Concepto de tecnología

Figure 1.5: The so-called Hillas plot for the different possible astrophysical accelerators.

One of the striking news which was released by the Auger collaboration at the end of 2007 is the discovery of their recorded 27 highest energy events (with Energy≥5.7×1019_{eV) which} are correlated with nearby (z ≤ 0.017) AGNs. Twenty out of twenty-seven events are within 3.1◦ from one AGN. While for isotropic arrival distribution, one expects only 5 coincidence events. Five of the non-correlating events come from less than12◦ galactic latitude, which can be understood as larger deflections in the galactic magnetic field. It is very surprising that there is no event coming from the direction of the Virgo cluster, which includes a large number of powerful galaxies, such as M87. The correlation between UHECRs and AGNs is impressive, however, the statistics is still limited. We can also interpret the arrival direction distribution of UHECRs observed by Auger correlating with the matter density distribution by the nearby Universe, for example like [29] [30] [31].

1.4

Very High Energy Photons

By Wien’s law, it can be easily proved that the average energy of the photon from a thermal radiation source at temperature T is about 2.3×10−10_{(T/K) MeV. In order to have photons with}

maximum energy of 1 GeV, thermal bodies with a temperature of∼1013K are necessary. This temperature cannot be found in any steady astrophysical objects, but only in explosive events such as supernovae or the big bang. Thus, typical astrophysical HEγ-ray spectra are continuum and have a non-thermal rather than a thermal origin.

There are several major mechanisms which can produce high energy photons.

• Synchrotron radiation: A relativistic particle moving in the magnetic field could be de- scribed by its ”pitch angle” θ. This is the angle between the particle trajectory and the direction of the magnetic field. According to the Lorentz law, the particle will rotate on the plane perpendicular to the field with the gyration frequencyνg = eB/2πm. The radi-

ated energy comes from the velocity component perpendicular to the field. The resulting synchrotron photon spectrum peaks at frequency

νc = 3 2γ

2_· eB

2πmsinθ (1.17)

The synchrotron radiation spectrum of an electron was first given by Ginzburg and Sy- rovatski in 1964. A high energy cosmic electrons in a typical interstellar magnetic field will radiate synchrotron photons at

Ehν ∼0.05·( E

(T eV))

2_· B

(3µG)(eV) (1.18)

Thus, higher magnetic fields and energies shift the peak of the photon energy to a higher value.

Synchrotron radiation is limited by the following condition:

Ee mec2 B Bcr 1 (1.19) where theBcr is 4.4×1013G.

• Curvature radiation: When the magnetic field is particularly strong, curvature radiation may occur. Since the synchrotron radiation is so effective, a particle within an intense magnetic field will dissipate the component of its momentum perpendicular to the line of the magnetic field and follow with a uniform motion along the line. In many astrophysical conditions, such as close by regions of pulsars and black holes, the magnetic lines are not straight, but curved. In such case, the particle follows a curved line and it also radiates. This is the curvature radiation.

• Bremsstrahlung: When a relativistic charged particle, like an electron, is accelerated in the electrostatic field of a nucleus or other charged particles, it will emit bremsstrahlung radiation. The spectrum remains flat up to roughly the electron kinetic energy. It drops sharply towards zero, which means effectively all the kinetic energy of the electrons is transferred to bremsstrahlung photons.

1.4 Very High Energy Photons 11

• Inverse Compton Scattering: When a high energy photon impinges on a charged particle, it transfers its momentum to the charge particle. This is the so-called Compton effect. The inverse process, Inverse Compton effect (IC), also exists. Energetic particles transfer momentum to low energy photons and endow them with a larger momentum and energy. If we assume that the energetic electron energy isand the coming photon energy isω0. The angle-averaged total cross-section of IC depends only on the product of the energies of

andw0. If the incoming photon energy is much smaller than the rest mass energy of the electron (ie: κ0 1), Compton scattering behaves as Thomson scattering, and the cross section is

σIC ∼σT(1−2κ0) (1.20)

However, the cross section of the IC must be computed by QED, when the incoming photon energy is closer to the electron rest mass energy. If this is the case, the cross section is described by a well known formula called Klein-Nishina formula. The total cross section could be described as follows:

σIC = 3σT 8κ0 [(1− 2 κ0 − 2 κ2 0 )ln(1 + 2κ0) + 1 2 + 4 κ0 − 1 2(1 + 2κ0)2 ] (1.21)

• Photo-meson production: The interaction of a highly relativistic proton with a photon can produce pions if the energy of the photons in the frame of the proton exceeds the threshold energyEth=134.7MeV, which is theπ0mass. This is the same process which degrades the

energy of the extra-galactic ultra-high energy cosmic rays to less then1020_{eV originating} at distances larger than 100 Mpc, the so-called Greisen-Zatsepin-Kuz’min (GZK) cut-off. The producedπ0 _{will decay to 2γs immediately due to the short life time.}

• π0_{decay from proton-proton interactions: The dominatedπproducing channels in hadronic} interactions are proton proton interactions. The following process is there:

p+p → p+p+π0 +π± (1.22)

π0 has a very short life time which is about 10−15s. We know that the electron-positron pairs are also created in the pp interactions because of theµdecay from theπ. These pairs can finally again generate γ-rays through relativistic Bremsstrahlung, IC or synchrotron processes depending on the environments.

• Annihilation : Annihilation of pairs of particles and antiparticles could also produce γ- rays. The simple case is the electron positron annihilation. The energy of produced two photons are 0.511 MeV in the rest frame. Another possibility is the π0 _{decay from the} proton-antiproton annihilation. The basic reaction chain is: p+p→η πand thenπ0 →2γ. Only a limited fraction of the universe is visible in γ-ray. Far distance VHEγ-rays will not be seen because of their absorption by the extra-galactic photon background. This implies that

all the objects beyond our Galaxy are not visible in PeVγ-rays and about 10 Mpc for 100 EeV

γ-rays. In other words, by reducing the energy threshold of the detectors down to 100 GeV, we can approach cosmological distances up to redshift z ∼ 1. Though TeVγ-rays could not be a direct probe of cosmological epochs, studying of the HE photons is still highly motivated.