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tokens: which may not be particularly informative, as Fig. 9 demonstrates. To this end, we do not interpret the total flux from the nebula in terms of any particular spectral model; rather, we present a total integrated flux of the nebula emission to interpret the overall source strength. The resulting spectral flux obtained for the nebula from the summation of the 38 independent 0.26°× 0.26° spectral boxes is shown in Fig. 10 . For illustration, the HESS J1825–137 spectrum obtained from Fermi -LAT ( Grondin et al. 2011 ) is also shown, although we recall that the region from which this spectrum is obtained, based on a morphological symmetric Gaussian fit template to the Fermi data, does not fully correspond to the same spatial region. The 2011 analysis and the 3FGL (the third LAT Catalogue, 20 MeV to 300 GeV, Acero et al. 2015 ) both use a Gaussian with σ = 0.56° as the spatial template, whereas the 3FHL (the third LAT High-Energy Catalogue, 10 GeV to 2 TeV, Ajello et al. 2017 ) uses a Gaussian with σ = 0.75°, which is consistent with the Fermi Galactic Extended Source catalogue best-fit Gaussian with σ = 0.79 ± 0.04 ( Ackermann et al. 2017 ). Whilst these templates are only approximations to the source morphology, in Fig. 10 the IC peak may be inferred to occur at around 200 GeV. Fig. 10 Spectral energy distribution of HESS J1825–137 showing the total combined H.E.S.S. energy flux with statistical error bars, as determined with the method outlined in Sect. A . The Fermi -LAT spectral fit from Grondin et al. (2011) is also shown, together with spectra from the 3FGL and 3FHL catalogues ( Acero et al. 2015 ; Ajello et al. 2017 ), both of which were best fit by a log-parabola spectral model. A plausible IC emission model with multiple contributing radiation fields is shown, using the parameterisation of the Popescu et al. (2017) radiation model and the Naima modelling package ( Zabalza 2015 ). The emission is mostly dominated by the FIR field; the CMB becomes dominant at a few tens of TeV and below ~ 10 GeV. The total flux above 300 GeV was found to be (6.07 ± 0.13 stat ± 0.07 sys ) × 10 −11 cm −2 s −1 , corresponding to ~54% of the flux of the Crab nebula. Above 1 TeV, a flux of (1.12 ± 0.03 stat ± 0.25 sys ) × 10 −11 cm −2 s −1 was obtained, such that the proportion relative to the Crab nebula increases slightly to ~ 64% of the Crab nebula flux. The source remains strong at ~61% of the Crab nebula flux above 10 TeV. It is impressive that the γ -ray flux is so strong given that the distance to HESS J1825–137 is approximately twice that of the Crab nebula (at 2 kpc), despite (or perhaps because of) the significantly older age of the system. Interestingly, as part of the second HAWC catalogue, it was found that the confused source associated with the combined emission from the region, 2HWC J1825–134, is brighter than the Crab nebula at 7 TeV ( Abeysekara et al. 2017 ). This point is included in Fig. 10 and is compatible within the HAWC systematic