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Despite advances in our understanding of low-luminosity active galactic nuclei (LLAGNs), the fundamental details about the mechanisms of radiation and flare/outburst in hot accretion flow are still largely missing. We have systematically analysed the archival Chandra and NuSTAR X-ray data of the nearby LLAGN M81*, whose Lbol ∼ 10−5LEdd. Through a detailed study of X-ray light curve and spectral properties, we find that the X-ray continuum emission of the power-law shape more likely originates from inverse Compton scattering within the hot accretion flow. In contrast to Sgr A*, flares are rare in M81*. Low-amplitude variation can only be observed in soft X-ray band (amplitude usually ≲2). Several simple models are tested, including sinusoidal-like and quasi-periodical. Based on a comparison of the dramatic differences of flare properties among Sgr A*, M31*, and M81*, we find that, when the differences in both the accretion rate and the black hole mass are considered, the flares in LLAGNs can be understood universally in a magnetohydrodynamical model.


Erratum in: Monthly Notices of the Royal Astronomical Society, August 2023, 523(2), 1728. In the originally published version of this manuscript, fig. 8 contained several unreadable codes. This error has now been corrected. See erratum publication for details at

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This article has been accepted for publication in the Monthly Notices of the Royal Astronomical Society ©: 2023 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.