Optical spectroscopy of GX 339-4 – 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 seven epochs. The equivalent width of Hα increased during the high state and then decreased during the low state. The equivalent width of He iiλ4686 decreased over both states. The full-width at half-maximum of Hα, Hβ, He iiλ4686 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 shows modulation of the radial velocities, equivalent width and V/R ratios of Hα on the 14.86-h orbital period. The equivalent width of He iiλ4686 also varies on the orbital period. The semi-amplitude of the Hα radial velocities is K₁= 14 km s⁻¹. Hence the mass function = 2 × 10⁻⁴ M_⊙.