NuSTAR hard X-ray spectra of radio galaxies

TL;DR

This study systematically analyzes NuSTAR X-ray spectra of 28 radio galaxies, measuring spectral features and comparing them with radio quiet sources to understand the corona and jet contributions.

Contribution

It provides the first uniform analysis of high-energy cut-offs and reflection features in a large sample of radio galaxies, revealing similarities with radio quiet sources and supporting the outflowing corona model.

Findings

E_{cut} measured for 13 sources; lower limits for others.

Radio galaxies show weaker reflection and Fe Kα EW compared to radio quiet sources.

Gamma-R correlation supports the outflowing corona model.

Abstract

NuSTAR observatory, with its 3 - 78 keV broadband spectral coverage, enables the detections 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 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 cut-off $E_{cut}$, photon index $\Gamma$, reflection factor R and Fe K$\alpha$ line equivalent width. Measurements of $E_{cut}$ are given for 13 sources, and lower limits for the rest. We find those $E_{cut}$ non-detections 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 corona emission, and the $E_{cut}$ distribution of the sample is indistinguishable from that of a radio quiet one in 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 K$\alpha$ line EW) comparing 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 $\Gamma$ - 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.