The X-Ray Coronae in NuSTAR Bright Active Galactic Nuclei

We present a systematic and uniform analysis of NuSTAR data of a sample of 60 SWIFT BAT-selected AGNs with 10-78 keV signal-to-noise ratio (S/N) > 50, 10 of which are radio loud. We measure their high-energy cutoff E (cut) or coronal temperature T (e) using three different spectral models to fit their NuSTAR spectra and show that 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 on E (cut) or T (e) are obtained. Strikingly, we find extraordinarily large E (cut) lower limits (>400 keV, up to >800 keV) in 10 radio-quiet sources, whereas we find none in the radio-loud sample. Consequently and surprisingly, we find a significantly larger mean E (cut)/T (e) of radio-quiet sources compared with radio-loud ones. The reliability of these measurements is carefully inspected and verified with simulations. We find a strong positive correlation between E (cut) and photon index Gamma, which cannot be attributed to the parameter degeneracy. The strong dependence of E (cut) on Gamma, which could fully account for the discrepancy of the E (cut) distribution between radio-loud and radio-quiet sources, indicates that 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 E (cut) 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.