Discovery of soft and hard X-ray time lags in extremely low-mass AGN

The scaling relations between the black hole (BH) mass and soft lag properties for both AGN and BH X-ray binaries (BHXRBs) indicate the same underlying physical mechanism at work in accreting BH systems spanning a broad range of mass. However, the low-mass end of AGN has never been explored. In this work, we extend the existing scaling relations to lower-mass AGN, which serve as anchors between the normal-mass AGN and BHXRBs. For this purpose, we construct a sample of extremely low-mass AGN from the XMM-Newton archive and measure frequency-resolved time delays between the soft and hard X-ray emission. We report that the soft band lags behind the hard band emission at high frequencies, which is interpreted as a sign of reverberation from the inner accretion disc in response to the direct coronal emission. At low frequencies, the hard band lags behind the soft band variations, which we explain in the context of the inward propagation of luminosity fluctuations through the corona. We find that the X-ray source for the sample extends at an average radius of around 6 gravitational radii and a median height of around 10 gravitational radii above the disc plane, consistent with gravitational microlensing observations. Our results confirm that the scaling relations between the BH mass and soft lag amplitude/frequency derived for higher-mass AGN can safely extrapolate to lower-mass AGN, and the accretion process is indeed independent of the BH mass. &nbsp