Circular No. 4043 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 GX 5-1 (4U 1758-250) M. van der Klis, European Space Agency, Noordwijk; F. Jansen, Space Research Laboratory, Leiden; J. van Paradijs, Astronomical Institute, Amsterdam; W. H. G. Lewin, Massachusetts Institute of Technology; and J. Trumper and M. Sztajno, Max-Planck-Institut fur Extraterrestrische Physik, Garching, communicate: "EXOSAT observations of the bright galactic-bulge source GX 5-1 made during 1984 Sept. 18.46-18.83 UT with a time resolution of 0.25 ms show the presence of quasiperiodic oscillations of the 1-10-keV flux with a typical period between 25 and 50 ms. The centroid frequency f of the highly significant (27-sigma) broad peak in the power spectrum is strongly correlated with the source intensity I with (d log f)/ (d log I) ~ 2.5 over a range in I from 2400 to 3100 counts/s (1 count/s ~ 0.3 ufu). With increasing source brightness the FWHM of the peak in the power spectrum (which is ~ 25 percent of the centroid frequency) increases, and its integrated excess power decreases from a maximum value equivalent to that of an 8-percent sinusoidal modulation to become undetectable for I > 3100 cts/s. Since this is the discovery of a new phenomenon, we urge observers to search for similar x-ray behavior in other sources and corresponding optical behavior in optically identified x-ray binaries." EXO 0748-676 H. Quintana, Department of Astronomy, Catholic University of Chile; and R. A. Wade, Steward Observatory, University of Arizona, report identification of this x-ray transient (IAUC 4039) with a faint stellar image on CCD records obtained with the Yale 1-m telescope at Cerro Tololo. This image is not seen on the SRC J Survey film. The position from offsets measured with the Cerro Tololo 4-m telescope from SAO Catalog stars is R.A. = 7h48m24s1, Decl. = -67deg37'27" (uncertainty 3"; equinox 1950.0). Preliminary magnitude estimates from CCD images on Mar. 1 gave Gunn r = 17.3, Gunn g = 17.2, Johnson B = 17.5. The source is variable in g over a period of 4 hr. Preliminary inspection of three 20-min spectra obtained with the 4-m telescope in the spectral region 720-880 nm shows a featureless continuum. The slope is similar to that of the Gunn-Oke standard star HD 84937. 1985 March 13 (4043) Brian G. Marsden
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