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Parsec-scale radio lobes as the site of X-ray production in AGN with newly born radio jets.

Presentation #102.26 in the session AGN Posters.

Published onMay 03, 2024
Parsec-scale radio lobes as the site of X-ray production in AGN with newly born radio jets.

Chandra, XMM-Newton, and NuSTAR data collected by our team were instrumental in establishing that X-ray emission of radio galaxies with compact (< 1 kpc) symmetric radio structures is ubiquitous. We laid the groundwork for uncovering the origins of this X-ray emission by leveraging the broad-band radio-to-γ-ray SED modeling. Here, we consider a sample of four ultra-compact radio sources (≈ 7-25 parsecs), classified as Compact Symmetric Objects (CSO) 2s by Readhead et al. (2023). PKS 1718-649 is γ-ray loud and X-ray unobscured, while the three remaining sources are γ-ray quiet and X-ray obscured with NH > 1023 cm-2. The SEDs of all four CSOs can be described well by the expanding radio lobe model, in which the high-energy emission is due to Inverse Compton scattering of soft photon fields (ultraviolet from an accretion disk and infrared from the circumnuclear dust) off ultra-relativistic electrons filling the expanding radio lobes. Furthermore, we propose that the 6.4 keV Fe Kα emission lines detected in our X-ray obscured CSOs may be produced via a reflection of the lobes’ continuum from the surrounding cold dust. This is supported by the parsec-scale radio sizes of these CSOs, which are smaller than the median diameters of AGN tori measured with ALMA (≈ 40 parsec, e.g. Combes et al. 2019; Garcia-Burillo et al. 2021). Finally, we find a correlation between the X-ray NH and the high-energy index of the electron energy distribution injected into the lobes, which suggests that the efficiency of particle acceleration decreases with the density of an ambient medium. As pointed out by Readhead et al., most CSO 2s do not evolve into large scale jetted-AGN. Our SED modeling constrains the kinetic jet power of the considered CSO 2s to 1042 - 1044 erg s-1, indicating that these jets are rather weak; while numerical simulations of Mukherjee et al. (2016, 2017) show that such weak jets may struggle to expand beyond their host galaxies. This finding offers a possible explanation for the observed tendency of most CSO 2s to spend their whole life-cycle being confined by their host galaxies; an alternative to the transient fueling via tidal disruption of giant stars proposed by Readhead et al. Our results have implications for the physics of the early AGN and host galaxy feedback through the interactions between the circumnuclear dust and expanding parsec scale radio lobes.

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