Photoionization instability of the Fe K absorbing plasma in the neutron star transient AX J1745.6-2901

TL;DR

This study investigates how photoionization instability affects the observable properties of ionized Fe K absorbers in the neutron star binary AX J1745.6-2901, revealing state-dependent stability that influences absorption features.

Contribution

It demonstrates that photoionization instability explains the presence or absence of ionized absorbers in different accretion states of neutron star binaries.

Findings

Ionized Fe K absorption is stable in the soft state.

Absorber becomes unstable in the hard state.

Instability may cause disappearance of winds in hard states.

Abstract

AX J1745.6-2901 is a Low Mass X-ray Binary with an accreting neutron star, showing clear evidence for highly ionized absorption. Strong ionized Fe K$\alpha$ and K$\beta$ absorption lines are always observed during the soft state, whereas they disappear during the hard states. We computed photoionization stability curves for the hard and the soft state, under different assumptions on the adopted spectral energy distributions and the physical parameters of the plasma. We observe that the ionized absorber lies always on a stable branch of the photoionization stability curve during the soft state, while it becomes unstable during the hard state. This suggests that photoionization instability plays a key role in defining the observable properties of the ionized absorber. The same process might explain the disappearance of the high ionization absorber/wind, during the hard state in other accreting neutron stars and black holes.