Rocket observations of Virgo XR-1

Abstract

Skylark 723, launched at 10.30 GMT on 12th June 1968 was an unstabilised rocket carrying two proportional counter detectors, each of 1385 cm2 effective area. Detector C1 covered the energy range 1.4 to 2.5 keV and detector C2 the range 2.0 to 18 keV. The detector outputs were analysed into 4 and 9 energy channels respectively. The field of view of each detector was 28° and 4° (FWHM) with the greater collimator extension mounted parallel to the longitudinal rocket axis for C2 and 6/10 of the C1 detector, the remaining 4/10 of the C1 collimator being canted at 40° to the major axis. During the flight, the rocket spun at a constant rate of 75° per sec whilst the spin axis slowly precessed about a flat cone, thereby surveying some 80% of the visible sky. On 25 consecutive scans the Vir XR-1 source, which by chance lay near the precession axis, crossed the field of view of both detectors. The count rate profiles for twenty scans in which the source passed within 21° of the centre of the field of view have been added together for each detector, resulting in the totals shown in Figure 1. Significant peaks are seen in the position of Vir XR-1 for both detectors and these have been fitted with the measured transmission profiles of the respective collimators assuming parallel incident radiation. The peaks are consistent with the observation of X-rays from a single point source.