On the Origin of the X-Ray Emission in Heavily Obscured Compact Radio Sources

Abstract

X-ray continuum emission of active galactic nuclei (AGNs) may be reflected by circumnuclear dusty tori, producing prominent fluorescence iron lines at X-ray frequencies. Here, we discuss the broadband emission of three radio-loud AGNs belonging to the class of compact symmetric objects (CSOs), with detected narrow Fe K α lines. CSOs have newly born radio jets, forming compact radio lobes with projected linear sizes of the order of a few to hundreds of parsecs. We model the radio-to- γ -ray spectra of compact lobes in J1407+2827, J1511+0518, and J2022+6137, which are among the nearest and the youngest CSOs known to date, and are characterized by an intrinsic X-ray absorbing column density of N _H > 10 ^23 cm ^−2 . In addition to the archival data, we analyze the newly acquired Chandra X-ray Observatory and Submillimeter Array (SMA) observations, and also refine the γ -ray upper limits from Fermi Large Area Telescope monitoring. The new Chandra data exclude the presence of the extended X-ray emission components on scales larger than 1.″5. The SMA data unveil a correlation between the spectral index of the electron distribution in the lobes and N _H , which can explain the γ -ray quietness of heavily obscured CSOs. Based on our modeling, we argue that the inverse-Compton emission of compact radio lobes may account for the intrinsic X-ray continuum in all these sources. Furthermore, we propose that the observed iron lines may be produced by a reflection of the lobes’ continuum from the surrounding cold dust.

Keywords

• Active galactic nuclei
• Active galaxies
• Radio lobes
• Jets
• High energy astrophysics
• Gamma-ray astronomy