A NICER View of a Highly-Absorbed Flare in GRS 1915+105

TL;DR

This study presents detailed spectroscopic analysis of a bright flare in GRS 1915+105 during its obscured state, revealing complex absorption features and proposing models for the obscuration mechanism.

Contribution

First detailed time-resolved spectroscopy of a flare in GRS 1915+105's obscured state, identifying a stratified absorber and exploring potential causes of the obscuration.

Findings

Evidence of high column density partial covering absorption.

Detection of extremely deep absorption lines with over 100 eV equivalent widths.

Identification of a radially-stratified absorber at ~1e11 cm from the black hole.

Abstract

After 26 years in outburst, the black hole X-ray binary GRS 1915+105 dimmed considerably in early 2018; its flux dropped sharply in mid-2019, and it has remained faint ever since. This faint period, the "obscured state," is punctuated by occasional X-ray flares, many of which have been observed by NICER as part of our regular monitoring program. Here we present detailed time-resolved spectroscopy of one bright flare, whose spectrum shows evidence of high column density partial covering absorption and extremely deep absorption lines (equivalent widths over 100 eV in some cases). We study the time-dependent ionization of the obscuring gas with XSTAR, ultimately attributing the absorption to a radially-stratified absorber of density 1e12-1e13 cm^-3 at a ~few x 1e11 cm from the black hole. We argue that a vertically-extended outer disk could explain this obscuration. We discuss several scenarios to explain the obscured state, including massive outflows, an increase in the mass accretion rate, and changes in the outer disk that herald the approach of quiescence, but none are entirely satisfactory. Alternative explanations, such as obscuration by the accretion stream impact point, may be testable with current or future data.