A Persistent Disk Wind in GRS 1915+105 with NICER

TL;DR

This study uses NICER observations to analyze the properties and variability of the disk wind in GRS 1915+105, revealing that the wind is persistent, varies rapidly, and correlates with X-ray spectral characteristics.

Contribution

First detailed NICER-based analysis of the GRS 1915+105 disk wind, demonstrating its persistence and rapid variability linked to X-ray spectral properties.

Findings

Fe XXVI detected in over 80% of observations

Wind properties vary rapidly on seconds timescales

Wind strength correlates with spectral hardness

Abstract

The bright, erratic black hole X-ray binary GRS 1915+105 has long been a target for studies of disk instabilities, radio/infrared jets, and accretion disk winds, with implications that often apply to sources that do not exhibit its exotic X-ray variability. With the launch of NICER, we have a new opportunity to study the disk wind in GRS 1915+105 and its variability on short and long timescales. Here we present our analysis of 39 NICER observations of GRS 1915+105 collected during five months of the mission data validation and verification phase, focusing on Fe XXV and Fe XXVI absorption. We report the detection of strong Fe XXVI in 32 (>80%) of these observations, with another four marginal detections; Fe XXV is less common, but both likely arise in the well-known disk wind. We explore how the properties of this wind depends on broad characteristics of the X-ray lightcurve: mean count rate, hardness ratio, and fractional RMS variability. The trends with count rate and RMS are consistent with an average wind column density that is fairly steady between observations but varies rapidly with the source on timescales of seconds. The line dependence on spectral hardness echoes known behavior of disk winds in outbursts of Galactic black holes; these results clearly indicate that NICER is a powerful tool for studying black hole winds.