Fermi/LAT observations of lobe-dominant radio galaxy 3C 207 and possible radiation region of γ-rays

Abstract

3C 207 is a lobe-dominant radio galaxy with one sided jet and the bright knots in kpc-Mpc scale were resolved in the radio, optical and X-ray bands. It was confirmed as a γ-ray emitter with Fermi/LAT, but it is uncertain whether the γ-ray emission region is the core or knots due to the low spatial resolution of Fermi/LAT. We present an analysis of its Fermi/LAT data in the past 9 years. Different from the radio and optical emission from the core, it is found that the γ-ray emission is steady without detection of flux variation over 2σ confidence level. This likely implies that the γ-ray emission is from its knots. We collect the radio, optical, and X-ray data of knot-A, the closest knot from the core at 1. ′′ 4, and compile its spectral energy distribution (SED). Although the single-zone synchrotron+SSC+IC/CMB model by assuming knot-A at rest can reproduce the SED in the radio-optical-X-ray band, the predicted γ-ray flux is lower than the LAT observations and the derived magnetic field strength deviates the equipartition condition with 3 orders of magnitude. Assuming that knot-A is relativistically moving, its SED from radio to γ-ray bands would be well represented with the single-zone synchrotron+SSC+IC/CMB model under the equipartition condition.These results likely suggest that the γ-ray emission may be from knot-A via the IC/CMB process and the knot should have relativistical motion. The jet power derived from our model parameters is also roughly consistent with the kinetic power estimated with the radio data.

Leptonic or Hadronic Emission: The X-Ray Radiation Mechanism of Large-scale Jet Knots in 3C 273

“…This is also the reason to rule out the IC/CMB model for large-scale jet of 3C 273 (Meyer et al 2015). The SED of knot-B1 apparently can be represented in this scenario, but the derived B value is 1.83 µG in case of δ = 1, which is much lower than the derived equipartition magnetic field strength of B eq = 10.5 mG. Zhang et al (2018) suggested that using the SSC process to explain the X-ray emission of large-scale jet substructures would result in an extremely high jet power. We estimate the powers of the non-thermal electrons using P e = πR 2 Γ 2 cU e (see also Zargaryan et al 2017 for the large-scale jet knots), and find that P e ranges from 1.9 × 10 47 erg s −1 to 2.3 × 10 49 erg s −1 in case of δ = 1 for the knots (as shown in Table 1).…”

“…In case of the knots do not have the relativistic motions, i.e., δ = Γ = 1, the IC component would be dominated by the SSC process since the energy density of the synchrotron radiation photon field (U syn ) is higher than U ′ CMB . A magnetic field strength (B) lower than the equipartition value (B eq ) is also needed (e.g., Kataoka & Stawarz 2005;Harris & Krawczynski 2006;Zhang et al 2010Zhang et al , 2018, and thus we do not take the equipartition condition into account in this scenario. The free parameters of the SED modeling are B, A 1 , p 1 , p 2 , E b .…”

“…The consistency between the X-ray spectrum and the extrapolation of the radio-optical synchrotron emission indicates the same synchrotron radiation origin of X-ray emission (Sambruna et al 2007;Zhang et al 2010Zhang et al , 2018b. But for most substructures, the hard spectra in the X-ray band require a new radiation component different from the low energy band, and then the inverse Compton (IC) scattering process is suggested to explain the X-ray emission (e.g., Kataoka & Stawarz 2005;Zhang et al 2010Zhang et al , 2018b, i.e., the synchrotron-self-Compton (SSC, Stawarz et al 2007) model and the IC scattering of the cosmic microwave background (IC/CMB, Georganopoulos & Kazanas 2003;Abdo et al 2010;McKeough et al 2016;Wu et al 2017;Zhang et al 2018a;Guo et al 2018) model. The X-ray emission may be produced by the synchrotron radiation of the second electron population different from the radio-optical emission (e.g., Zhang et al 2009;Zargaryan et al 2017;Sun et al 2018).…”

“…Comparing with the variability of the core radiation, the emission of substructures in large-scale jets almost does not show any variation, which is also used to estimate the origin of the γ-ray emission (e.g.,Zhang et al 2018a;Guo et al 2018;Meyer et al 2019). …”