Left: ROSAT/PSPC significance map using the reflected background method and a correlation radius of R_spec = 0.6° (represented by a yellow circle). The dashed green circles are the 1RXS and 2RXS sources that have been masked in the ROSAT/PSPC count map (0.9–2.4 keV). Right: mean magnetic field as a function of the TeV photon spectral index and the column density. Thefull and dashed horizontal lines are the TeV spectral index and its associated statistical errors, as reported in the HGPS. The vertical dashed lines correspond to the column density range derived from Sano et al. (2015).