Stratified Wind Geometries of GRS 1915+105: Density Profiles and Radii

Abstract

The low-mass X-ray binary GRS 1915+105 exhibits dramatic spectral and flux variability, often accompanied by highly ionized outflows. In this work, we analyze four archival Chandra/HETG observations spanning over a decade, which vary greatly in their ionizing spectrum from soft (low inner disk temperature) to hard (high inner disk temperature). We measure the wind properties, using the insightful tool of the absorption measure distribution (AMD, a distribution of column density with ionization). While the soft epoch reveals a steep, monotonically rising AMD, hard epochs feature a distinct turnover at high ionization parameter ($\logξ>3.3$). This is the first time we observe a negative AMD slope in an outflow, which we interpret as a localized density inversion, which could point to radiation-pressure compression zones. We utilize the AMD shape and total column density measurements to calculate the density profile and distance of the wind from the central source. For the hard states, we obtain densities of $10^{12}-10^{13}$ cm$^{-3}$ at radii of $10^{10}-10^{11}$ cm. Such measurements are usually not directly possible in X-ray spectroscopy. Here we constrain the inner radius and density of the outflow to a factor of two. The high-$ξ$ values up to $\logξ\sim6.5$ with no turnover in the soft state imply a smaller radius of a few $10^{8}$cm, which is in tension with the slow velocities observed. We demonstrate that this is a generic tension between compact launch radii and sub-escape velocities of high-column, high-ionization outflows observed in stellar-mass black hole binaries.