Using the Faint Object Spectrograph ( FOS ) aboard the Hubble Space Telescope , we have obtained the first time-resolved spectra of the King et al .
ultraviolet-bright counterpart to the 11-minute binary X-ray source in the core of the globular cluster NGC 6624 .
This object can not be readily observed in the visible , even from HST , due to a much brighter star superposed < 0.1 ^ { \prime \prime } distant .
Our FOS data show a highly statistically significant UV flux modulation with a period of 11.46 \pm 0.04 min , very similar to the 685 s period of the known X-ray modulation , definitively confirming the association between the King et al .
UV counterpart and the intense X-ray source .
The UV 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 predicts periodic UV fluctuations in this shortest-known period binary system , due to 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 formalism , which invokes a number of different physical assumptions , we infer a system orbital inclination 35 ^ { \circ } \lesssim i \lesssim 50 ^ { \circ } .
Amongst the three best-studied UV/optical counterparts to the intense globular cluster X-ray sources , two are now thought to consist of exotic double-degenerate ultrashort period binary systems .