Variability of the X-ray Broad Iron Spectral Features in Active Galactic Nuclei and Black-hole Binaries

TL;DR

This study investigates the short-term X-ray spectral variability of black-hole binary GRS 1915+105 and compares it with AGN, providing insights into the origins of broad iron spectral features and supporting the partial covering model.

Contribution

First detailed short-time scale spectral variability analysis of BHB GRS 1915+105 using Suzaku's P-sum mode, contrasting its behavior with AGN to constrain spectral models.

Findings

GRS 1915+105 shows no iron feature variability at sub-second timescales.

AGN like 1H0707--495 exhibit reduced variation amplitude at iron K-energy band.

Results support the partial covering scenario over relativistic disc reflection.

Abstract

The "broad iron spectral features" are often seen in X-ray spectra of Active Galactic Nuclei (AGN) and black-hole binaries (BHB). These features may be explained either by the "relativistic disc reflection" scenario or the "partial covering" scenario: It is hardly possible to determine which model is valid from time-averaged spectral analysis. Thus, X-ray spectral variability has been investigated to constrain spectral models. To that end, it is crucial to study iron structure of BHBs in detail at short time-scales, which is, for the first time, made possible with the Parallel-sum clocking (P-sum) mode of XIS detectors on board Suzaku. This observational mode has a time-resolution of 7.8~ms as well as a CCD energy-resolution. We have carried out systematic calibration of the P-sum mode, and investigated spectral variability of the BHB GRS 1915+105. Consequently, we found that the spectral variability of GRS 1915+105 does not show iron features at sub-seconds. This is totally different from variability of AGN such as 1H0707--495, where the variation amplitude significantly drops at the iron K-energy band. This difference can be naturally explained in the framework of the "partial covering" scenario.