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Session 23 - Degenerate Stars, Pulsars.
Display session, Tuesday, June 10
South Main Hall,
We examine HST images of the field of X1832--330, the luminous (L_X\sim1\times10^36\ erg\ s^-1) X-ray source near the center of the globular cluster NGC\,6652. A precise (few arcsecond accuracy) X-ray position is available from ROSAT. As four of the five known optical counterparts to luminous globular cluster X-ray sources were discovered from HST due to considerable UV excess, (U-B)_0\sim-1.0, we take this same approach, searching for unusual objects in a color-magnitude diagram. Although not all of the images cover the entire X-ray error circle, an optical counterpart as luminous as AC\,211 in M\,15 can certainly be ruled out. We set more detailed constraints on the optical luminosity of the counterpart with the available data.
In an effort to clarify the expected magnitude and color of the counterpart, we consider the currently-known group of five optical counterparts to high-luminosity globular cluster X-ray sources, present a homogeneous set of HST photometry on these objects, and compare their optical properties with those of field low-mass X-ray binaries. The mean (U-B)_0 color of the cluster sources is identical to that of the field sources, and the mean M_B_0 is similar to bursters in the field. However, the ratio of optical to X-ray flux of cluster sources shows a significantly larger dispersion than field sources. The optical counterpart of the source in NGC\,6652 is probably within reach of further HST observations, which we intend to pursue. This work has been supported by NASA Grant NAG5-1630.
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