# The X-ray binary GX 339-4/V821 Ara: the distance, inclination, evolutionary status and mass transfer

## TL;DR

This paper constrains the distance, inclination, and masses in the X-ray binary GX 339-4, models its evolution, and discusses the discrepancy between mass transfer and accretion rates, proposing irradiation and non-conservative transfer as solutions.

## Contribution

It provides detailed constraints on system parameters and introduces models explaining the mass transfer rate discrepancy in GX 339-4.

## Key findings

- Donor mass estimated at 0.5-1.4 solar masses.
- Inclination angle around 40-60 degrees.
- Mass transfer rate is lower than accretion rate, explained by irradiation or outflows.

## Abstract

We consider constraints on the distance, inclination and component masses in the X-ray binary GX 339--4 resulting from published works, and then construct detailed evolutionary models for the donor. From both considerations, and assuming the black-hole nature for the compact object (i.e., its mass $>3\rm{M}_{\odot}$), the possible donor mass is $\approx$0.5--$1.4\rm{M}_{\odot}$, the inclination is $\approx{40}^\circ$--$60^\circ$ and the distance is $\approx$8--12\,kpc. The corresponding mass of the compact object is $\approx$4--$11\rm{M}_{\odot}$. We then confirm a previous estimate that the theoretical conservative mass transfer rate in GX 339--4 is $\lesssim{10^{-9}}\rm{M}_{\odot}$ yr$^{-1}$. This is $\gtrsim$10 times lower than the average mass accretion rate estimated from the long-term X-ray light curve. We show that this discrepancy can be solved in two ways. One solution invokes irradiation of the donor by X-rays from accretion, which can temporarily enhance the mass transfer rate. We found that absorption of a $\sim$1 per cent of the irradiating luminosity results in the transfer rate equal to the accretion rate. The time scale at which the transfer rate will vary is estimated to be $\sim$10 yr, which appears consistent with the observations. The other solution invokes non-conservative mass transfer. This requires that $\approx$70 per cent of the transferred mass escapes as a strong outflow and carries away the specific angular momentum comparable to that of the donor.