IAUC 4043: GX 5-1 (4U 1758-250); EXO 0748-676

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                                                  Circular No. 4043
Central Bureau for Astronomical Telegrams
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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