NuSTAR Hard X-Ray Spectra of Radio Galaxies

Abstract The Nuclear Spectroscopic Telescope Array (NuSTAR) observatory, with its 3–78 keV broadband spectral coverage, enables the detection of the high-energy cutoff in a number of active galaxies, including several individual radio-loud ones. In this work, we present systematic and uniform analyses of 55 Nuclear Spectroscopic Telescope Array (NuSTAR) spectra for a large sample of 28 radio galaxies, 20 of which are FR II galaxies. We perform spectral fitting to measure the high-energy cutoff E cut, photon index Γ, reflection factor R, and Fe Kα line equivalent width (EW). Measurements of E cut are given for 13 sources, and lower limits are given for the rest. We find that those E cut nondetections could primarily be attributed to the obviously smaller net photon counts in their spectra. This indicates that the NuSTAR spectra of the majority of our sample are dominated by the thermal coronal emission, and the E cut distribution of the sample is indistinguishable from that of a radio-quiet one in the literature. The flatter NuSTAR spectra we observed, comparing with radio-quiet sources, are thus unlikely due to jet contamination. The radio galaxies also show weaker X-ray reflection (both in R and Fe Kα line EW) compared with radio-quiet ones. Combining with the radio-quiet sample, we see a correlation between R and EW but with considerably large scatter. Notably, the radio-loud and -quiet sources appear to follow a common Γ–R correlation trend, supporting the outflowing corona model for both populations in which higher bulk outflowing velocity yields weaker reflection and flatter X-ray slope.