Circular No. 4101 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION Postal Address: Central Bureau for Astronomical Telegrams Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. TWX 710-320-6842 ASTROGRAM CAM Telephone 617-495-7244/7440/7444 AC 211 M. Auriere, European Southern Observatory, Garching, and Pic du Midi Observatory; J. P. Cordoni, Laboratoire d'Astronomie, Montpellier; and L. Koch-Miramond, C.E.N. Saclay, communicate: "We obtained esposures of AC 211, the possible optical counterpart for the M15 globular cluster x-ray source (Auriere et al. 1984, A.Ap. 138, 415), on the three consecutive nights July 21-23, in good to excellent seeing, using UBV filters with the CCD at the f/25 Cassegrain focus of the 2-m Pic du Midi telescope. The star was generally in a bright state (e.g., the averaged observed magnitude and colors between July 22.00 and 22.08 UT were U = 14.6, U-B = -1.2, B-V = -0.1, all +/- 0.2). We observed a sudden decrease in U on July 22, the star fading to U = 14.6 during the hour preceding July 22.12. These observations confirm that AC 211 has photometric properties that do not fit those of normal globular cluster stars but are consistent with those of some low-mass x-ray binaries. The data obtained up to now suggest a binary composed of a neutron star and a post-main-sequence companion, and possibly an eclipsing system. Because AC 211 is included in the 98-percent- confidence error circle of the Einstein Observatory's HRI observations (Grindlay et al. 1984, Ap.J. 282, L13) it can be considered as the optical counterpart of the M15 x-ray source." GX 349+2 W. H. G. Lewin, Massachusetts Institute of Technology; J. van Paradijs, University of Amsterdam; F. Jansen, Space Research Laboratory, Leiden; M. van der Klis, European Space Agency; and M. Sztajno and J. E. Trumper, Max-Planck Insitut fur Extraterrestrische Physik, telex: "We have detected low-frequency noise (LFN), below ~ 4 Hz, and may have discovered quasiperiodic oscillations (QPO) in the average power spectrum of the fluctuations in the x- ray flux of the bright bulge source GX 349+2 = 4U 1702-36. The observations were made using EXOSAT during 1984 Sept. 11.81-12.04 UT. The significance of the peak in the power spectrum is only 3.0V. The centroid frequency and FWHM of the peak are ~ 11 and ~ 7.5 Hz, respectively. The integrated excess power in the peak and the LFN (integrated down to 0.5 Hz) correspond to rms variations of the signal of 2 and 6 percent, respectively. This may be the fifth bright x-ray bulge source to show the LFN/QPO phenomenon." 1985 September 6 (4101) Brian G. Marsden
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