Optical Spectroscopy of GX 339 - 4 - Paper I : Orbital Modulation

Optical spectroscopic observations of GX 339-4 were carried out between 1998 May (X-ray high state) and 1999 May (X-ray low state) over 7 epochs. The equivalent width of Hα increased during the high state then decreased during the low state. The equivalent width of HeII λ4686 decreased over both states. The full-width half-maximum of Hα, Hβ, HeII λ4686 and the Bowen blend increased from 1998 to 1999 indicating that the emission line regions moved closer to the compact object. Hβ shows a redshifted absorption feature at λ4880 at all epochs. This line remains unidentified. Analysis of individual spectra from 1998 May 28-31 show modulation of the radial velocities, equivalent width and V/R ratios of Hα on the 14.86 hour orbital period. The equivalent width of HeII λ4686 also varies on the orbital period. This is the first time since the study of Callanan et al. (1992) that spectroscopic data has confirmed the orbital period. The semi-amplitude of the Hα radial velocities is K 1 = 14 km s −1. Hence the mass function = 2 x 10 −4 M ⊙ .