D. R. Parsignault

Observational constraints on the models for Cygnus X-3

Observations of Cygnus X-3 made by the ANS and Uhuru satellites have shown significant X-ray flux variability on a time scale of hours. No spectral change was observed associated with these fluctuations. Significant intensity and spectral changes were found on a time scale of months with an inverse correlation between the source intensity and the temperature, and possibly the low-energy cutoff. The spectrum studies have confirmed the existence of an excess above the fitted continuum which can be interpreted as line emission of Fe XXIV and/or Fe XXV at about 6.7 keV. Phenomenological models for Cygnus X-3 are discussed; the observational data are best interpreted by assuming that a scattering and absorbing shell is symmetrically placed around the X-ray source.

ANS observations on the X-ray burster MXB1730—335

A NEW type of time variability of cosmic X-ray sources (‘bursters’) was discovered from Astronomical Netherlands Satellite (ANS) observations on the source 3U1820–30, associated with the globular cluster NGC6624 (refs 1, 2). These X-ray bursts are characterised by a rapid rise in flux ( ∼ 0.5 s) to a relative intensity comparable to the Crab Nebula, followed by an exponential decay at a time scale of ∼ 10 s. Certain characteristic features of the bursts, in particular the shape and spectral hardening, are consistent with a model where the X-ray emitting region is situated in a cloud of hot gas near a massive black hole ( ∼ 1,000M\cirċ) (ref. 3). On March 14, 15 and 16, 1976 the ANS spacecraft was pointed in the direction of the remarkable new X-ray burster MXB1730–335 discovered by SAS-3 (ref. 4) in the constellation Scorpius at 5° from the galactic centre in the galactic plane. The source is characterised by rapidly repetitive X-ray bursts of varying intensity. The energy in a given burst is approximately linearly proportional to the time interval to the next burst5. We report here an improved position of this source and give the energy spectrum from data obtained in four observations for which quick-look data are available. An upper limit for the steady source level in between the bursts is obtained.

ANS observations of Cygnus X-1

Cygnus X-1 was observed from 3 November until 9 November, 1974, using the hard X-ray (1–28 keV) and soft X-ray (1–8 keV) experiment on board the Astronomical Netherlands Satellite. On three occasions, on 4 and 5 November, the X-ray spectrum was observed to be harder, while the flux intensity in the 1–7 keV ranges decreased by ∼50% from its quiescent value. These events occurred near and following the time of superior conjunction of the likely optical counterpart HDE 226 868. These events appear to be the same as previously reported absorption dips and reveal this phenomenon to be more complex than had been believed. A systematic study of the X-ray spectral variations in these energy bands, on a time scale of 64 seconds over the period of the spectroscopic binary, is presented.

Si XIII and Si XIV line emission search with the ANS crystal spectrometer

The final results of the search for Si XIII and Si XIV line emission, using the ANS Bragg crystal spectrometer, are presented. No positive line detection was achieved in any of the measurements made on several types of cosmic object; i.e., a transient X-ray source, close binary systems, supernova remnants, GX sources, and clusters of galaxies. Three-sigma upper limits for discrete line emission from these objects are reported and compared with line strengths inferred from other proportional-counter and Bragg spectrometer data. The present upper limit of 1.0 eV for the equivalent width of a narrow Si XIV line emission from the X-ray nova A0620-00 is an order of magnitude lower than previous limits set by other experiments.

Spectral and intensity variations in cygnus X-3 by the Astronomical Netherlands Satellite

Measurements obtained with the Utrecht (1–8 keV) and Cambridge (1–28 keV) instruments on board the Astronomical Netherlands Satellite are discussed. Particularly, the 4.8 hr period is investigated.