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Session 44 - Stellar X-Ray Sources.
Display session, Tuesday, January 14
Metropolitan Ballroom,

[44.03] First Detection of an 11 Minute UV Modulation from the Counterpart to the Luminous Globular Cluster X-ray Source in NGC\,6624

S. F. Anderson, B. Margon, E. W. Deutsch (U. Wash.), R. A. Downes (STScI), R. G. Allen (U. Az.)

Using the Faint Object Spectrograph (FOS) aboard the Hubble Space Telescope, we have obtained the first time-resolved spectra of the ultraviolet-bright counterpart to the 11-minute binary X-ray source in the core of the globular cluster NGC\,6624 (King et al., ApJ, 413, L117, 1993). As reported in IAU Circ. 6475, our data show a highly statistically significant UV flux modulation with a period very similar to the 11.4 minute period of the known X-ray modulation, definitively confirming the association between the King et al. UV counterpart and the X-ray source. However, 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 amplitudes of order 15--20% in the 125 to 250 nm range. These FOS data also have spectral resolution of 0.7 nm, and so are sensitive to spectral-dependent fluctuations in this amplitude. Arons and King (ApJ, 413, L121, 1993) have predicted 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 FOS observations, this predicted modulation had not previously been detected. Employing the Arons and King formalism, which invokes a number of different physical assumptions, we infer a system orbital inclination between about 35 and 55 degrees. Additional UV observations with HST and X-ray observations with RXTE are currently intended by several groups, and multiwavelength comparisons should yield intriguing further insights into this exotic system.

This work has been supported by NASA Grant NAG5-1630.

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