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Beyond the EGRET model

Several questions about the correct modeling of the interstellar medium gamma-ray emissivity are still open. The measured spectrum appear to have a 40% of excess with respect to the theoretical prediction for energy above 1 GeV [Hunter et al., 1997]. Many hypothesis have been proposed to solve this problem. Most theories explain this feature assuming that the galactic cosmic ray spectrum is on average different from that obtained from local measurements.
Pohl & Esposito showed that the energy losses suffered by high energy electrons imply that these electrons are confined in regions close to their sources [Pohl & Esposito, 1998]. This would imply that the spectrum is highly inhomogeneous (both in space and in time) and therefore the local spectrum cannot be assumed as representative of the whole Galaxy. Moreover, according to this model, electrons would have a spectrum on average harder than the local spectrum. This would increase the contribution due to inverse Compton and would explain the event excess for gamma-ray energy above 1 GeV.
Other models, instead, link the excess of diffuse radiation to the hadronic component of cosmic rays. Mori has calculated the gamma emission due to proton-proton interactions using numeric simulation based on the Monte Carlo method [Mori, 1997]. This work confirmed the results obtained when the production of secondary pions is evaluated in a simpler way [Dermer, 1986a]. He concluded that a harder proton spectrum (with spectral index 2.4 - 2.5) would explain, at least in part, the excess measured at high energies. A model able to reproduce the whole spectrum of diffuse emission of the inner Galaxy in the full range between 10 keV and 100 TeV is presented in [Aharonian & Atoyan, 2000]. In this model the excess at GeV is reproduced by summing to the contribution due to electron inverse Compton emission, the $ \pi ^0$ decay emission produced by protons with a quite flat (spectral index 2.1) energy distribution and with a break around 10 GeV. Busching et al.,2001 show that a spectrum of Galactic protons on average harder than the local spectrum can be explained by assuming a dispersion of the spectrum index of injection for nucleons accelerated by supernova remnants. In [Strong et al., 2000] a study of several possible scenarios, in which both the electrons and protons spectra are modified, is presented also taking in consideration the local measure of atomic abundance in cosmic rays, of secondaries (as antiprotons and positrons) and of synchrotron emission. From this study one can deduce that changing only the proton spectrum would produce a rate of antiprotons and positrons not compatible with the measurements. On the contrary, taking a proton spectrum consistent with the limits imposed by secondaries measurements and a harder spectrum for electrons (injection index 1.9), it is possible to reproduce the GeV excess and to fit the radio observations of synchrotron radiation. [Strong et al., 2004a] proposed a model that assumes an electron and a proton spectrum slightly harder than the local ones (electrons injection index 1.5 below rigidity 20 GV and 2.42 above and the same indexes for protons but with the break at rigidity 10 GV). This model can fit the observations in the radio and gamma bands as well as the primary and secondary cosmic rays abundances. Other works propose that the excess is caused by non resolved sources with a harder spectrum than that of diffuse emission, as supernova remnants [Berezhko & Völk, 2000] or pulsars [Pohl et al., 1997]. In [De Boer et al., 2004] the excess is interpreted as WIMP annihilation emission coming from a dark matter halo and disk.
Finally we cannot exclude that the theoretical knowledge about gamma-ray emission due to hadronic component of cosmic rays needs to be improved.
Beside the GeV excess other problems are still waiting for a solution. For example, the gradient of diffuse emission in galactic longitude is significantly different from the gradient derived from supernova remnants, which are assumed to be cosmic-ray sources. This discrepancy could be linked to propagation effects of cosmic rays. In particular, it can be supposed either that the thickness of the Galactic halo in which the cosmic rays are confined is a function of the distance from the center of Galaxy [Breitschwerdt et al., 2002] or that the propagation is deeply affected by turbulence of interstellar medium [Erlykin & Wolfendale, 2002]. Another possibility is that the factor X (which is used to derive the $ H_2$ density from CO emission intensity) has a strong gradient along the distance from the Galactic center (about an order of magnitude from 0 to 10 kpc) [Strong et al., 2004b].
Due to the poor angular resolution of previous missions it is not possible to know how the observed diffuse emission is contaminated by the presence of unresolved sources. Many Galactic sources of gamma rays have been proposed as possible contributors to the diffuse emission, as, pulsars [Pohl et al., 1997], supernova remnants [Berezhko & Völk, 2000] or molecular clouds [Aharonian, 1991].
next up previous contents
Next: The AGILE diffuse gamma-ray Up: Modeling the Gamma-Ray Emission Previous: The EGRET observations and   Contents
Andrea Giuliani 2005-01-21