Distinct High Energy Cutoff Variation Patterns in Two Seyfert Galaxies

Abstract

Investigating how the cutoff energy $E_{\rm cut}$ varies with X-ray flux and photon index $\Gamma$ in individual AGNs opens a new window to probe the yet unclear coronal physics. So far $E_{\rm cut}$ variations have only been detected in several AGNs but different patterns have been reported. Here we report new detections of $E_{\rm cut}$ variations in two Seyfert galaxies with multiple NuSTAR exposures. While in NGC 3227 $E_{\rm cut}$ monotonically increases with $\Gamma$, the $E_{\rm cut}$-$\Gamma$ relation exhibits a $\Lambda$ shape in SWIFT J2127.4+5654 ($E_{\rm cut}$ increasing with $\Gamma$ at $\Gamma$ $\lesssim$ 2.05, but reversely decreasing at $\Gamma$ $\gtrsim$ 2.05), indicating more than a single underlying mechanism is involved. Meanwhile both galaxies show softer spectra while they brighten in X-ray, a common phenomenon in Seyfert galaxies. Plotting all 7 AGNs with $E_{\rm cut}$ variations ever reported with NuSTAR observations in the $E_{\rm cut}$-$\Gamma$ diagram, we find they could be unified with the $\Lambda$ pattern. Although the sample is small and SWIFT J2127.4+5654 is the only source with $\Gamma$ varying across the break point thus the only one exhibiting the complete $\Lambda$ pattern in a single source, the discoveries shed new light on the coronal physics in AGNs. Possible underlying physical mechanisms are discussed.