The Feasibility of Using Fe XXIII Metastable Transitions as a Density Diagnostic for LMXB Disk Winds

TL;DR

This study investigates the potential of using Fe XXIII metastable transitions as a density diagnostic tool for outflowing winds in low mass X-ray binaries, considering different spectral energy distributions and ionization conditions.

Contribution

It introduces a new density diagnostic method based on Fe XXIII metastable absorption features and assesses its applicability across various LMXB spectral states.

Findings

Fe XXIII can serve as a density diagnostic at specific ionization parameters.

The technique is more feasible for black hole LMXBs than neutron star LMXBs.

Detectability depends on the ionizing spectrum and outflow density.

Abstract

Low mass X-ray binaries (LMXBs) occasionally show signs of outflowing material from the accretion disk. Studying these outflows can inform the understanding of the geometry of the systems, as well as the dynamics and energetics of accretion. One key variable for determining the location of these disk winds is the density of the outflowing material. In this paper we explore a density diagnostic based upon the absorption of ionizing photons by density-sensitive metastable states of Fe XXIII. This can yield a blue shifted complex of absorption features in the region of $6.61-6.64$ keV. We use the photoionization code {\sc pion} to test how varying the ionizing spectrum affects the detectability and interpretation of these features. We base these ionizing spectral energy distributions on GX~13$+$1 to represent a bright thermally dominated spectrum; 4U 1735$-$44 representing a harder, fainter LMXB spectrum; and MAXI J1820$+$070 representing a black hole LMXB spectrum completely dominated by Comptonized emission. For each of these, we find that the regime where Fe XXIII can be used as a density diagnostic is with an ionization parameter $\log{(\xi/{\rm erg~cm~s^{-1}})}\sim2-3$ and an outflow density $\log{(n_H/{\rm cm^{-3})}}\gtrsim14$. The typical range of ionization parameters for LMXBs indicates that this technique is more feasibly achieved with BH LMXBs than their NS counterparts.