Scaling accretion flow models from BHB to AGN - Why doesn't it work?

TL;DR

This paper investigates why accretion flow models from stellar-mass black holes to supermassive black holes don't scale straightforwardly, highlighting missing physics and using spectral-timing techniques to analyze the soft X-ray excess.

Contribution

It introduces new spectral-timing methods to constrain the nature of the soft X-ray excess in AGN, challenging previous reflection-based interpretations.

Findings

Soft X-ray excess likely due to low temperature Comptonisation

Spectral-timing data supports a Comptonisation origin over reflection

Variability differences highlight missing physics in simple scaling models

Abstract

Black holes depend only on mass and spin, while what we see from the accretion flow in steady state depends also on mass accretion rate and (weakly) inclination. Hence we should be able to scale the accretion flow properties from the stellar to the supermassive black holes. But the data show significant differences between these two types of systems, suggesting that we are missing some crucial physics in AGN. One of these differences is the soft X-ray excess which is seen ubiquitously in bright AGN, but only occasionally in BHB. Another is the much faster variability seen in the high energy tail of high mass accretion rate AGN compared to that seen in the tail of BHB. We show that while this variability is not understood, it can be used via the new spectral-timing techniques to constrain the nature of the soft X-ray excess. The coherence, lag-frequency and lag-energy results strongly support this being an additional low temperature Comptonisation component rather than extreme relativistically smeared reflection in the 'simple' Narrow Line Seyfert 1 PG1244+026.