Optical Observations of the Black Hole Candidate GX 339−4 (V821 Arae)

GX 339-4 has long been known as a black hole candidate because of its rapid variability and high/low X-ray states, which are similar to those of Cyg X-1. Although GX 339-4 is assumed to be a binary, the orbital period has not yet been convincingly determined, and hence little is known about the nature of the component stars. In this study simultaneous photometric and spectroscopic observations have been made in an effort to address these problems. Although a definitive period has not been found, we present evidence that it lies near ∼0.7 days. The I-band light curve has an amplitude of only ∼0.2 mag, with flickering of >0.1 mag superposed, making it difficult to determine the orbital period from light variations alone. The emission-line velocity amplitudes are small, suggesting that GX 339-4 is seen at a low orbital inclination angle. Even though the period is still not determined, the relative phasing of the velocities and light minima are known, since the photometric and spectroscopic observing runs overlapped. Assuming the orbital period is near ∼0.7 days, we can set limits on the mass ratio and stellar masses. For reasonable assumptions about the inclination angle, the mass ratio appears to lie between q ∼ 3 and 15, where q = M_X/M_donor. If the donor star mass is greater than ∼0.3 M_⊙, the X-ray source is likely to be a black hole.