Appendix is only available in electronic form at http://www.edpsciences.org

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We emphasize that the use of a homogeneous model is an assumption, but is justified in the compactness of the innermost starburst region. We are basically assuming that there is a homogeneous distribution of supernovae in the central hundreds of pc, which is supported observationally (Ulvestad & Antonucci 1997).

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... 200 GeV

To further ease the comparison, we here note some typos in the Paglione et al. (1996) paper: The factor b(E) should be elevated to minus one in their Eq. (4), as well as the term $\tau_{\rm c}$ in their Eq. (3). The y-axis of their Fig. 1 is not the emissivity, but the emissivity divided by the density; units need to be modified accordingly , see e.g. Abraham et al. (1966). $E_{\rm p}$ in their Eq. (7) and x-axis of Figs. 2 and 3 is the kinetic energy, but the generic E in Eq. (1) is the total energy. The y-axis of Fig. 2 is in units of cm^-3 GeV^-1.

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... $M_\odot$

In Romero & Torres (2003), higher mass loss rates up to 10^-5 $M_\odot$ , i.e., grammages between 50 and 150 g cm^-2, were allowed. Although found in perhaps one or two Galactic early O stars, these values are uncommon. Since the size of the base of the wind for each star, the grammage, and the ambient enhancement of cosmic rays were independently allowed to take values within their assumed ranges in the Monte Carlo simulation of Romero & Torres (2003), the stars with the most favorable parameters would dominate the sum, overestimating the relative importance of their fluxes.

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... 1000 GeV)

HESS site latitude provides that NGC 253 can be observed very close to the zenith (the minimum zenith angle for NGC 253 from HESS site is 2 degrees). As a consequence, HESS observations of NGC 253 can be performed with the minimum energy threshold of the experiment. The MAGIC Telescope, although at a northern hemisphere site, is also able to observe NGC 253 at a larger zenith angle, about 53 degrees.

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... 3 TeV

This figure enlarges the comparison of the Blattnig et al. (2000) inclusive cross section with experimental data (see their Fig. 4), where only three low-energy data points from Whitmore (1974) were considered.

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