Mid-infrared excess from the west hot spot of the radio galaxy Pictor A unveiled by WISE
Naoki Isobe, Shoko Koyama, Motoki Kino, Takehiko Wada, Takao Nakagawa,, Hideo Matsuhara, Kotaro Niinuma, Makoto Tashiro

TL;DR
This study reports the detection of a mid-infrared excess in the west hot spot of Pictor A using WISE data, suggesting a complex electron population and magnetic field structure beyond simple synchrotron models.
Contribution
It presents the first mid-infrared detection of the hot spot and identifies a significant excess indicating a distinct electron component and magnetic field variation within the hot spot.
Findings
Mid-infrared excess detected at 22 μm with 4.8σ significance.
The excess is modeled as a separate electron population with a lower cutoff frequency.
Magnetic field in the excess region is stronger than the minimum energy condition.
Abstract
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 m with a statistical significance of 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…
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