Effect of the spectral model and energy bin. Left panel: SEDs of the Geminga pulsar halo emission for different values of the spectral parameters γ and E_c of the electron’s source term Q(E)∝E^−γexp( − E/E_c). When not specified, curves are obtained for the benchmark value of γ = 1.9 and E_c = 10³ TeV. Angular integration as in Fig. 1. Right panel: Surface brightness of the Geminga’s pulsar halo obtained by integrating in different energy intervals, corresponding to the XMM-Newton (0.5−8 keV) data analysis, and to the NuSTAR ones. All curves are rescaled to their value at θ = 0.