Investigating the impact of vertically extended coronae on X-ray reverberation mapping

TL;DR

This paper introduces a model with a vertically extended corona, including a second lamp post, to better explain X-ray reverberation and spectral features in black hole systems, surpassing the limitations of the lamp post paradigm.

Contribution

The authors develop a new model incorporating a second, extended corona source to improve understanding of X-ray reverberation and spectral variability in black hole accretion systems.

Findings

Extended corona model fits NICER data better than single lamp post.

Vertical corona explains both spectral and timing properties.

Variability linked to jet-related processes.

Abstract

Accreting black holes commonly exhibit hard X-ray emission, originating from a region of hot plasma near the central engine referred to as the corona. The origin and geometry of the corona are poorly understood, and models invoking either inflowing or outflowing material (or both) can successfully explain only parts of the observed phenomenology. In particular, recent works indicate that the time-averaged and variability property might originate in different regions of the corona. In this paper we present a model designed to move beyond the lamp post paradigm, with the goal of accounting for the vertical extent of the corona. In particular, we highlight the impact of including self consistently a second lamp post, mimicking for example an extended jet base. We fully include the effect that the second source has on the time-dependent disk ionization, reflection spectrum, and reverberation lags. We also present an application of this new model to NICER observations of the X-ray binary MAXI J1820+070 near its hard-to-soft state transition. We demonstrate that in these observations, a vertically extended corona can capture both spectral and timing properties, while a single lamp post model can not. In this scenario, the illumination responsible for the time-averaged spectrum originates close to the black hole, while the variability is likely associated with the ballistic jet.