould be liberated at all.would be liberated at all.However, it is mostHowever, it is mostunlikely that the matter falls straight in,unlikely that the matter falls straight in,because it must acquire some angularbecause it must acquire some angularmomentum, or rotation.momentum, or rotation.The matter canThe matter cancollapse along the axis of rotation and formscollapse along the axis of rotation and formsan accretion disc about the axis of rotation.an accretion disc about the axis of rotation.

Supernovae•Supernova occur at the end of astar's lifetime, when its nuclear fuelis exhausted and it is no longersupported by the release of nuclearenergy.•If the star is particularly massive,then its core will collapse and ahuge amount of energy is released.•This will cause a blast wave thatejects the star's envelope intointerstellar space.•The result of the collapse may be arapidly rotating neutron star thatcan be observed many years later asa radio pulsar.

Supernovae are rare events in our galaxy.There are many remnants of Supernovaeexplosions in our galaxy, that are seen as X-ray shell like structures caused by the shockwave propagating out into the interstellarmedium.A famous remnant is the Crab Nebula whichexploded in 1054: pulsar which rotates 30times a second and emits a rotating beam ofX-rays (like a lighthouse).

Neutron stars

Neutron stars may appear in supernova remnants, as isolated objects, or in binary systems.When a neutron star is in a binary system, astronomers are able to measure its mass. Forbinary systems containing an unknown object, this information helps distinguish whether theobject is a neutron star or a black hole, since black holes are more massive than neutron stars.

PulsarsRadio pulsars were discovered in 1967.Pulsars are isolated, rotating, magnetised neutron stars.They have jets of particles moving almost at the speed of light streaming outabove their magnetic poles.

Crab Nebula: example of a neutron star formedduring a supernova explosion. Figures show thediffuse emission of the Crab Nebula surroundingthe bright pulsar in both the "on" and "off"states, i.e. when the magnetic pole is "in" and"out" of the line-of-sight from Earth. The periodis 33 ms.

Rotation axisRotation axis

Magnetic axisMagnetic axis

The pulses detected separately for the firstThe pulses detected separately for the firsttimetime

Mullard Mullard Radio Astronomy ObservatoryRadio Astronomy Observatory

One secondtime markers

Gamma-ray bursts are short-lived burstsof gamma-ray photons.At least some of them are associated witha special type of supernovae, theexplosions marking the deaths ofespecially massive stars.

Lasting anywhere from a few millisecondsto several minutes, gamma-ray bursts(GRBs) shine hundreds of times brighterthan a typical supernova.GRBs are detected roughly once per dayfrom wholly random directions of the sky.

GRBs were discovered in the late 1960sby U.S. military satellites which were onthe look out for Soviet nuclear testing inviolation of the atmospheric nuclear testban treaty.

Gamma Ray Bursts

The interstellar mediumThe region between the stars in a galaxy have very low densities (they constitute avacuum far better than can be produced artificially on the surface of the Earth),but are filled with gas, dust, and charged particles.

Approximately 99% of the mass of the interstellar medium is in the form of gaswith the remainder primarily in dust. The total mass of the gas and dust in theinterstellar medium is about 15% of the total mass of visible matter in the MilkyWay.

Of the gas in the Milky Way, 90% by mass is hydrogen. The gas appears primarilyin two forms

1. Cold clouds of atomic or molecular hydrogen 2. Hot ionized hydrogen near hot young stars

Interstellar dust grains are typically a fraction of a micron across (approximately thewavelength of blue light), irregularly shaped, and composed of carbon and/or silicates.These dust clouds are visible if they absorb the light coming through them.

The clouds of cold molecular and atomic hydrogen represent the raw material from whichstars can be formed in the disk of the galaxy if they become gravitationally unstable andcollapse.

Example: Orion Nebula

The Orion Nebula is relatively nearby, about 1500 lightyears away in the same spiral arm of the galaxy as ourown Sun.

Example: The Pleiades Cluster

The Pleiades Cluster is a young cluster of predominantly blue stars that is visible to the naked eye.There is still some dust left from the nebula in which they formed, and light reflecting from that dustcauses the blue haze around each star of the cluster.

Magnetic fieldsThe Milky Way galaxy contains an ordered, large-scale magnetic field of the value ofabout 3 µGauss (Earth's field at ground level is about 1 Gauss.)

Observation methods: analysis of starlight polarization, modeling pulsar or Faraday rotation,Zeeman splitting of atomic or molecular lines, radio synchrotron emission of electrons

A spiral galaxy like the Milky Way has three basic components to its visible matter which include thedisk (containing the spiral arms), the halo, and the nucleus or central bulge. Because of the varyingdensity in the galaxy's components, the magnetic field has a range of values.

Magnetic field derived from galaxy simulationoverlaid on the galaxy NGC 4151. The blue 'ribbons'are components of a vertical magnetic field whilethe green arrows depict both the axisymmetric andbisymmetric magnetic fields observed in galaxies ofthis morphological type.

Estimations for extra-galacticmagnetic field varies from 1 nGaussto 100 nGauss

The strongest, naturally-occurring, fields arefound on a new kind of neutron star called amagnetar. These fields can exceed 1015 Gauss.

Dark matterThe basic principle for observations is that if we measure velocities in some region,then there has to be enough mass there for gravity to stop all the objects flying apart.Velocity measurements -> the amount of inferred mass is much more than can beexplained by the luminous stuff -> Dark MatterPrecise measurements of the cosmic microwave background -> dark matter makes upabout 25% of the energy budget of the Universe; visible matter in the form of stars,gas, and dust only contributes about 4%.

The leading candidate for this "dark matter" is the neutralino, the lightestsupersymmetric particle.On astrophysical scales, collisions of neutralinos with ordinary matter arebelieved to slow them down. The scattered neutralinos, whose velocity isdegraded after each collision, may then be gravitationally trapped byobjects such as the Sun, Earth, and the black hole at the center of theMilky Way galaxy, where they can accumulate over cosmic time scales.

Origin of cosmic rays: Cosmicaccelerators?

Origin of cosmic rays• The origin of CR is one of the major unsolved

astrophysical problems.• Their origin is most likely to be associated with

the most energetic processes in the Universe.• There is a large number of models for cosmic ray

acceleration. The acceleration mechanism is notcompletely understood yet.

• In top-down scenarios energetic cosmic rays canalso be produced by decay of massif particleswhich can be related to early Universe.

General principles of accelerationThe acceleration mechanisms may be classified as dynamic, hydrodynamicand electromagnetic.Dynamic: Acceleration takes place through the collision of particles withclouds.Hydrodynamic: Acceleration of whole layers of plasma to high velocities.Electromagnetic: Particles accelerated by electric fields (magnetospheres ofneutron stars).

Acceleration of particles in electric and magnetic field: d/dt (γmv) = e(E+v x B)

Static electric fields difficult to maintain due to the very high conductivity of ionisedgases. Acceleration mechanism can only be associated with non-stationary electricfields. Static magnetic fields don’t do any work but if the magnetic field is time-varyingwork is done by induced electric field.

Recent years: most effort to study particle acceleration in strong shock waves.

Cyclotron mechanism

• Even normal stars can accelerate chargedparticles up to GeV range.

• This acceleration can occur in time-dependent magnetic fields.

• These magnetic sites appear as star spots oror sunspots: B typically 1000 Gauss.

• Particles from the Sun with energies up to100 GeV has been observed.

Fermi acceleration(E. Fermi, Phys. Rev. 75 (1949) 1169)

Magnetic clouds (M>>mp), with typicalvelocity v in the ISM

When a particle is reflected off a magneticmirrorcoming towards it, in a head-on collision, it gainsenergyWhen a particle is reflected off a magneticmirrorgoing away from it, in an overtaking collision, itloses energy

Net energy gain, on average (stochastic process)

Fermi mechanism of 2nd order (more generally Fermi mechanism) describes theinteraction of CR with magnetic clouds.

Schema of Fermi mechanism

particle gas cloud

v -u

particle gas cloud

v u

Case 1

Case 2

ΔE1 = 1/2m(v+u)2 - 1/2mv2 = 1/2m(2uv + u2)

ΔE2 = 1/2m(v-u)2 - 1/2mv2 = 1/2m(-2uv + u2)

On the average, net energy gain is:

ΔE = ΔE1 + ΔE2 = mu2

ΔE/E = 2u2/v2

Characteristics

Yields a power law spectrum (OK)

However:

Since the cloud velocity is low compared to particle velocities(u acceleration requires a long time (about 108 years!)

CR particles loose some of their gained energy between twocollisions => requires a minimum injection energy abovewhich particles can only be effectively accelerated.

Shock acceleration

Example: The ejected envelope of a supernova represents a shock front withrespect to interstellar medium.

u2

v

u1 u1-u2

Shock front

Shock front velocity

Incident particle

Gas streaming awayfrom the shock front

Velocity of the gasin the lab frame

A particle of velocity v colliding with the shock front and being reflected gains theenergy:

ΔE = 1/2m(v+(u1 - u2))2 - 1/2mv2

= 1/2m(2v+(u1 - u2)+(u1 - u2)2 )

Suggested in early 70, under developmentSee for example J. R. Jokipii, Ap. J., 313 (1987) 443

Shock accelerationSince the linear terms dominate (v » u1, u2, u1>u2):

ΔE/E ≈ 2(u1 - u2)/v

A more general, relativistic treatment including also variableangles yield:

ΔE/E = 3/4(u1 - u2)/c,particle velocity ≈ c

More efficient than 2nd order Fermi acceleration.

Astrophysical shocks

Maximum energy?Energy losses◆ Friction with ambient medium◆ Synchrotron, IC◆ Pair production◆ photo-π

Escape◆ Keep the particle inthe accelerator◆ Diffusion, gyroradius

Destruction◆ Photo-disintegration

⇒ 10 15 eV1 pc

B = 3 µ GSupernova remnant

SN

⇒ For energies of 1020 eV a chock of 1 Mpc needed !

Maximum energy: Example

E. Parizot

AGN hot spots

3C 219

A candidate for acceleration up to extreme energies

-

Ω

B

Pulsars as cosmic accelerators

However, it is not at all obvious how pulsars manage in detail totransform the rotational energy into acceleration of particles.

Spinning magnetized neutron stars (pulsars) areremnants of supernova explosions. They shrink undergravitational collapse to a size about 20 km! Theirdensities are comparable to nuclear densities!

Gravitational collapse conserve angular momentum=> short rotational periods (milliseconds)!

The gravitational collapse amplifies the magneticfield (magnetic flux is conserved):Bstar = 1000 Gauss => Bpulsar ≈1012 Gauss = 108 TeslaFor a 30 ms pulsar vrot = 4 106 m/sE = v x B with v⊥B => E ≈ vB = 1015 V/mA single charged particle can gain 1 PeV per meter!

Production of particles and radiationby cosmic accelerators

p γ,ν

p

Accelerator

Target:Radiation and gas aroundthe accelerator

Acceleration of charges particlesProduction of gammas andneutrinos in the radiation and gasaround the accelerator

Acceleration power

Origin of cosmic rays:Something exotic?

Exotic Particles? Other Sources?

Long living X-particles

Clustering in galactic halo, Dark Matter candidate

Decay of topological defects:super heavy relic particles from Big-Bang

X-particles (m ≈ 1021-1025 eV)

W,Z bosons

γ, ν, p…

i.e.: proof of evidence for neutrino background radiation

UHE ν relic ν (mν≠0)

Z0→ p,n,γ,π,ν

ZevatronZ-burst model

Conclusions

Cosmic ray Physics started 1912 when Victor Hessdiscovered that mysterious radiation is coming from space.

Today, the origin of cosmic rays is one of the most importantquestion in modern astrophysics.

There are astrophysical objects that can probably acceleratecosmic rays. However, it is unclear how they can accelerateup to energies of 1020 eV!

Another possibility would be that some of these cosmic raysare decay products of yet unknown particle.