Overview

In this Appendix further results of a periodicity search project in a sample of EXOSAT Medium Energy light curves are reported.

The European Space Agency's X-ray observatory EXOSAT was operational from May 1983 to Apr. 1986. Its low energy imaging telescopes (LE1 and LE2), used in conjunction with the Channel Multiplier Array (CMA), covered the 0.05-2 keV band and provided broad band filter spectroscopy. The Argon chambers of the Medium Energy (ME) proportional counter array operated over the 1-20 keV band and produced spectra (with a resolution of ) and light curves with high throughput. The ME was often operated with one half of the detector array offset from the source in order to provide a simultaneous monitor of the particle background; the pointing directions of the two halves were usually swapped every 3-4 hours in order to minimise the systematic uncertainties in the background subtraction. Usually these changes in the pointing direction are called array swaps (hereafter AS). The Gas Scintillation Proportional Counter (GSPC) provided a factor of 2 improved spectral resolution in the 2-20 keV band, with a factor of 5 lower effective area than the ME (see White & Peacock 1988 for more details).

The project was threefold:

Ultrasoft X-ray sources

A search aimed at detecting fast periodicities in the light curves of X-ray sources characterised by an Ultrasoft (US) spectrum in the ME was performed. This leads to the discovery of 8.7 s pulsations in 4U0142+614 (Israel, Mereghetti & Stella 1994). Data were accumulated using the ESA-ESTEC's on-line interactive analysis system (IA). The characteristics of this source were soon noticed to have striking similarities with a subsample of X-ray pulsators which was proposed as a new class of objects (Mereghetti & Stella 1995; van Paradijs, Taam & van den Heuvel 1995; Mereghetti, Stella & Israel 1996; Stella, Mereghetti & Israel 1996).

Orbital modulations in LMXBs

A search for sinusoidal modulations in the X-ray flux of a sample of 25 low mass X-ray binaries (LMXBs), for periods ranging from a few minutes to hours, were performed (Stella et al. 1996). In the case of 4U1636-549 an X-ray orbital modulation was detected at the known optical period (Israel, Parmar & Stella 1996). We found evidence of an X-ray modulation at the optical orbital period is present also in GX9+9 and LMC X-2. The data were obtained from the EXOSAT database available within High Energy Astrophysics Database Service at the Astronomical Observatory of Brera (Tagliaferri & Stella 1993).

NGC5548

Papadakis & Lawrence (1993a) reported the likely detection of QPOs in five out of eight EXOSAT ME light curves of the bright close-by Seyfert 1 galaxy NGC5548 suggesting that they may arise from instability or variability in an accretion disc around a black hole of only a few solar masses. On the basis of the newly developed technique (see Chapter 3) a detailed search for QPOs was performed by artificially decreasing the original Fourier resolution.

In the next three sections the results outlined above are briefly reported.