Left panel: Swift/XRT X-ray power-law spectral index vs flux in the 2–10 keV band. Right panel: measured VHE power-law spectral index vs VHE Flux above 0.2 TeV. Red points represent VERITAS data and black points MAGIC data. The data are EBL corrected using Franceschini et al. (2008). The Swift/XRT spectrum of Mrk 501 is often curved, and can be described at keV energies with a spectral index that is typically between 1.8 and 2.1, while the VHE spectral index measured with MAGIC and VERITAS during typical non-flaring activity is about 2.5 (Abdo et al. 2011; Aleksić et al. 2015a). The typical spectral indices at X-ray and VHE are marked with a dashed line. For comparison purposes, the panels depict with solid lines the result of a fit with a constant to the X-ray and VHE spectral indices.