Spectral analysis and interpretation of the \gamma-ray emission from the
  Starburst galaxy NGC 253
  Very-high-energy (VHE; E >100 GeV) and high-energy (HE; 100 MeV < E < 100
GeV) data from \gamma-ray observations performed with the H.E.S.S. telescope
array and the Fermi-LAT instrument, respectively, are analysed in order to
investigate the non-thermal processes in the starburst galaxy NGC 253. The VHE
\gamma-ray data can be described by a power law in energy with differential
photon index \Gamma=2.14 \pm 0.18_stat \pm 0.30_sys and differential flux
normalisation at 1 TeV of F_0 = (9.6 \pm 1.5_stat (+5.7,-2.9)_sys) x 10^{-14}
TeV^{-1} cm^{-2} s^{-1}. A power-law fit to the differential HE \gamma-ray
spectrum reveals a photon index of \Gamma=2.24 \pm 0.14_stat \pm 0.03_sys and
an integral flux between 200 MeV and 200 GeV of F(0.2-200 GeV) = (4.9 \pm
1.0_stat \pm 0.3_sys) x 10^{-9} cm^{-2} s^{-1}. No evidence for a spectral
break or turnover is found over the dynamic range of both the LAT instrument
and the H.E.S.S. experiment: a combined fit of a power law to the HE and VHE
\gamma-ray data results in a differential photon index \Gamma=2.34 \pm 0.03
with a p-value of 30%. The \gamma-ray observations indicate that at least about
20% of the energy of the cosmic rays capable of producing hadronic interactions
is channeled into pion production. The smooth alignment between the spectra in
the HE and VHE \gamma-ray domain suggests that the same transport processes
dominate in the entire energy range. Advection is most likely responsible for
charged particle removal from the starburst nucleus from GeV to multiple TeV
energies. In a hadronic scenario for the \gamma-ray production, the single
overall power-law spectrum observed would therefore correspond to the mean
energy spectrum produced by the ensemble of cosmic-ray sources in the starburst
region.
