Thermal X-ray iron line emission from the advection dominated accretion flow in the galactic binary GX 339-4

TL;DR

This study models thermal X-ray iron line emission from the accretion flow in GX 339-4, showing that outflows significantly enhance line strength, which can reveal the accretion mode and outflow presence.

Contribution

It introduces detailed calculations of thermal iron line equivalent widths in ADAF models with and without outflows, linking line strength to outflow strength and accretion parameters.

Findings

Strong thermal iron lines indicate significant outflows in the accretion flow.

Equivalent widths can reach over 500 eV with outflows, providing observable signatures.

Degeneracy exists between accretion rate and outflow strength, which can be broken with line data.

Abstract

We explore thermal X-ray iron line emission from the galactic X-ray binary GX 339-4 in the off state, using the models of the advection dominated accretion flow (ADAF) without or with outflows. The equivalent widths of hydrogen-like and helium-like thermal iron lines are calculated with different model parameters including viscosity parameter $\alpha$, mass accretion rate at the outer radius of the ADAF $\dot{m}_{\rm out}$ and outflow strength parameter $p$. Our calculations show that the equivalent widths of thermal iron lines emitted from the pure ADAF, i.e., the ADAF without outflows, should be very small, assuming a solar metallicity for the accreting gas in the accretion flow. Strong thermal iron lines are expected to be emitted from the ADAF with relatively strong outflows. {For reasonable choice of parameters, the total equivalent width of the He-like and H-like thermal iron lines reaches to $\gtrsim 500$ eV for accreting gas with solar metallicity. The observation of strong thermal X-ray lines from GX 339-4 at the off state may give a clue to the accretion mode of the source and provide evidence for the presence of outflows/winds in the accretion flow around the black hole in GX 339-4. It is found that the values of $\dot{m}_{\rm out}$ and $p$ are degenerate, i.e., the observed X-ray continuum spectrum can be fairly well reproduced with different sets of the parameters $\dot{m}_{\rm out}$ and $p$. Such {degeneracy} can be broken when the thermal X-ray line emission data is available. We also compare our results with those in the previous similar work.}