The X-ray high-energy cutoff in Compact Symmetric Object Mrk 348

Abstract

Compact radio AGN are thought to be young radio active galactic nuclei (AGN) at the early stage of AGN evolution, thus are ideal laboratory to study the high-energy emission throughout the evolution of radio AGN. In this work, we report for the first time the detection of the high-energy cutoff ($E_{\rm cut}$), a direct indicator of thermal coronal radiation, of X-ray emission in Mrk 348 ($z$ = 0.015), a young radio galaxy classified as compact symmetric object. With a 100 ks NuSTAR exposure, we find that the high-energy cutoff ($E_{\rm cut}$ ) is firmly detected ($218^{+124}_{-62}$ keV). Fitting with various Comptonization models indicates the presence of a hot corona with temperature $kT_{\rm e}$ = 35 -- 40 keV. These strongly support the corona origin for its hard X-ray emission. The comparison in the $E_{\rm cut}$ -- spectra index $\Gamma$ plot of Mrk 348 with normal large-scale radio galaxies (mostly FR II) yields no difference between them. This suggests the corona properties in radio sources may not evolve over time (i.e., from the infant stage to mature stage), which is to-be-confirmed with future sample studies of young radio AGN.