IAUC 4140: GX 5-1; 1985P

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                                                  Circular No. 4140
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
     M. van der Klis, Space Science Department, European Space
Agency, Noordwijk; F. Jansen, Laboratory for Space Research,
Leiden; J. van Paradijs, Astronomical Institute, University of
Amsterdam; W. H. G. Lewin, Massachusetts Institute of Technology;
and J. Truemper and M. Sztajno, Max-Planck-Institute for
Extraterrestrial Physics, Garching, telex:  "A 4-ms-time-resolution
EXOSAT observation of GX 5-1 (= 4U 1758-25) between Aug. 27.6-28.6
UT shows no quasi-periodic oscillations (QPO; cf. IAUC 4043 and
van der Klis et al. 1985, Nature 316, 225), with an upper limit on
the strength of a peak in the power spectrum (centroid frequency,
c, between 6 and 60 Hz and 0.1 < FWHM/nuc < 1.0) corresponding to
an rms variation in the source flux of roughly 1 percent.  Low-
frequency noise is weak (roughly 2 percent rms above 0.03 Hz) and
only detected below 0.5 Hz.  The source flux varied over the same
range as during the first observation when QPO were seen (IAUC
4043).  However, the x-ray spectral state is different.  In both
cases, the average x-ray spectrum, as measured with the gas
scintillation proportional counter (GSPC), is well fitted between 2 and
20 keV by a function approximating an unsaturated Comptonization
spectrum (cf. White et al. 1985, Ap. J. 296, 475; GAMMA = 1) plus
interstellar absorption with NH = 4-5 x 10**26 m**-2 (more complex
models cannot be excluded, but are not required).  When no QPO are
seen, the average spectrum is significantly softer (with a
characteristic temperature of 2.9 keV) than when they are seen (
temperature 4.5 keV).  When no QPO are seen, the (6-10, 3-6 keV) GSPC
hardness ratio, H, is strongly correlated with the 3-10 keV count
rate, I, with dH/dI ~ 2 x 10**3 (behavior like that of Sco X-1).
When they are seen, dH/dI is ~ -0.5 x 10**-3 ('horizontal branch';
cf. Shibazaki and Mitsuda 1983, ISAS Research Note 234).  Observations
in 1985 Apr. and Sept. confirm the characteristics of the
hard/QPO state, while observations on Aug. 29 and 30 confirm those
of the soft/no-QPO state.  Thus, the data strongly suggest that
there is a correlation between the spectral state and the
occurrence of QPO in GX 5-1."

     J. Hillier, M. Cropper, and F. Freeman, Anglo-Australian
Observatory, report that a low-dispersion spectrogram obtained on
Nov. 3 with the Anglo-Australian Telescope confirms that the
supernova is probably of type II (cf. IAUC 4124).

1985 November 18               (4140)            Daniel W. E. Green

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