2021MNRAS.502...80K

Investigating how the cut-off energy E_cut varies with X-ray flux and photon index Γ in individual active galactic nuclei (AGNs) opens a new window to probe the yet unclear coronal physics. So far E_cut variations have only been detected in several AGNs but different patterns have been reported. Here, we report new detections of E_cut variations in two Seyfert galaxies with multiple Nuclear Spectroscopic Telescope Array (NuSTAR) exposures. While in NGC 3227 E_cut monotonically increases with Γ, the E_cut-Γ relation exhibits a Λ shape in SWIFT J2127.4+5654 (E_cut increasing with Γ at Γ <= 2.05, but reversely decreasing at Γ >= 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 seven AGNs with E_cut variations ever reported with NuSTAR observations in the E_cut-Γ diagram, we find they could be unified with the Λ pattern. Although the sample is small and SWIFT J2127.4+5654 is the only source with Γ varying across the break point thus the only one exhibiting the complete Λ pattern in a single source, the discoveries shed new light on the coronal physics in AGNs. Possible underlying physical mechanisms are discussed.