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

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 $α$, 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.}