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تسجيل مستخدم جديدHerschel SPIRE discovery of far-infrared excess synchrotron emission from the west hot spot of the radio galaxy Pictor A
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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 $mu$m. 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, $ u_{rm b} = 1.6_{-1.0}^{+3.0} times 10^{12}$ Hz, the magnetic field was estimated as $Bsimeq1$-$4$ 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.
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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 catalo
g. 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.
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