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

Abstract

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.

15 pages including 2 tables and 4 figures; to be published in The Astrophysical Journal, Volume 493, 1998 Feb 1. Preprint with full-resolution figures available at http://www.astro.washington.edu/deutsch/pubs/

References in corpus (3)

• Discovery of Extended Blue Horizontal Branches in Two Metal-Rich Globular Clusters
• Time-Resolved Ultraviolet Observations of the Globular Cluster X-ray Source in NGC 6624: The Shortest Known Period Binary System
• An Ultraviolet-Excess Optical Candidate for the Luminous Globular Cluster X-ray Source in NGC1851

Cited by in corpus (12)

• Relativistic Binaries in Globular Clusters
• The gas-to-extinction ratio and the gas distribution in the Galaxy
• Optical Identification of Multiple Faint X-ray Sources in the Globular Cluster NGC 6752: Evidence for Numerous Cataclysmic Variables
• Ultracompact X-ray Binaries in Globular Clusters: Variability of the Optical Counterpart of X1832-330 in NGC 6652
• NuSTAR Observations of the Accreting Atolls GX 3+1, 4U 1702-429, 4U 0614+091, and 4U 1746-371
• Identification of the LMXB and Faint X-ray Sources in NGC 6652
• Discovery of a ~1 Hz quasi-periodic oscillation in the low-mass X-ray binary 4U 1746-37
• Optical Identification of the X-ray Burster in the Globular Cluster NGC1851
• Relativistic Binaries in Globular Clusters
• A Search for the Optical Counterpart of the Luminous X-ray Source in NGC 6652
• Very Large Telescope Observations of the peculiar globular cluster NGC6712. Discovery of a UV, H-alpha excess star in the core
• The Optical Identification of the X-ray Burster X1746-370 in the Globular Cluster NGC6441