Detection of an Ultraviolet and Visible Counterpart of the NGC 6624 X-Ray Burster
Boksenberg, A.; Barbieri, C.; Weigelt, G.; Nota, A.; Crane, P.; Baxter, D.; Sparks, W. B.; Albrecht, R.; Blades, J. C.; Deharveng, J. M.; Disney, M. J.; Jakobsen, P.; Kamperman, T. M.; King, I. R.; Macchetto, F.; Mackay, C. D.; Paresce, F.; Greenfield, P.; Jedrzejewski, R.; Stanford, S. A.; Sosin, C.
United States, Germany, Italy, United Kingdom, France, Netherlands
Abstract
We have detected, in images taken with the HST FOC, the UV and optical counterpart of the X-ray source 4U 1820-30 in the globular cluster NGC 6624. Astrometric measurements place this object 2 sigma from the X-ray position of 4U 1820-30. The source dominates a far-UV FOC image and has the same flux at 1400 A as was seen through the large IUE aperture by Rich et al. (1993). It has a B magnitude of 18.7 but is not detected in V. It is 0.66 arcsec from the center of NGC 6624, a fact that may change the interpretation of the P-average of the 11 minute binary orbit. The flux drops between 1400 and 4300 A at a rate that is nearly as steep as that of a Rayleigh-Jeans curve. The flux is far too large to come from the neutron star directly but could accord with radiation from a heated accretion disk and/or the heated side of the companion star.