Unraveling the enigmatic soft x‐ray excess: Current understanding and future perspectives

Abstract

This article explores various theoretical models proposed to explain the soft x‐ray excess phenomenon, including warm Comptonization, ionized reflection models, and ionized outflowing disc winds. The soft x‐ray excess is better understood thanks to crucial observations made by ROSAT, XMM‐Newton, and eROSITA. The article briefly touches upon historical discoveries in this field, specifically the initial detection of the soft x‐ray excess in Narrow Line Seyfert 1 galaxies. While progress has been made in comprehending the soft x‐ray excess, the article also acknowledges the challenges and limitations of the different models. We emphasize the importance of further studies to refine theoretical models by, for example, building up a large database of reverberation time lags between the disc and coronal x‐ray emission to achieve a more comprehensive understanding of this intriguing phenomenon.