Adv. Space Res. Vol.1, pp.2112 14. 0273—1177/81/04010211$O5.OO/O © COSPAR, 1981. Printed in Great Britain. LOW ENERGY OBSERVATIONS OF CYGNUS X-2 BY ARIEL VI A.M. Cruise,’ C.V. Goodall,2 D.K. Bedford,2 D.J. Campbell,2 G.F. Carpenter,2 R.E. Cole,’ J.L. Cuihane,’ J. Osborne,’ A.M.T. Pollock,2 A.P. Willmore2 and J. Zarnecki’ ‘Mullard Space Science Laboratory, Department of Physics and Astronomy, University College London, UK 2 of Space Research, University of Birmingham, UK ABSTRACT Ariel VI observations of Cygnus X-.2 have revealed a rather flat spectrum between 0.1 and 1.5 keV with variable emission at low energy. Of the two conflicting interpretations of this object in termé of i) a distant high—luminosity (L,~~,.~1038 ergs s-i) binary and ii) a nearby low—luminosity (L~..,1o35 ergs s_i) degenerate dwarf system, our measurements support the latter. INTRODUCTION The ~riel VI satellite, launched on behalf of the U.K. Science Research Council on June 2nd 1979 includes, among its three instruments, a set of 4 low energy (nominally 0.1 - 2 key) telescopes built jointly by the Mullard Space Science Laboratory and the University of Birmingham. Each telescope consists of a single paraboloidal mirror, a series of apertures allowing the choice of a field of view with a diameter between 0.2° and 3.90, a shutter for charged particle background determination and a ~A~tm polypropylene-windowed, propane-filled proportional counter. There is no spatial resolution within the field of view and the total geometric area of all 4 telescopes is 65 cm2. During the Cygnus X-2 observation the instrument operated in a mode yielding a succession of 64 second, 32—channel pulse height energy spectra with the shutter alternately open and closed to allow simultaneoua determination of X—ray spectrum and particle background. Contamination by particles is low because the use of a reflector allows a detector of small active volume, protected by additional counters operated in anti—coincidence. The satellite is epin—stabilised with a magneto-torquer pointing system. Due to imperfect balancing of the spacecraft before launch, the spin and telescope axes are misaligned by 1.08° necessitating the use of large apertures with a consequent large contribution from the diffuse X-ray background. RESULTS Cygnus X—2 was observed on 1979 September 18 and 19. It was subsequently deter- mined that, due to an error in the attitude-reconstruction programme which existed at that time, the spin axis was pointing towards a = 325.6°a = 40.3°, about halfway between Cygnus X-2 and SS Cygni. X-ray count rate data for the telescopes ~3A and Q5B are shown in Figure 1 along with the subtracted particle signal. These 211