摘要：Context.NGC 253 is one of only two starburst galaxies found to emitγ-rays from hundreds of MeV to multi-TeV energies. Accurate measurements of the very-high-energy (VHE;E> 100 GeV) and high-energy (HE;E> 60 MeV) spectra are crucial to study the underlying particle accelerators, probe the dominant emission mechanism(s) and to study cosmic-ray interaction and transport.Aims.The measurement of the VHEγ-ray emission of NGC 253 published in 2012 by H.E.S.S. was limited by large systematic uncertainties. Here, the most up to date measurement of theγ-ray spectrum of NGC 253 is investigated in both HE and VHEγ-rays. Assuming a hadronic origin of theγ-ray emission, the measurement uncertainties are propagated into the interpretation of the accelerated particle population.Methods.The data of H.E.S.S. observations are reanalysed using an updated calibration and analysis chain. The improvedFermi–LAT analysis employs more than 8 yr of data processed using pass 8. The cosmic-ray particle population is evaluated from the combined HE–VHEγ-ray spectrum using NAIMA in the optically thin case.Results.The VHEγ-ray energy spectrum is best fit by a power-law distribution with a flux normalisation of (1.34 ± 0.14stat± 0.27sys) × 10−13cm−2s−1TeV1at 1 TeV – about 40% above, but compatible with the value obtained inAbramowski et al. (2012). The spectral index Γ = 2.39 ± 0.14stat± 0.25sysis slightly softer than but consistent with the previous measurement within systematic errors. In theFermienergy range an integral flux ofF(E> 60 MeV) = (1.56 ± 0.28stat± 0.15sys) × 10−8cm−2s−1is obtained. At energies above ∼3 GeV the HE spectrum is consistent with a power-law ranging into the VHE part of the spectrum measured by H.E.S.S. with an overall spectral index Γ = 2.22 ± 0.06stat.Conclusions.Two scenarios for the starburst nucleus are tested, in which the gas in the starburst nucleus acts as either a thin or a thick target for hadronic cosmic rays accelerated by the individual sources in the nucleus. In these two models, the level to which NGC 253 acts as a calorimeter is estimated to a range offcal= 0.1 to 1 while accounting for the measurement uncertainties. The presented spectrum is likely to remain the most accurate measurements until the Cherenkov Telescope Array (CTA) has collected a substantial set of data towards NGC 253.
