X-ray radiation mechanisms and beaming effect of hot spots and knots in active galactic nuclear jets

The observed radio-optical-X-ray spectral energy distributions (SEDs) of 22 hot spots and 45 knots in the jets of 35 active galactic nuclei are complied from the literature and modeled with single-zone lepton models. It is found that the observed luminosities at 5GHz (L _{5 GHz}) and at 1keV (L _{1 keV}) are tightly correlated, and the two kinds of sources can be roughly separated with a division of L _{1 keV} = L _{5 GHz}. Our SED fits show that the mechanisms of the X-rays are diverse. While the X-ray emission of a small fraction of the sources is a simple extrapolation of the synchrotron radiation for the radio-to-optical emission, an inverse Compton (IC) scattering component is necessary to model the X-rays for most of the sources. Considering the sources at rest (the Doppler factor δ = 1), the synchrotron-self-Compton (SSC) scattering would dominate the IC process. This model can interpret the X-rays of some hot spots with a magnetic field strength (B ^{δ = 1} _ssc) being consistent with the equipartition magnetic field (B ^{δ = 1} _eq) in 1 order of magnitude, but an unreasonably low B ^{δ = 1} _ssc is required to model the X-rays for all knots. Measuring the deviation between B ^{δ = 1} _ssc and B ^{δ = 1} _eq with ratio R_B ≡ B ^{δ = 1} _eq/B ^{δ = 1} _ssc, we find that R _B is greater than 1 and it is tightly anti-correlated with ratio R_L ≡ L _{1 keV}/L _{5 GHz} for both the knots and the hot spots. We propose that the deviation may be due to the neglect of the relativistic bulk motion for these sources. Considering this effect, the IC/cosmic microwave background (CMB) component would dominate the IC process. We show that the IC/CMB model well explains the X-ray emission for most sources under the equipartition condition. Although the derived beaming factor (δ) and co-moving equipartition magnetic field (B′_eq) of some hot spots are comparable to the knots, the δ values of the hot spots tend to be smaller and their B′_eq values tend to be larger than that of the knots, favoring the idea that the hot spots are jet termination and knots are a part of a well-collimated jet. Both and δ are tentatively correlated with R_L . Corrected by the beaming effect, the L′ _{5 GHz}-L′_{1 keV} relations for the two kinds of sources are even tighter than the observed ones. These facts suggest that, under the equipartition condition, the observational differences of the X-rays from the knots and hot spots may be mainly due to the differences on the Doppler boosting effect and the co-moving magnetic field of the two kinds of sources. Our IC scattering models predict a prominent GeV-TeV component in the SEDs for some sources, which are detectable with H.E.S.S. and Fermi/LAT.