Cosmic rays at the Galactic Centre

To conclude the section on the CR, we here briefly present three outstanding evidences of the presence of CR in the central region of our Galaxy; as discussed above, galactic CR are likely to have energies lower than 1015 _{eV, and therefore will be in the GeV-TeV}

regime. However, the propagation of such energetic particles cannot be traced directly to the acceleration source, since the interaction with the galactic magnetic fields change the original trajectory. To study the distribution of CR in the Galaxy it is possible to use the radiation that is produced by the interaction of these particles with both the surrounding ISM and the magnetic fields. CR interaction with a magnetic field results in the production of radio photons via synchrotron emission; one of the best and most evident example of synchrotron emission in the GC region can be seen in the non-thermal radio filaments and threads running perpendicular to the Galactic plane. In the left panel of Fig.1.13 we present a 90-cm view of the GC region as observed with the Very Large Array (LaRosa et al., 2000); it is surprising how the GC region is populated by these magnetic structures, the most prominent of which is the giant radio arc that is located at l∼0.2 deg and runs for at least 30 pc. The presence of such unique structures is a peculiar property of the GC region, and it contributes to make the surrounding diffuse emission from radio to the more energetic X-rays andγ-rays particularly difficult to disentangle in its original components. To infer the energetics of the CR involved in the radio emission at 90 cm (ν=330 MHz) we can write the formula for the characteristic frequency of the synchrotron emission produced by the interaction of an electron of energy E with a magnetic field of intensity B. This is (e.g. Rybicki & Lightman, 1979)

1.2 Cosmic rays 33

Figure 1.13: a: 90 cm view of the innermost GC region as measured with the VLA (LaRosa et al., 2000); in yellow are highlighted the brightest radio structures in the region, especially the giant radio arc. b: TeV map of the GC region. The colour map show the intensity of the TeV γ-rays once the brightest point sources have been subtracted. The contours represent the distribution of the CS J=2-1 molecular emission, smoothed in order to match the angular resolution of the HESS instrument (Aharonian et al., 2006). c: distribution of the TeV emission in galactic longitude. The red line traces the density of the molecular gas (CS emission), the black points show the TeV emission after point source subtraction, with the best fit model shown by the dashed green curve (Aharonian et al., 2006). To notice the mismatch between the black-green curve and the red one at longitudes higher than ∼1.2 deg. νc = 3 e B γ2 _sinα 16π m ≈1.6 GHz B_⊥ 10−4 _Gauss E GeV 2 , (1.39) where e is the electron charge, B_⊥ is the magnetic field perpendicular to the electron motion, γ is the electron Lorenz factor, α is the pitch angle and m is the electron mass. Magnetic fields in the GC non-thermal radio filaments have been measured to be in the order of a few mGauss. Therefore, the electrons to be associated with this radio emission are those with∼GeV energies; for example, assuming B=0.1 mG, the energy of the electron emitting synchrotron emission at 90 cm is ∼0.9 GeV.

More generally, the γ-ray satellites EGRET and FERMI have discovered the presence of diffuse emission in the inner region of the Galaxy, due to the interaction of CR with the ISM and the photon field there (e.g. Strong et al., 2007). The main mechanisms which produceγ-ray emission from CR are: inverse Compton from the interaction of energetic CR

with a low energy photon field, non-thermal bremsstrahlung due to the Coulomb collisions of e+-e− with the ionised nuclei and pion decay from interaction of CR hadrons with the ISM (Strong et al., 2007).

One of the most outstanding results found in the last decade is the discovery of diffuse TeV emission correlating with the distribution of the most massive complexes in the inner CMZ (Aharonian et al., 2006); these authors, together with mapping the TeV diffuse emission with an unprecedented angular resolution (better than 0.1 deg) using the H.E.S.S. atmospheric Cherenkov telescope, found that the inner ∼200 pc of the Galaxy is filled with an extra component of recently accelerated hadronic cosmic rays. In the H.E.S.S. energy range, the dominant component of diffuse γ-ray emission is the pion decay, an electromagnetic process with a lifetime of 8.4×10−17_{(basically immediate) whose diagram}

can be written as

pCR +pISM → π0 → γ+γ (1.40)

The probability of decay in 2 γ-ray photons is ∼0.988. The π0 mass, the minimum energy that the decay products can have, is about 135 MeV/c2_{; as a result, 2 γ-rays are}

produced in this electromagnetic process. The right panel of Fig.1.13 shows the results of the TeV study of the inner CMZ. Panel b shows that the TeV diffuse emission is ex- tended for about 2 degrees along the Galactic plane, and well correlates with the contours of the CS molecular emission, which represents the location of the most massive MCs in the region. This emission is not only extended in longitude, but also in latitude, with a characteristic root mean squared width of 0.2 deg, corresponding to about 30 pc, remark- ably similar to the height distribution of the molecular target material (Aharonian et al., 2006). The correlation between TeV photons and target material is a strong indication that this emission is due to the interaction of CRs with the MCs. The spectrum of the TeV emission has been measured to be harder than expected, resulting in a CR spectrum with a spectral indexα=2.3, significantly lower than the one measured in the solar neigh- bourhood (α=2.7 see above). This result is readily understood considering that in the GC region the propagation effects are likely to be less pronounced than in the whole Galaxy, and therefore the hardening of the spectrum can be thought to be due to the vicinity of the CR source and accelerators; moreover, the flux of the TeV γ-rays has been measured to be higher than expected from models of ISM and γ-ray distribution (Aharonian et al., 2006). The higher than expected TeV flux, together with the harder than expected CR spectrum, indicates that there is a CR component other than the one filling the Galaxy. In the panel c of Fig.1.13 we present the correlation study between theγ-ray flux and the CS molecular emission; it is noticeable that, whereas the correlation is very good within 1 deg in longitude, at about 1.2-1.3 deg this correlation breaks, indicating that the additional CR component has not yet propagated outwards. The energy required to sustain this addi- tional bunch of CR has been estimated to be about 1050 erg, therefore being approximately the 10% of the energy released by a SN explosion (Aharonian et al., 2006). If we represent the diffusion of protons through TeV energies with the diffusion coefficient