Broadband X-ray spectra and X-ray variability of young radio sources

Understanding what triggers AGN radio activity is of utmost astrophysical importance because the properties and evolution of AGN jets are among the key factors influencing the host galaxy and active nucleus feedback process. As the jets expand into the interstellar medium, they mix and shock the circumnuclear matter, affect the rate at which the supermassive black hole is fed, and regulate the rate at which the stars are formed in the host galaxy. By definition, Compact Symmetric Objects (CSOs), characterized by radio lobe emission on both sides of an active nucleus and an overall radio size below 1 kpc, are particularly well suited for the studies of the young radio jets. Here, we report on our X-ray study of three nearby CSOs (z < 0.2). With projected radio sizes in the 2-25 parsec range, our sources represent the most compact extragalactic radio jets known to date. Combined with hotspot expansion velocities, derived from the multi epoch radio observations, these radio sizes imply that the radio activity started only 100-450 years ago. We obtained the first CSO detections above 10 keV with NuSTAR, and combined the new NuSTAR data with the archival soft X-ray data to perform a broadband X-ray spectroscopy. We constrained the intrinsic X-ray absorbing column densities, and we found that they exceed 10²³ cm^-2. We detected a 6.4 keV Fe Kα emission line, and concluded that the underlying X-ray continua are consistent with the reprocessing of the primary power law emission by a cold toroidal obscurer. The primary power law emission, parametrized by a relatively hard photon index, Gamma < 2, could originate from a number of emitting regions expected in a radio AGN, including an X-ray corona, an X-ray jet, and expanding radio lobes. We found evidence for X-ray variability on the timescale of years, after carefully accounting for cross-calibration uncertainties between various X-ray instruments used to collect the data. We note that none of our targets have been detected in the gamma-ray band to date. We compared the X-ray and radio properties of our sources with those of CSOs with low intrinsic X-ray absorption, and with those of the gamma-ray loud CSOs, to evaluate the impact of the environment on the evolution and high-energy radiative output of a young radio source. Finally, based on the X-ray photon indices, Eddington luminosity ratios, the presence of a young radio jet, and the timescale of the X-ray flux variability, we speculate that the active nuclei of our CSOs are in the spectral state reminiscent of the hard/low or hard/intermediate state of accreting stellar mass black holes in X-ray binary systems.