The X-ray variation of M81*resolved by Chandra and NuSTAR

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 L-bol similar to 10(-5)L(Edd). 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 less than or similar to 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.

Automated summary

Despite advances in our understanding of LLAGNs, we still lack fundamental details about the mechanisms of radiation and flare/outburst in hot accretion flow. By analyzing the X-ray data of nearby LLAGN M81*, we find that the X-ray continuum emission likely originates from inverse Compton scattering in the hot accretion flow and flares are rare in M81*. Based on a comparison with Sgr A* and M31*, we propose a magnetohydrodynamical model to universally understand flares in LLAGNs when considering differences in accretion rate and black hole mass.