X-ray Radiation Mechanisms and Beaming Effect of Hot Spots and Knots in Active Galactic Nuclear Jets

TL;DR

This study models the X-ray emission mechanisms of hot spots and knots in AGN jets, revealing the importance of relativistic beaming and magnetic fields, and predicting detectable high-energy components.

Contribution

It provides a comprehensive analysis of X-ray emission mechanisms, emphasizing the role of relativistic motion and beaming effects in AGN jet features.

Findings

IC/CMB model explains most X-ray emissions when considering beaming.

Deviations from equipartition magnetic fields are linked to relativistic effects.

Predicted GeV-TeV emissions are detectable with current telescopes.

Abstract

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 literature and modeled with single-zone lepton models. It is found that the observed luminosities at 5 GHz (L_5GHz) and at 1 keV (L_1keV) are tightly correlated, and the two kinds of sources can be roughly separated with a division of L_1keV=L_5GHz. Our SED fits show that the mechanisms of the X-rays are diverse. Considering the sources at rest, 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_ssc^delta=1) being consistent with the equipartition magnetic field (B_eq^delta=1) in one order of magnitude, but an unreasonably low B_ssc^delta=1 is required to model the X-rays for all knots. The ratio R_B=B_eq^delta=1/B_ssc^delta=1 is greater than 1 and it is tightly anti-correlated with ratio R_L= L_1keV/L_5GHz 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, we show that the IC/CMB model well explains the X-ray emission for most sources. Both B_eq' and delta are tentatively correlated with R_L. Corrected by the beaming effect, the L'_5GHz-L'_1keV 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 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.