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More than softer-when-brighter: The X-ray powerlaw spectral variability in NGC 4051

Cai, Zheng; Kang, Jia-Lai; Cai, Zhen-Yi +3 more

The powerlaw X-ray spectra of active galactic nuclei at moderate to high accretion rates normally appear softer when they brighten, for which the underlying mechanisms are yet unclear. Utilizing XMM-Newton observations and excluding photons < 2 keV to avoid contamination from the soft excess, in this work we scrutinize the powerlaw spectral var…