A far-infrared counterpart to the west hot spot of the radio galaxy Pictor A is discovered with the Spectral and Photometric Imaging REceiver ( SPIRE ) onboard Herschel .
The color-corrected flux density of the source is measured as 70.0 \pm 9.9 mJy at the wavelength of 350 \micron .
A close investigation into its radio-to-optical spectrum indicates that the mid-infrared excess over the radio synchrotron component , detected with WISE and Spitzer , significantly contributes to the far-infrared band .
Thanks to the SPIRE data , it is revealed that the spectrum of the excess is described by a broken power-law model subjected to a high-energy cutoff .
By applying the radiative cooling break under continuous energy injection ( \Delta \alpha = 0.5 ) , the broken power-law model supports an idea that the excess originates in 10-pc scale substructures within the hot spot .
From the break frequency , \nu _ { b } = 1.6 _ { -1.0 } ^ { +3.0 } \times 10 ^ { 12 } Hz , the magnetic field was estimated as B \simeq 1 – 4 mG .
This is higher than the minimum-energy magnetic field of the substructures by a factor of 3 – 10 .
Even if the origin of the excess is larger than \sim 100 pc , the magnetic field stronger than the minimum-energy field is confirmed .
It is proposed that regions with a magnetic field locally boosted via plasma turbulence are observed as the substructures .
The derived energy index below the break , \alpha \sim 0.22 ( conservatively < 0.42 ) , is difficult to be attributed to the strong-shock acceleration ( \alpha = 0.5 ) .
Stochastic acceleration and magnetic reconnection are considered as a plausible alternative mechanism .