Speaker
Mr
Massimo Cavadini
(Insubria University)
Description
We present a two-parameter model of the extragalactic $\gamma$-ray background (EGB)
in the
0.1-100 GeV range as measured by the Large Area Telescope (LAT) onboard the {\it
Fermi} satellite.
The EGB can be fully explained as the sum of three distinct components, namely blazars,
non-beamed AGNs (Seyfert galaxies and QSOs), and cosmic rays from star-forming galaxies.
The contribution to the background from beamed sources is obtained by fitting
the {\it Fermi}-LAT blazar differential number counts assuming that the $\gamma$-ray
luminosity function
is directly proportional to the radio luminosity function of FRI and FRII galaxies.
The high energy emission from non-beamed AGNs is instead determined
by popular synthesis models of the observed X-ray background. Finally, the EGB is fit
by adding a third component
arising from pion decay in cosmic rays, assuming that such component is closely
linked to the cosmic star formation history.
We find that blazars dominate at energies $\gsim$ 10 GeV, for $E\lsim 0.2$ GeV the
main contribution is from
non-beamed AGNs, while cosmic rays are required in between.
Because of absorption due to interaction of $\gamma$-rays with
the extra-galactic background light, our model falls short at the highest energies
probed by LAT, ($\gsim 70$ GeV), leaving
room to a possible contribution from dark matter particle annihilation. As an example,
a particle of mass $\simeq 0.5$ TeV and cross section $\langle \sigma v \rangle
\simeq 5 \times 10^{-26}$ cm$^3$ s$^{-1}$
can accomodate the data.
Primary author
Mr
Massimo Cavadini
(Insubria University)
Co-authors
Dr
Francesco Haardt
(Insubria University)
Dr
Ruben Salvaterra
(Insubria University)
