Correlated variability of the reflection fraction with the X-ray flux and spectral index for Mkn 478

TL;DR

This study reveals that in Mkn 478, the reflection fraction's anti-correlation with flux and spectral index across multiple time scales supports a model where gravitational light bending influences variability by moving the corona closer or farther from the black hole.

Contribution

It demonstrates that both long-term and short-term variability in Mkn 478 are driven by the same light bending mechanism affecting the reflection fraction and flux.

Findings

Reflection fraction anti-correlates with flux and spectral index.

Variability driven by corona movement near the black hole.

Consistent behavior observed on hours to years time scales.

Abstract

The X-ray spectrum of Mkn 478 is known to be dominated by a strong soft excess which can be described using relativistic blurred reflection. Using observations from {\it XMM-Newton}, {\it AstroSat} and {\it Swift}, we show that for the long-term ($\sim$ years) and intermediate-term (days to months) variability, the reflection fraction is anti-correlated with the flux and spectral index, which implies that the variability is due to the hard X-ray producing corona moving closer to and further from the black hole. Using flux-resolved spectroscopy of the {\it XMM-Newton} data, we show that the reflection fraction has the same behaviour with flux and index on short time-scales of hours. The results indicate that both the long-term and short-term variability of the source is determined by the same physical mechanism of strong gravitational light bending causing enhanced reflection and low flux as the corona moves closer to the black hole.