Hard X-ray observations near Ariel 1118–61

Abstract

OBSERVATIONS made by Ariel V in the region of Cen X-3 have revealed a new transient X-ray source with a periodicity of 6.75 min (refs 1 and 2). Tentative identifications are: with a Mira-type variable star by Fabian et al.3, who suggest the 6.75 min periodicity is associated with the rotation period of a compact companion; and with a heavily reddened, variable ultraviolet star4. As well as the proportional counter observations during the period 1974 December 17 to 1975 January 31, the Imperial College crystal scintillator, sensitive to the photon energy range 26 keV to 1.2 MeV, viewed the same region of the sky. This detector has a sensitive area of 8 cm2, an opening angle of 8° FWHM defined by an active collimator and is offset from the spacecraft spin axis by an angle of 3°. A source a few degrees from the direction of the spin axis can be detected by determining the modulation in the counting rate seen in four equal angle spin sectors with a time resolution of 512 s. A single channel (50–190 keV) mode with a 48 s time resolution was also used. Since a spurious modulation is induced in the apparatus by cosmic rays, two pointing direction offsets on opposing sides of the source have to be used, and the X-ray contribution is obtained by a subtraction technique.