New insight into the orbital parameters of the gamma-ray binary HESS J0632+057

Abstract: The gamma-ray binary HESS J0632+057 consists of a Be star and an undetected compact object in a $\sim$317 day orbit. The interpretation of the emission from this system is complicated by the lack of a clear orbital solution, as two different and incompatible orbital solutions were obtained by previous radial velocity studies of this source. In order to address this, we report on 24 new observations, covering $\sim$60 per cent of the orbit which we have undertaken with the Southern African Large Telescope (SALT). We obtained new radial velocity measurements from cross-correlation of the narrower spectral features, and by fitting Voigt profiles to the wings of the Balmer emission lines. Additionally, we find an indication of orbital variability in the equivalent widths and V/R of the Balmer lines. Using the combined data from this study, as well as archival data from the earlier radial velocity studies, we have derived updated orbital solutions. Using reported H $\alpha$ emission radial velocities - previously not considered for the orbital solution - along with the new SALT data, a solution is obtained where the brighter peak in the X-ray and gamma-ray light curves is closer to periastron. However, continuing sparse coverage in the data around the expected phases of periastron indicates that the orbital solution could be improved with further observation.