The X-ray coronae in NuSTAR bright active galactic nuclei

TL;DR

This study systematically analyzes NuSTAR data of 60 AGNs to measure coronal properties, revealing differences between radio-loud and radio-quiet sources and challenging existing coronal models.

Contribution

It provides the first uniform measurements of high energy cutoffs in a large AGN sample, highlighting differences based on radio loudness and testing coronal physics models.

Findings

Radio-quiet AGNs have higher Ecut/Te than radio-loud AGNs.

Strong positive correlation between Ecut and photon index {Gamma}.

Many sources exceed pair production limits, challenging current coronal models.

Abstract

We present systematic and uniform analysis of NuSTAR data with 10-78 keV S/N > 50, of a sample of 60 SWIFT BAT selected AGNs, 10 of which are radio-loud. We measure their high energy cutoff Ecut or coronal temperature Te using three different spectral models to fit their NuSTAR spectra, and show a threshold in NuSTAR spectral S/N is essential for such measurements. High energy spectral breaks are detected in the majority of the sample, and for the rest strong constraints to Ecut or Te are obtained. Strikingly, we find extraordinarily large Ecut lower limits ( > 400 keV, up to > 800 keV) in 10 radio-quiet sources, whereas none in the radio-loud sample. Consequently and surprisingly, we find significantly larger mean Ecut/Te of radio-quiet sources compared with radio-loud ones. The reliability of these measurements are carefully inspected and verified with simulations. We find a strong positive correlation between Ecut and photon index {\Gamma}, which can not be attributed to the parameter degeneracy. The strong dependence of Ecut on {\Gamma}, which could fully account for the discrepancy of Ecut distribution between radio-loud and radio-quiet sources, indicates the X-ray coronae in AGNs with steeper hard X-ray spectra have on average higher temperature and thus smaller opacity. However, no prominent correlation is found between Ecut and {\lambda}edd. In the l-{\Theta} diagram, we find a considerable fraction of sources lie beyond the boundaries of forbidden regions due to runaway pair production, posing (stronger) challenges to various (flat) coronal geometries.