Origin of the X-ray Broad Iron Spectral Feature in GRS 1915+105

TL;DR

This study investigates the spectral variations of GRS 1915+105 at very short timescales using Suzaku data, revealing that the iron spectral feature does not vary in amplitude, supporting the partial covering model and highlighting differences in wind mechanisms between AGNs and BHBs.

Contribution

First-time analysis of GRS 1915+105 spectral variations at millisecond timescales, demonstrating independent variability of flux and iron features, supporting the partial covering model.

Findings

Variation amplitude does not drop at the iron K-energy band across all timescales.

The entire X-ray flux and iron feature vary independently at hours timescales.

Results support the partial covering model with different wind mechanisms in AGNs and BHBs.

Abstract

The X-ray spectrum of GRS 1915+105 is known to have a ``broad iron spectral feature'' in the spectral hard state. Similar spectral features are often observed in Active Galactic Nuclei (AGNs) and other black-hole binaries (BHBs), and several models have been proposed for explaining it. In order to distinguish spectral models, time variation provides an important key. In AGNs, variation amplitude has been found to drop significantly at the iron K-energy band at timescales of ~10 ks. If spectral variations of black-holes are normalized by their masses, the spectral variations of BHBs at timescales of sub-seconds should exhibit similar characteristics to those of AGNs. In this paper, we investigated spectral variations of GRS 1915+105 at timescales down to ~10 ms. This was made possible for the first time with the Suzaku XIS Parallel-sum clocking (P-sum) mode, which has the CCD energy-resolution as well as a time-resolution of 7.8 ms. Consequently, we found that the variation amplitude of GRS 1915+105 does not drop at the iron K-energy band at any timescales from 0.06 s to 63000 s, and that the entire X-ray flux and the iron feature are independently variable at timescales of hours. These are naturally understood in the framework of the ``partial covering'' model, in which variation timescales of the continuum flux and partial absorbers are independent. The difference of energy dependence of the variation amplitude between AGN and BHB is presumably due to different mechanisms of the outflow winds, i.e., the partial absorbers are due to UV-line driven winds (AGN) or thermally-driven winds (BHB).