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Session 44 - Stellar X-Ray Sources.
Display session, Tuesday, January 14
With the recent identification of the optical counterpart of the X-ray source in the globular cluster NGC 1851 by Deutsch et al., less than a handful of the luminous (L_x>10^36 erg s^-1) X-ray sources in globular cluster cores remain optically unidentified. All but the optically bright star AC 211 in M15 have required the resolution of HST for confident identification. Here we discuss HST images of the field of the luminous X-ray source in the globular cluster NGC 6441, and suggest a candidate for the optical counterpart.
Although the X-ray position is known to a precision of a few arcseconds, this source is only \sim4'' from the cluster core, and the field is very crowded. Our multicolor WFPC2 images reveal a markedly UV-excess object with m_336=17.69,\ m_439=18.09, well within the X-ray error box. Correcting for substantial reddening (E_B-V=0.42) and assuming a T_eff=20,000 K blackbody spectrum, we infer B_0=17.0,\ (U-B)_0=-1.0, clearly an unusual star for a globular cluster; its luminosity is similar to M15 AC 211. The identification with the X-ray source is not totally secure, however, due to a second unusual aspect of these data: the cluster exhibits a surprisingly large population of UV-excess objects, spanning a wide range of luminosities. The probability of a chance coincidence of one of these objects in the small X-ray positional error box is still significant, and further study will be needed to confirm the identification.
This star should not be confused with another interesting and very rare object in NGC 6441, just 25'' distant, namely a planetary nebula discovered by Jacoby et al. Curiously, the UV magnitude and colors of the nebula's central star are similar to that of our X-ray candidate.
This work has been supported by NASA Grant NAG5-1630.
Program listing for Tuesday