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."