The Probable Optical Counterpart of the Luminous X-ray Source in NGC 6441

Details The Probable Optical Counterpart of the Luminous X-ray Source in NGC 6441 The Probable Optical Counterpart of the Luminous X-ray Source in NGC 6441

1997-04-16

arxiv.org/abs/astro-ph/9704157v2

Astrophys.J. 493 (1998) 775

Astronomy and Astrophysics

Astrophysics

15 pages including 2 tables and 4 figures; to be published in The Astrophysical Journal, Volume 493, 1998 Feb 1. Preprint with

Scientific paper

10.1086/305140

We report results from Hubble Space Telescope WFPC2 imaging of the field of the luminous, bursting X-ray source in the globular cluster NGC 6441. Although the X-ray position is known to a precision of a few arcseconds, this source is only ~6'' from the cluster center, and the field contains hundreds of stars within the 3'' X-ray error circle, making it difficult to isolate the optical counterpart. Nevertheless, our multicolor images reveal a single, markedly UV-excess object with m_{336}=19.0, m_{439}=19.3, within the X-ray error circle. Correcting for substantial reddening and bandpass differences, we infer B_0=18.1, (U-B)_0=-1.0, clearly an unusual star for a globular cluster. Furthermore, we observe an ultraviolet intensity variation of 30% for this object over 0.5 hr, as well as an even greater variation in m_{439} between two HST observations taken approximately one year apart. The combination of considerable UV-excess and significant variability strongly favors this object as the optical counterpart to the low-mass X-ray binary X1746-370. With a group of five optical counterparts to high-luminosity globular cluster X-ray sources now known, we 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 seems to show a significantly larger dispersion than field sources.

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