C.1.2 (Ultra-) A. Zech, Physics & Detection of AstroParticles, C1 Cosmic Rays 63+1

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1 C.1.2 C.1.2 (Ultra-) (Ultra-)high high energy energycosmic cosmicrays rays 63+1

2 The ultra-high energy range Limite LHC The "ultra-high" energy range begins at around 1017 ev. There is no strict definition, though. An energy of about 1015 ev corresponds to the upper limit that will be reached at the LHC (Large Hadron Collider) in the center of mass frame for Pb nuclei. Theoretical mechanisms to explain cosmic ray acceleration (e.g. in shockwaves, AGN jets, etc.) above ~1019 ev are still very uncertain. At ~6 x 1019 ev a flux suppression is expected to be observed in the extragalactic cosmic ray spectrum due to the GZK effect (photo pion-production with the CMBR). 63+2

3 The "GZK Controversy" Until recently, two experiments had measured the cosmic ray spectrum at ultra-high energies: HiRes (two air fluorescence telescopes) and AGASA (a 100 km2 array of scintillators). AGASA observed a continuation of the cosmic ray spectrum up to the highest energies without any sign of the GZK feature. HiRes did observe the GZK cut-off in the spectrum. The Auger experiment combines the two techniques. It has confirmed the flux suppression discovered by HiRes. 63+3

4 C C Detection Detectionof of(ultra-) (ultra-) high highenergy energycosmic cosmicrays rays 63+4

5 Hadronic air showers (1) Air showers initiated by hadrons (protons, neutrons, nuclei...) have at their center a hadronic cascade that produces pions in nuclear interactions. The pions will then decay, mainly in the following way: ; 0 2 The charged pions generate muons, which can decay into electrons (positrons) or reach the ground. The neutral pions feed electromagnetic cascades, which produce electrons, positrons and photons, as we saw. A part of the electrons, positrons and photons will reach the ground. Hadronic showers develop earlier in the atmosphere than electromagnetic showers because of the additional nuclear interactions. They also have a large muon fraction and are wider than e.m. showers. 63+5

6 Hadronic air showers (2) 63+6

7 cascade equations Analytically, one-dimensional hadron transport is described by the following coupled differential equations: dn i E, X N E, X N E, X N E ', X de ' = i i j F ij E, E ' dx d i E E i j E j E ' i, j = p, n,, K,... F ij = E i = 1 d ij E, E ' de m atom,medium i inclusive cross-section for i + target -> j + anything interaction length N (E,X) is the number of particles at energy E and slant depth X (atmospheric depth in g/cm2 along the shower direction). d(e) is the decay length. Hadronic cascades are usually modelled with the use of sophisticated Monte Carlo simulations. 63+7

8 The superposition principle The relations we found with the Heitler model for a simple electromagnetic air shower are still a good approximation for hadronic showers initiated by a single proton or neutron: N max E 0 ; X max log E 0 If the primary particle is a nucleus with mass number A and energy E 0, we can treat it approximately as a superposition of A nucleons with energy E 0 / A. We then assume that the A nucleons interact independently with the atmosphere and generate a superposition of A sub-showers. The N max of the sum of these sub-showers will still be the same: N max E 0 This Nmax is now the sum of the Nmax of the A sub-showers: X max / N max = A 2 X max log E 0 / A This means that an air shower that comes from a nucleus will reach its maximum size higher up in the atmosphere than an air shower coming from a single proton! Xmax will thus help us distinguish between different primary particles for different showers with the same N max (=same energy E0). It should be noted that the superposition principle is a stark simplification of the real process, which can only be described with the proper nuclear processes. It can thus only be used for a qualitative statement. 63+8

9 Photon, Proton and Iron Induced Air Showers photon proton iron Vertical (z-) axis range is 30 km. First interaction at a height of 30 km. The shower is projected onto the x-z plane. Horizontal (x-) axis range is +/- 5 km around the shower core. Energy: 100 TeV. Vertical injection of the cosmic ray particle. Colors: e+,e-,photons (red) / muons (green) / hadrons (blue) (red+green -> yellow) ( 63+9

10 The Ground Array Technique Detection of lateral particle profile (muons, electrons/positrons, photons) on ground. A large array of detectors, usually sctintillators or water Cherenkov tanks, is used. Reconstruction of the shower geometry from signal strengths and timing information from the different detectors. Reconstruction of the cosmic ray energy by comparison of the lateral density profile with simulations. Reconstruction of the cosmic ray composition from analysis of the recorded pulse shape and the curvature of the shower front and/or from the muon fraction. =>100% duty cycle (detectors can run 24 hours a day). => fluctuations of the lateral profile can be important => energy reconstruction is very model dependent 63+10

11 Fluorescence and Cherenkov light from air showers The charged particles of an air shower (>90% electrons and positrons) excite air molecules, which emit UV "fluorescence" light. The fluorescence light emission is istotropic. Since the charged particles have velocities close to the vacuum speed of light, they also emit Cherenkov radiation in the atmosphere (in the UV range). The Cherenkov radiation is emitted in cones along the direction of the particles. This leads to an emission in the forward direction of the shower axis (non-isotropic!). Scattering of Cherenkov photons in the atmosphere distributes a part of the "Cherenkov cone" also at larger angles

12 "Fluorescence" Light from Air Showers Usually, the term "fluorescence" refers to the process by which atoms absorb photons of one wavelength and emits photons at a longer wavelength. The passage of charged particles in an extensive air shower through the atmosphere results in the ionisation and excitation of the gas molecules (mostly nitrogen). Some of this excitation energy is emitted in the form of visible and UV radiation. Strictly speaking, this is a "luminescence" or "scintillation" process. Much to the horror of the optical physicists, the name "Air Fluorescence" has been adopted by the astrophysics community to describe the scintillation light from extensive air showers. On the positive side, this usage makes it easy to distinguish between a fluorescence detector from a scintillation detector

13 The Air Fluorescence Technique Detection of the longitudinal shower profile in the atmosphere via UV fluorescence light. Optical detectors (mirrors and cameras with PMTs) Reconstruction of shower geometry from the observed track and the trigger times of the PTMs. Best reconstruction in stereo mode. Reconstruction of the cosmic ray energy from an integral over the observed light flux. The cosmic ray deposits ~90% of its energy into the electromagnetic cascade. The atmosphere is thus used like a huge calorimeter. Light flux is proportional to the energy deposit. Reconstruction of composition from Xmax measurements. => 10% - 15% duty cycle (Observation only possible on clear, moonless nights) => atmosphere needs to be well understood => energy dependent aperture 63+13

14 Cherenkov Technique Imaging Atmospheric Cherenkov Telescopes (IACTs): Telescopes like HESS, MAGIC and VERITAS observe the Cherenkov signal from γ-ray induced showers. Hadronic showers are background for these experiments, but they can also be observed as signals. The irregular shape of the shower image on the camera makes it difficult to reconstruct their geometry. A flux measurement at the highest energies is not possible, since the Cherenkov light is non-isotropic and the fraction of showers that hit the detectors is too small. Non-Imaging Atmospheric Cherenkov Telescopes: These are ground arrays of PMTs that register the Cherenkov photons from the air shower. This technique is used in combination with other surface detectors to provide an additional estimate of the energy of the shower (i.e. Haverah Park, Yakutsk, Blanca...)

15 The Radio Detection Technique Technique discovered in the 1960s and tested in several experiments. The radio signal of an air shower comes from coherent synchrotron emission from e+e- pairs that gyrate in the earth's magnetic field. An additional effect comes from the fact that there is a certain surplus of electrons in the air shower, which means that the shower front carries a net charge. The radio signals can be picked up with arrays of antennae at energies > ~1 PeV. Reconstruction of geometry and energy seem very promising. Several prototype experiments are currently testing this technique: Lopes/Lofar in combination with the ground array Kascade, Codalema (at the Nançay site of the Paris Observatory)... Radio antennae have also been added to the Auger Observatory. The CODALEMA experiment in Nançay

16 a bit of history...some UHECR experiments (1) late 1940s: ground array technique developed by scientists from MIT (USA). 1959: Volcano Ranch (New Mexico, USA), 10 km2 array of plastic scintillators, energies estimated to be up to 1020 ev. 1962: Haverah Park (England), 12 km2 array of water Cherenkov detectors, spectrum measured from 0.3 to 10 EeV 1964: Fluorescence technique tested by scientists from Cornell (USA). 1968: First successful air fluorescence detection near Tokyo. 1976: Air fluorescence observed in coincidence with Volcano Ranch detectors. Yakutsk (Siberia), 18 km2 of muon counters, scintillators and PMTs for Cherenkov light detection, highest E event: 1.3 x 1020 ev SUGAR (Australia), 60 km2 of muon counters Akeno (Japan), 20 km2 of muon counters and scintillators, later extended to AGASA 1982: Fly's Eye (Utah, USA), two air fluorescence detectors, spectrum measured above 0.4 EeV, the highest ever energy event was measured at 3 x 1020 ev. 1990: AGASA, extension of Akeno to 100 km2, energy spectrum did not show any sign of the GZK feature, 11 events above 1020 ev

17 a bit of history...some UHECR experiments (2) : HiRes/MIA was the first experiment to employ a hybrid (fluorescence + ground array) technique, using a HiRes prototype telescope and the MIA muon counter array : HiRes (High Resolution Fly's Eye) consisted of two air fluorescence telescopes located in Utah (USA), measurement of the energy spectrum above ~0.1 EeV, discovery of the GZK cutoff in the spectrum. 2004: The Pierre Auger Observatory (Auger South) is taking the first data 2006: Telescope Array (Utah, USA) begins operation. It combines fluorescence telescopes with an array of 1000 km2 of plastic scintillators. Its unique advantage is a large coverage of energies from ~30 PeV to the highest energies. 2008: Auger South array completed at present: development of radio detection methods are very promising 63+17

18 The Pierre Auger Observatory 63+18

19 Hybrid Detection Technique Hybrid technique: combination of surface detectors and fluorescence telescopes very large area (~3000 km2) for good statistics at the highest energies (around GZK threshold) Study of the energy spectrum, arrival directions and composition of ultrahigh energy cosmic rays

20 The Auger South array Auger South is located in the province of Mendoza (Argentina), near Malargüe. Layout: 1600 surface detectors and 4 fluorescence stations (6 telescopes each). Advantages of the site: Far from light-pollution, clear atmosphere, large flat area, good infrastructure

21 Instrumentation - Auger More than 1600 surface detectors are arranged in a grid with a 1.5 km spacing between two stations. They cover ~3000 km2. 4 stations with 6 fluorescence telescopes each view the atmosphere above the site. Several stations (laser, lidar, weather station...) are included in the array to collect information on the local weather conditions. The Pierre Auger Observatory South site started taking data in January 2004 with a small proportion of the full array. A similar, but larger array was planned for the northern hemisphere (Colorado): Auger North (however no funding is available at present) 63+21

22 Surface Detectors (SD) The surface detectors are water Cherenkov tanks. They are each filled with 12 tons of ultra-clean water. Charged particles from the air shower will leave a trace of Cherenkov light when traversing the tank. e+,e-: provide a signal, but are quickly absorbed (E~10MeV) muons: provide a strong signal as they traverse the tank (E~GeV) photons: pair produce e+,e-> advantages to scintillators: larger zenith angle coverage, scintillators only respond to charged particles & do not distinguish muons 3 PMTs observe the Cherenkov photons and record traces. solar panel for power supply antenna for GPS & communication 63+22

23 Signal A water Cherenkov detector solar panel Antenne Temps electronics reflecting bag photomultiplier Cherenkov cone batteries ultra-clean water 63+23

24 FADC traces seen in the SD detectors Station selection : shape of the signal ns muon ns muon + electromagnetic 63+24

25 Reconstruction of the Shower Geometry (SD) When an air shower triggers several SD stations, their pulse sizes and trigger times are used in the reconstruction of the shower front and of the complete shower geometry. Below: a footprint of a highly energetic shower. The angular resolution of the shower axis and thus of the cosmic ray direction is < 2.2o

26 Lateral distribution function Energy reconstruction LDF = distribution of signal size as a function of distance from shower axis The signal size at 1000 m is called "S1000". It is used as an estimator of the primary energy E. S1000 ~ const x E. The distance of 1000 m is determined from Monte Carlo simulations for the given array configuration. For the tank distance of 1.5 km, fluctuations in the LDFs are at a minimum at 1000 m. The conversion factor between S1000 and E has to be determined from simulations or from additional shower measurements (FD, radio...). S1000 recorded signal distance to axis (m) 63+26

27 Radius of Curvature (SD) Composition Measurement Deeper showers have smaller radii of curvature. This helps to distinguish photon showers from hadronic showers. One can also try to distinguish between proton and iron showers, which is more difficult. Xmax Xmax Proton photon proton 63+27

28 Risetime (SD) Composition Measurement Photon showers develop deeper in the atmosphere and over a longer range => longer risetime of the FADC pulse. There is an additional effect from the difference in muon fraction. The risetime of the pulse can be used to distinguish photons and hadrons. One can also use it to try and distinguish between protons and iron nuclei, which is more difficult. Photon Proton 63+28

29 Air Fluorescence Detectors (FD) 4 fluorescence stations view the atmosphere above the ground array. Each station consists of 6 telescopes. Operation only during clear, moonless nights. Antenna for communication with surface detectors and with central trigger. Each time an FD is triggered, SD stations are read out as well to yield hybrid information

30 Mirrors UV fluorescence light from the air shower passes through the aperture (with filter and a corrector lens for spherical aberration) and hits the mirror. (Schmidt optics) The mirror reflects the light onto a camera in its focal point

31 Camera Each camera consists of 440 (20 x 22) PMTs arranged in a hexagonal pattern. Each PMT sees ~1.5o. One telescope covers 30o in azimuth and 30o in elevation. The six telescopes of each station view 180o in azimuth. Reflectors similar to Winston cones are put in front of the PMTs to enhance their angular range. When an air shower traverses the sky, its image traverses the camera. A track of triggered pixels can be recorded

32 Reconstruction of the Shower Geometry (FD) 1 The first step in the geometry reconstruction is the reconstruction of the "showerdetector-plane". above: shower track seen in two telescopes; below: reconstruction of a stereoscopic event (seen by two stations) Next, the position of the axis inside the plane has to be determined with stereo information or timing information of the PMTs on the track

33 Reconstruction of the Shower Geometry (FD, Hybrid) (2) The geometry of the axis is determined (in the absence of stereo information) from a time vs. angle plot. The additional information from the tanks improves this fit considerably. (Hybrid reconstruction) PMT traces of a stereo event. The pulse sizes give information on the longitudinal profile of the air shower

34 Reconstruction of the cosmic ray energy (FD, Hybrid) The recorded signals are transformed into a photon flux at the detector. This flux is traced back through the atmosphere to the shower axis. Atmospheric attenuation is taken into account. The photon flux is converted into a distribution of charged particles per slant depth (= the longitudinal shower profile). Cherenkov light from the shower is subtracted in an iterative procedure. The integral over the shower times the mean ionisation loss rate de/dx ~2.9 MeV / (g cm-2) = energy deposit of the shower. ~10% is added for "missing energy" to get the total energy of the cosmic ray. (The exact fraction of missing energy is determined from simulations.) 63+34

35 Using hybrid information for SD reconstruction 10EeV ) Ve E/ E( gol The information from hybrid events can be used to determine the conversion factor between S1000 and the primary energy. One determines this factor for a subset of hybrid events with well reconstructed S1000 (from SD) and energy (from FD). One reconstructs the S1000 of all SD events and applies the factor to determine their energy. => combination of large statistics from the SD and the very good energy reconstruction of the FD 1 EeV Log (S(1000)/VEM) 63+35

36 Measurement of the Cosmic Ray Composition (FD) top of atmosphere Xmax FD Xmax Xmax is used as an estimator for the composition of the cosmic ray. Heavy nuclei lead to showers that develop higher up in the atmosphere than light nuclei. The measured Xmax are compared to Monte Carlo simulations. A big problem lies in the fact that fluctuations of the Xmax between different showers of the same composition and energy are large. Longitudinal profile of a 1018 ev proton shower

37 Calibration (FD) photon flux -> p.e. : each PMT is calibrated against NIST calibrated photodiode p.e. -> ADC counts : absolute calibration every few months with an LED light source. The light source illuminates a drum, which is plugged onto the aperture of each telescope. The mirror is illuminated uniformly. verification with laser shots of known energy nightly relative calibration with light sources at the mirrors, camera, aperture

38 Calibration (SD) The SD stations are "self-calibrating" Atmospheric muons (from air showers) are used to calibrate all the stations. There are counts per minute from this background. The number of ADC counts at the peak is the signature of a vertically traversing muon. The muon velocity is close to the speed of light v~c and the Cherenkov light output depends on v, not on the Lorentz factor. The peak ADC value defines a "VEM" (vertically equivalent muon). All signals are then expressed in multiples of 1 VEM

39 Atmospheric Monitoring There are two main processes for atmospheric attenuation in the UV band of interest (~ nm): molecular (Rayleigh) scattering aerosol (Mie) scattering The molecular density of the atmosphere is measured with ~5 radiosonde launches per month. The air pressure is measured at different altitudes. The aerosol density and distribution has to be measured during the nights when data are taken, since aerosols can change rather rapidly. Several detectors are used (lasers, lidars, horizontal attenuation detectors) Clouds are observed with infrared cameras. A weather station records temperature, wind speed, precipitation

40 The Future of Cosmic Ray Physics JEM/EUSO is a Japanese/European proposal for a detector to be deployed on the international space station. It will observe extensive air showers in the atmosphere from above and significantly increase the experimental aperture at the highest energies. (planned to be launched in 2013) 63+40

41 More Information HiRes: AGASA: AUGER: TA: JEM/EUSO:

42 C.2.2 C.2.2 Topics Topicsinin(ultra-) (ultra-)high highenergy energy cosmic cosmicray rayphysics physics 63+42

43 Potential Sources There are several possible sources for the energy needed for acceleration of the cosmic ray particle: kinetic energy: translation (shock fronts in supernova remnants, GRB, AGN => Fermi acceleration) rotation (pulsars, black holes, neutron stars) gravitational energy: material falling onto accretion disks (AGN, binary systems...) Galactic cosmic rays are supposed to be accelerated in shock waves in supernova remnants. No direct proof has been found so far. (Evidence from diffuse γ-ray emission.) One assumes that the cosmic rays at the highest energies (> ~10 EeV) are of extragalactic origin. Their sources must be particularly violent objects to provide their enormous energies. AGN, GRBs, pulsar wind bubbles are being explored as possible sources. electromagnetic energy: magnetars... In a different approach, the socalled "TOPDOWN" models, ultra-high energy cosmic rays are the decay products of supermassive exotic particles

44 Acceleration mechanisms There are several possible acceleration mechanisms for charged particles, e.g.: stochastic acceleration (second order Fermi acceleration) e.g. scattering on magnetized clouds diffusive shock acceleration (first order Fermi acceleration) scattering on magnetic turbulences in shock waves shear acceleration Fermi-type acceleration on magnetic turbulences in gradual shear flows magnetic reconnection reconnection of magnetic field lines acceleration in rotating B-fields 63+44

45 Which acceleration mechanism for cosmic rays? Galactic Cosmic Rays: Total power required to accelerate cosmic rays in Galactic disc: W CR = E R D 2 10 J yr Average power output from type II supernovae: W SN J yr 1 => efficiency of a few percent for 1st order Fermi acceleration in SN shocks would be sufficient. Difficult to explain acceleration up to the high energy end of the Galactic CR spectrum of ev. Non-linear effects, leading to magnetic field amplification, might help. Extragalactic Cosmic Rays: The most discussed acceleration sites are GRB, radio-loud AGN and supper-bubbles. In the case of AGN for example, acceleration could take place in the plasma jets (1st order Fermi acceleration on shocks, shear acceleration, magnetic reconnection), in the large radio lobes (Fermi) or very close to the supermassive black hole (centrifugal acceleration). The above sources could probably provide the needed power, but acceleration to 1020 ev is still very difficult to explain

46 1st order Fermi acceleration (1) Fermi Acceleration at shock waves is an important mechanism to explain the energies of Galactic and maybe extra-galactic cosmic rays. Energy from a shock wave is transferred to single particles in repeated encounters. The energy of the particle increases at each encounter. We assume that an infinite plane shock wave is moving with velocity -u1 through the interstellar medium. Its velocity is non-relativistic. The shocked (downstream) plasma has a velocity V = -u1 + u2, with u2 < u1. An "encounter" happens when a particle crosses the shock front from upstream into the downstream region and is then scattered back again into the upstream region. A shock wave moving to the left through the interstellar medium with velocity -u1. The charged particle is scattered on irregularities in the turbulent magnetic field that is carried along with the downstream plasma. The particle gains energy in each encounter. A magnetic field alone cannot change the energy of a particle, as we know. However, here the magnetic field is moving relative to the upstream frame, which corresponds to an electric field : E = V B for V c 63+46

47 1st order Fermi acceleration (2) The acceleration process can be described as a "Lorentz boost" by changing the frame of reference through the encounter: The energy of the particle in the upstream region is E1 ~ pc, i.e. the particle is relativistic. Its energy in the rest frame of the downstream plasma is thus: p 1 E 2 = E 1 V E 2 = E 1 1 cos 2 =V/c In this frame, the energy of the particle does not change. It interacts only with magnetic fields at rest, which change its direction. Its energy does change with respect to the initial rest frame. Transforming back into the upstream frame: E 3 = E 2 1 cos 1 In the same way, the particle will get reflected again in the magnetic field lines of the upstream region. A shock wave moving to the left through the interstellar medium with velocity -u1. The fractional energy gain per encounter is: 2 E E 3 E 1 1 cos 2 cos 1 cos 2 cos 1 = = 1 2 E1 E

48 1st order Fermi acceleration energy gain For an isotropic flux, the angular distribution of particles hitting a plane is: cos sin d If we assume an isotropic upstream flux, the mean cosθ is given by: 3 / 2 cos 2 = /2 cos cos sin d = = 2 / 2 cos 2 d cos = 2[1/3 cos3 ] /2 = 2/3 In an analogous way, the mean cosθ for the reflected particle flux is: cos 1 = 2 /3 One obtains for the average fractional energy gain: 2 E 1 4/3 4/9 = 1 2 E1 1 If the shock wave is non-relativistic, i.e. β << 1 E 4 4V E1 3 3c 63+48

49 1st order Fermi acceleration some conclusions This is called the "First Order Fermi Acceleration", since it is linear in β. The fractional energy gain depends only on the difference of velocities V between the upstream and downstream plasma. How many encounters does a particle need to reach an energy E? energy gain per encounter: E = E0 n final energy after n encounters: E = E 0 1 n= ln E / E 0 ln 1 The finite lifetime of the accelerator puts thus an upper limit on the total energy gain. What is the energy distribution of a flux of particles after acceleration? P is the probability that a particle escapes from the accelerating region. The proportion of particles accelerated to energies > E is thus: n 1 P m N E 1 P = P m=n substituting n yields: N E P 1 E E0, = ln 1/ 1 P ln 1 The Fermi Acceleration process leads to a power law spectrum of particle energies

50 2nd order Fermi acceleration (1) Before working out acceleration at shock waves, Fermi developped an acceleration mechanism of cosmic rays using magnetic fields carried in clouds. This leads to a "second order fermi acceleration" (energy gain is second order in β ). Acceleration on shock waves (1st order Fermi acceleration) is a more efficient mechanism. Reflection of cosmic rays in magnetized clouds ("magnetic mirrors"), e.g. clouds of ionised interstellar gas in random motion. We suppose the cloud is infinitely massive and does not change its velocity in the interaction (M>>m). The center of momentum frame is thus the reference frame of the cloud. The energy of the particle (with momentum p, energy E) in this frame is: E ' = E V p cos θ The component of the particle momentum in direction of V is in the CM frame: VE p ' x = p ' cos ' = p cos 2 c Elastic collision => v, m head-on encounter following encounter E ' before = E ' after p'x p' x 2 Transforming back to the observer's frame: E ' ' = E ' Vp ' x = E 1 where: p= V, M 2 V v cos c 2 V / c 2 E v 2 c 63+50

51 2nd order Fermi acceleration (2) We expand this term to second order in V/c: E = E ' ' E E 2 V v cos 2 2 V /c c2 We then have to average over the angle θ. The probabilities of "head-on" and "following" collisions are proportional to the relative velocities between particle and cloud: v + V cosθ and v - V cosθ. For v ~ c => 1 + (V/c) cosθ with 0<θ<π. The first term becomes in the relativistic limit v ~ c: 1 cos [1 V / c cos ]d cos 1 1 [1 V / c cos ]d cos 2 V cos 2V = c c 1 The average energy gain per collision is thus: = 2 / 3 V / c 2 E 2 = 8/ 3 V / c E There is a net energy gain, since "head-on" collisions are more likely than "following" collisions, but this gain is only second order in V/c. A correct treatment of Fermi acceleration needs to take into account the stochastic diffusion of particles in momentum-space, which leads to a broadening of their energy distribution. 2nd order Fermi acceleration leads to power law spectra that are typically flatter ( Γ < 2) than for 1st order Fermi

52 Fermi acceleration and cosmic rays More quantitative calculations show that Fermi acceleration leads to a universal spectral index of ~2. The steeper observed CR spectrum (Γ~2.7) might be explained by energydependent escape probabilities. Energy loss mechanisms (e.g. synchrotron radiation) play an important role in gradual acceleration mechanisms and can greatly lower the net energy gain. Estimates of the acceleration on supernova shock fronts yield an upper energy limit of ~ 100 TeV (T. Gaisser). Acceleration on relativistic shock waves could lead to a higher energy limit. Amplification of the magnetic fields by turbulences and instabilities seems to improve the upper energy limit for SNR to several PeV. Shock waves powered by supernovae are assumed to provide only sufficient energies for Galactic cosmic rays roughly up to the "knee" region. Similar (maybe relativistic) shocks in extragalactic objects (AGN, radio galaxy lobes, GRB...) might reach significantly higher energies. Shear acceleration inside a gradual shear flow with a velocity gradient u = uz(x) ez might be a promising mechanism to explain high energy particle acceleration in plasma jets (AGN, GRB,...). Charged particles are scattered in the magnetic turbulences that are carried with the flow at different velocities. The process ressembles 2nd order Fermi acceleration, but has a different acceleration time scale

53 magnetic reconnection Magnetic reconnection is thought to be an important acceleration process in solar flares and in the Earth's magnetosphere or in the atmosphere of magnetars. Magnetic field lines are bundled into domains inside electrically conductive plasma. Reconnection of field lines in different domains allows magnetic fluxes to cancel out. The topology of the magnetic domains is changed to an energetically more favourable configuration. The energy difference is released as kinetic and thermal energy. A detailed description of magnetic reconnection theory is given in the framework of magneto-hydro dynamics (MHD)

54 acceleration in rotating B-fields Particle acceleration can also take place in electric fields, induced by rotating magnetic fields: E = r B/ c This is the basis e.g. of acceleration processes in the magnetosphere of pulsars or in plasma corotating with super-massive black holes. The basic mechanisms for acceleration at pulsars are: in the co-rotating plasma (charged particles!) in the pulsar magnetosphere, the generated electric field is shorted out. Acceleration can only take place in charge depletion regions ("gaps") at open magnetic field lines, where the electric field can develop. charged particles are accelerated in a "gap" by the electric field component parallel to the magnetic fieldlines. A.K.Harding, NASA,GSFC M.A Ruderman, P.G Sutherland, 1975, ApJ, 196;

55 The Hillas Diagram Only very few sites can be considered for Fermi-like acceleration of the most energetic cosmic rays. A potential site needs to have a long enough lifetime. The extension and the magnetic field of the site need to permit the particle to be confined long enough. The particle's gyro-radius needs to be smaller than the accelerating region: r G [kpc ] = E [ EeV ] L Z B [ G ] The Hillas diagram shows sites that fulfill these basic criteria. For a given energy E, the product of extension L and magnetic field B has to be larger than E/Z. All the potential sites are thus above the diagonal lines for a certain E and Z. The Hillas diagram does not give any information on the acceleration process

56 Top-Down Models Top-Down models present an alternative to the "bottom-up" acceleration of particles in astrophysical sources. Here, the ultra-high energy cosmic rays are suggested to be decay products of very heavy exotic particles (i.e. often unknown particles beyond the Standard Model). Candidates are: Topological Defects: defects in the early space-time topology after the Big Bang, which decay into exotic heavy particles and then into UHECRs. (predicted in Grand Unified Theories; e.g. magnetic monopoles, cosmic strings,...) Superheavy Dark Matter: very heavy exotic particles (~ 1013 GeV) that are gravitationally bound in the galactic halo or inside heavy objects and decay into UHECRs. Z-bursts: neutrinos interact with the cosmological neutrino background to generate Z bosons. Those decay into UHECRs. Top-Down models explain UHECRs through decays. Part of the decay products contain always pions, which decay further, producing photons and neutrinos. A considerable photon and neutrino flux are thus distinct signatures of Top-Down models, which distinguish them from the "bottom-up" models. By measuring upper limits on the photon or neutrino flux, one can test the Top-Down hypothesis

57 Some Scientific Results from Auger Photon Flux Limit Auger (latest results = black arrows) puts a stringent upper limit on the photon fraction at ultra-high energies. Previous measurements from Haverah Park, Yakutsk, Agasa and previous Auger results (A and FD) are also shown. These upper limits rule out many "Top-Down" models (Super Heavy Dark Matter, Topological Defects)

58 The Cosmic Ray Composition at High Energies (1) Evidence for a transition from heavy to light above ~ 1017 ev in the HiRes data. Transition from light to heavy around the knee (1-10 PeV), seen by KASCADE (and other experiments). -> Rigidity dependent cutoffs

59 The Cosmic Ray Composition at High Energies (2) There is a discrepancy between measurements by HiRes and Auger at the highest energies: - HiRes measures a light, protonic composition - Auger data indicate an increasingly heavy composition

60 Propagation Effects magnetic fields (galactic, extragalactic) => one expects an isotropic flux for energies below ~1 EeV for charged cosmic rays => only at the highest energies (> several 10 EeV) can charged cosmic rays point back to their sources adiabatic energy loss (red-shifting) e+e-- pair production with CMB 1: p + redshift + e+e2: 1 + GZK 3: Fe + e+e4: 3 + photo-spallation photo-spallation of nuclei (Ethreshold ~ atomic number) GZK effect with CMB (photo pion-production) => should lead to a cut-off in the spectrum around 60 EeV 63+60

61 galactic/extragalactic transition Extragalactic Galactic The current data on the CR spectrum at the highest energies and on the CR composition allow different interpretations of the "ankle": - if the composition at highest energies is protonic, the "ankle" would be due to pair production - if the composition at highest energies is heavy or mixed, the "ankle" would indicate the transition between Galactic and extragalactic cosmic rays 63+61

62 GZK effect (1) REMINDER: Resonances are excited hadronic states of a very short lifetime. In astrophysics the + (1232) - resonance, an excited proton state of mass 1232 MeV, plays an important role for the propagation of cosmic rays through the CMB. This resonance has a lifetime of the order of s. In the rest frame of ultra-high energy cosmic ray protons, the CMB photons can reach sufficiently high energies to excite the resonance: p C M B The resonance then decays quickly, following mostly two principal channels: p 0 p 2 n n p e e 63+62

63 GZK effect (2) This interaction makes the CMB effectively opaque for ultra-high energy protons with energies above ~ 6 x 1019 ev, which travel farther than ~50 Mpc ("GZK horizon"). The graph shows the effect on the energy of extragalactic particles as a function of the distance from the source. Details of the curve depend on the assumptions on the magnetic fields in the intergalactic medium. It can be seen that one expects a certain maximum energy at a distance that is far enough from the source. This would lead to an effective cut-off of the spectrum of ultra-high energy cosmic rays. This effect has been named after the theorists who found it (independently): Greisen, Zatsepin and Kuzmin. It has been discovered in the cosmic ray spectrum by the High-Resolution Fly's Eye Experiment

64 Some Scientific Results from Auger - Spectrum The energy spectrum measured by Auger using the SD data (and calibrated against the FD measurements) confirms the flux suppression detected by HiRes. However, the composition measurement with Auger seems to indicate that the cutoff in the spectrum might not come from protons interacting with the CMB. It might be a cutoff in a spectrum dominated by heavier nuclei

65 Anisotropy Searches left: AGASA sees excess towards G.C. at EeV energies -> claim refused by Auger right: indirect evidence from HESS for cosmic ray acceleration in GC and G. Plane left: SUGAR sees excess toward G.C. at EeV energies -> claim refused by Auger right: AGASA sees clusters of events at > 40 EeV. -> claim refused by HiRes 63+65

66 Some Scientific Results from Auger - Anisotropies (1) Auger does not see any anisotropy towards the Galactic Center (as claimed by SUGAR, AGASA) at EeV energies 63+66

67 Some Scientific Results from Auger - Anisotropies (2) Auger sees for the first time some evidence for a correlation between the arrival direction of UHECRs of the highest energies (circles) and a catalogue of nearby AGN (red stars). With more statistics in the Auger data, the correlation becomes less significant, but an anisotropy remains

68 Bibliography Cosmic rays and air showers "High energy astrophysics", Malcolm S. Longair, Cambridge University Press, 1994 "Cosmic Rays and Particle Physics", T. Gaisser, Cambridge University Press, 1990 "Introduction to Ultrahigh Energy Cosmic Ray Physics", P. Sokolsky, Westview, 2004 Fermi acceleration Longair "High Energy Radiation from Black Holes", C.D.Dermer & G. Menon, Princeton Series in Astrophysics,

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