Distinct high-energy cutoff variation patterns in two Seyfert galaxies

Abstract

ABSTRACT Investigating how the cut-off energy Ecut 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 Ecut variations have only been detected in several AGNs but different patterns have been reported. Here, we report new detections of Ecut variations in two Seyfert galaxies with multiple Nuclear Spectroscopic Telescope Array (NuSTAR) exposures. While in NGC 3227 Ecut monotonically increases with Γ, the Ecut–Γ relation exhibits a Λ shape in SWIFT J2127.4+5654 (Ecut 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 Ecut variations ever reported with NuSTAR observations in the Ecut–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.