An extreme gravitationally redshifted iron line at 4.8 keV in Mrk 876

TL;DR

This paper reports the detection of an extremely redshifted iron emission line at 4.8 keV in Mrk 876, suggesting it originates from very close to a supermassive black hole, providing insights into the extreme environments of AGN.

Contribution

The study presents a rigorous Monte Carlo simulation approach to confirm the significance of a highly redshifted iron line in Mrk 876, supporting the hotspot scenario near a rotating black hole.

Findings

Detection of a 4.8 keV iron line at 99% confidence

Line likely originates within 6 gravitational radii of black hole

Supports the hotspot model of accretion disk emission

Abstract

X-ray spectral lines at unforeseen energies are important because they can shed light on the extreme physical conditions of the environment around the supermassive black holes of active galactic nuclei (AGN). Mrk 876 displays such a line at 4.8 keV rest-frame energy. A possible interpretation of its origin can be found in the hotspot scenario. In this scenario the primary radiation from a flare in the hot corona of an AGN illuminates a limited portion of the accretion disk that emits by fluorescence. In this context the line can represent an extreme gravitationally redshifted Fe line originating on the accretion disk below 6 gravitational radii from a rotating supermassive black hole. The correct estimate of the line significance requires a dedicated approach. Based on an existing rigorous approach, we have performed extensive Monte Carlo simulations. We determine that the line is a real feature at ~99 confidence level.