Herschel SPIRE Discovery of Far-infrared Excess Synchrotron Emission from the West Hot Spot of the Radio Galaxy Pictor A
Abstract
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) on board Herschel. The color-corrected flux density of the source is measured as 70.0 ± 9.9 mJy at the wavelength of 350 μm. A close investigation into its radio-to-optical spectrum indicates that the mid-infrared excess over the radio synchrotron component, detected with Wide-field Infrared Survey Explorer 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 (Δα = 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, ${\nu }_{{\rm{b}}}={1.6}_{-1.0}^{+3.0}\times {10}^{12}$ Hz, the magnetic field was estimated as B ≃ 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 ∼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, α ∼ 0.22 (conservatively <0.42), is difficult to be attributed to the strong-shock acceleration (α = 0.5). Stochastic acceleration and magnetic reconnection are considered as a plausible alternative mechanism.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 2020
- DOI:
- 10.3847/1538-4357/ab9d1c
- arXiv:
- arXiv:2006.10737
- Bibcode:
- 2020ApJ...899...17I
- Keywords:
-
- Galaxy jets;
- Non-thermal radiation sources;
- Radio hot spots;
- High energy astrophysics;
- Active galaxies;
- Far infrared astronomy;
- Fanaroff-Riley radio galaxies;
- 601;
- 1119;
- 1344;
- 739;
- 17;
- 529;
- 526;
- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 13 pages, 3 figures, 4 tables, accepted for ApJ