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A far-infrared counterpart to the west hot spot of the radio galaxy Pictor Ais 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 $\mu$m. A close investigation intoits radio-to-optical spectrum indicates that the mid-infrared excess over theradio synchrotron component, detected with WISE and Spitzer, significantlycontributes to the far-infrared band. Thanks to the SPIRE data, it is revealedthat the spectrum of the excess is described by a broken power-law modelsubjected to a high-energy cutoff. By applying the radiative cooling breakunder continuous energy injection ($\Delta \alpha = 0.5$), the broken power-lawmodel supports an idea that the excess originates in 10-pc scale substructureswithin the hot spot. From the break frequency, $\nu_{\rm b} = 1.6_{-1.0}^{+3.0}\times 10^{12}$ Hz, the magnetic field was estimated as $B\simeq1$-$4$ mG. Thisis higher than the minimum-energy magnetic field of the substructures by afactor 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. Itis proposed that regions with a magnetic field locally boosted via plasmaturbulence are observed as the substructures. The derived energy index belowthe break, $\alpha \sim 0.22$ (conservatively $<0.42$), is difficult to beattributed to the strong-shock acceleration ($\alpha = 0.5$). Stochasticacceleration and magnetic reconnection are considered as a plausiblealternative mechanism.

Herschel SPIRE discovery of far-infrared excess synchrotron emission from the west hot spot of the radio galaxy Pictor A