Upper: 0.45°× 0.2° three color image of the IR emission around σ Ori AB. Red is WISE-4 24 μm, green is WISE-3 12 μm and blue is Hα from the SHASSA mission, smoothed over 5 pixels in order to remove star residuals. Overplotted are the Spitzer/IRS LL2 regions. Lower: continuum subtracted and integrated intensity of the S[III] 18.7 μm line inside the IRS regions as a function of distance towards σ Ori AB (a positive distance corresponds to the region in front of the star). The dashed line marks the position of σ Ori AB. The IR arc is located at ~0.1–0.4 pc (the peak IR value being at ~0.1 pc), corresponding to 50–200 arcsec in front of σ Ori AB.