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, 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 NGC6624. This object cannot be readily observed in the visible, even from HST, due to a much brighter star superposed < 0.1 distant. Our FOS data show a highly statistically significant UV flux modulation with a period of 11.46±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 ∼< i ∼< 50 . 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. Subject headings: X-rays: stars — binaries: close — globular clusters: general Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.