Time-resolved ultraviolet observations of the globular cluster X-ray source in NGC 6624: The shortest known period binary system

Using the Faint Object Spectrograph (FOS) aboard the Hubble Space Telescope (HST), we have obtained the first time-resolved spectra of the King et al. (1993) ultraviolet-bright counterpart to the 11 minute binary X-ray source in the core of the globular cluster NGC 6624. This object cannot be readily observed in the visible, even from HST, because of a much brighter star superposed less than 0.″1 away. Our FOS data show a highly statistically significant ultraviolet flux modulation with a period of 11.46 ± 0.04 minutes, very similar to the 685 s period of the known X-ray modulation, definitively confirming the association between the King et al. (1993) ultraviolet counterpart and the intense X-ray source. The ultraviolet amplitude is very large compared with the observed X-ray oscillations: X-ray variations are generally reported as 2%-3% peak to peak, whereas our data show an amplitude of about 16% in the 126-251 nm range. A model for the system by Arons & King (1993) predicts periodic ultraviolet fluctuations in this shortest known period binary system, because of the cyclically changing aspect of the X-ray heated face of the secondary star (perhaps a very low mass helium degenerate). However, prior to our observations, this predicted modulation has not been detected. Employing the Arons & King (1993) formalism, which invokes a number of different physical assumptions, we infer a system orbital inclination 35° ≲ i ≲ 50°. Among the three best-studied ultraviolet/optical counterparts to the intense globular cluster X-ray sources, two are now thought to consist of exotic double-degenerate ultrashort-period binary systems.