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

TL;DR

This study extends the understanding of X-ray time lags in low-mass AGNs, confirming that accretion processes and scaling relations observed in higher-mass AGNs and BHXRBs also apply to lower-mass systems, revealing consistent physical mechanisms.

Contribution

It provides the first detailed analysis of soft and hard X-ray time lags in low-mass AGNs, extending existing scaling relations to this less-explored mass regime.

Findings

Soft lags at high frequencies indicate reverberation from the inner accretion disc.

Hard lags at low frequencies suggest inward propagation of luminosity fluctuations.

X-ray source extends at ~10r_g height and ~6r_g radius, supporting lamppost geometry assumptions.

Abstract

The scaling relations between the black hole (BH) mass and soft lag properties for both active galactic nuclei (AGNs) and BH X-ray binaries (BHXRBs) suggest the same underlying physical mechanism at work in accreting BH systems spanning a broad range of mass. However, the low-mass end of AGNs has never been explored in detail. In this work, we extend the existing scaling relations to lower-mass AGNs, which serve as anchors between the normal-mass AGNs and BHXRBs. For this purpose, we construct a sample of low-mass AGNs ($M_{\rm BH}<3\times 10^{6} M_{\rm \odot}$) from the XMM-Newton archive and measure frequency-resolved time delays between the soft (0.3-1 keV) and hard (1-4 keV) X-ray emissions. We report that the soft band lags behind the hard band emission at high frequencies $\sim[1.3-2.6]\times 10^{-3}$ Hz, which is interpreted as a sign of reverberation from the inner accretion disc in response to the direct coronal emission. At low frequencies ($\sim[3-8]\times 10^{-4}$ Hz), 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. Assuming a lamppost geometry for the corona, we find that the X-ray source of the sample extends at an average height and radius of $\sim 10r_{\rm g}$ and $\sim 6r_{\rm g}$, respectively. Our results confirm that the scaling relations between the BH mass and soft lag amplitude/frequency derived for higher-mass AGNs can safely extrapolate to lower-mass AGNs, and the accretion process is indeed independent of the BH mass.