  Mid-infrared properties are reported of the west hot spot of the radio galaxy
Pictor A with the Wide-field Infrared Survey Explorer (WISE). The mid-infrared
counterpart to the hot spot, WISE J051926.26-454554.1, is listed in the AllWISE
source catalog. The source was detected in all the four WISE photometric bands.
A comparison between the WISE and radio images reinforces the physical
association of the \wise\ source to the hot spot. The WISE flux density of the
source was carefully evaluated. A close investigation of the multi-wavelength
synchrotron spectral energy distribution from the object reveals a mid-infrared
excess at the wavelength of $\lambda=22$ $\mu$m with a statistical significance
of $4.8 \sigma$ over the simple power-law extrapolation from the synchrotron
radio spectrum. The excess is reinforced by single and double cutoff power-law
modeling of the radio-to-optical spectral energy distribution. The synchrotron
cutoff frequency of the main and excess components was evaluated as $7.1 \times
10^{14}$ Hz and $5.5 \times 10^{13}$ Hz, respectively. From the cutoff
frequency, the magnetic field of the emission region was constrained as a
function of the region size. In order to interpret the excess component, an
electron population different from the main one dominating the observed radio
spectrum is necessary. The excess emission is proposed to originate in a sub
structure within the hot spot, in which the magnetic field is by a factor of a
few stronger than that in the minimum energy condition. The relation of the
mid-infrared excess to the X-ray emission is briefly discussed.
