Mid-infrared Excess from the West Hot Spot of the Radio Galaxy Pictor A Unveiled by WISE

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 λ = 22 µm with a statistical significance of 4.8σ 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 × 10 14 Hz and 5.5 × 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.

Chandra Imaging of the Western Hotspot in the Radio Galaxy Pictor A: Image Deconvolution and Variability Analysis

“…The above equation, with the expected β ∼ 0.3 characterizing the downstream of the reverse shock (e.g., Meisenheimer et al 1989;Kino, & Takahara 2004), and so Γ ∼ δ ∼ 1, as well as B 100 µG 1 (see Meisenheimer et al 1989Meisenheimer et al , 1997Isobe et al 2017), gives rad 3 pc for keV energies of the observed synchrotron photons. Interestingly, such a small spatial scale would be in rough agreement with the observed variability timescale as well, since the corresponding light-crossing timescale rad /c 10 yr.…”

“…In the context of the former, one may reiterate on the presence of the extended radio/optical filament perpendicular to the jet axis and located several kpc upstream of the termination shock (Perley et al 1997;Thomson et al 1995;Saxton et al 2002), as well as of the high-brightness temperature compact (pc-scale) radio knots within/around the termination shock (Tingay et al 2008). Regarding the latter, one may note the mid-infrared excess over the integrated radio-to-optical continuum of the entire structure (Isobe et al 2017), as well as the intense power-law X-ray emission, for which both the observed flux and the best-fit slope challenge all simple "one-zone" emission scenarios, ascribing the production of the observed keV-energy photons to either synchrotron or inverse-Compton processes (e.g., Wilson et al 2001).…”

“…The other particularly bright and extended hotspot can be found at the edge of the Western lobe of the nearest FR II type (Fanaroff, & Riley 1974) radio galaxy, Pictor A, located at redshift z = 0.035 (Eracleous, & Halpern 2004). The Western hotspot, which is much brighter than the Eastern hotspot located at the counter-jet side, and which is located at the projected distance of 240 kpc from the nucleus, has been investigated in detail: at cm radio wavelengths using the Very Large Array (Perley et al 1997), at mid-infrared frequencies with the Spitzer Space Telescope (Werner et al 2012) and the Wide-field Infrared Survey Explorer (WISE; Isobe et al 2017), in near-infrared using groundbased data (Meisenheimer et al 1997), at optical wavelengths with the Faint Object Camera onboard the Hubble Space Telescope (Thomson et al 1995), and finally in X-rays with the Chandra X-ray Observatory (Hardcastle et al 2016, and references therein). The hotspot was also the subject of high-resolution radio imaging with the Very Long Baseline Array (VLBA; Tingay et al 2008).…”

Complex Structure of the Eastern Lobe of the Pictor A Radio Galaxy: Spectral Analysis and X-ray/Radio Correlations

“…The large-scale radio/X-ray jet in Pictor A originates in the galaxy nucleus, and extends up to hundreds of kiloparsecs beyond the host galaxy to the West (Perley et al 1997); the counter-jet is not prominent at radio frequencies, but can be spotted in deep X-ray maps by the Chandra X-ray Observatory . The hotspots located at both sides of the core at the lobes' edges, mark the termination points of the jet (to the West) and of the counter-jet (to the East); the bright Western hotspot is clearly detected and even, in some cases, resolved at radio, infrared, optical, and Xray frequencies (Roeser & Meisenheimer 1987;Thomson et al 1995;Perley et al 1997;Wilson et al 2001;Werner et al 2012;Isobe et al 2017;Thimmappa et al 2020). The radio lobes appear in X-rays as a low-surface brightness cocoon surrounding the large-scale jets (Grandi et al 2003;Migliori et al 2007;Hardcastle et al 2016).…”

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

“…In addition, a new spectral component was identified from the west hot spot of Pictor A in the MIR band (Isobe et al 2017). From a careful analysis of the MIR data obtained with the Wide-field Infrared Survey Explorer (WISE), a significant MIR excess was unveiled over the power-law (PL) like extrapolation of the synchrotron radio spectrum smoothly connecting to the optical one.…”