High resolution temporal studies of Cygnus X-1.
thesisposted on 19.11.2015, 09:19 authored by Alan B. Giles
Rapid variability in X-ray sources implies a compact object such as a white dwarf, neutron star or black hole. Following a review of the best known examples and their associated models, the observational history of Cygnus X-1 is outlined. This source is generally believed to be a black hole since it appears to exhibit rapid variability (millisecond bursts) and has a mass above the theoretical neutron star limit. This thesis is concerned with an attempt to study the rapid variability of the source using a multiwire array deploying detector system with a sensitive area of 4,000 cm2 (1.5 - 15 keV). The chance expectation value of millisecond bursts as seen by previous smaller experiments would be decreased from 10-2 to 10-9. The anticipated high count rate required the use of a data interactive electronics system together with a large buffer store to optimise use of the available telemetry data rate. This allowed timing of the X-ray events to 2 mS together with a large transient handling capacity so that the system was no longer severely limited at the very moment that the information was of most interest. The experiment was flown from Woomera in November 1976 and functioned correctly but the attitude control system failed providing only 6 seconds of Cygnus X-1 data at reduced sensitivity. No other interesting observations were made while the experiment's field of view drifted across the celestial sphere. Investigations of the shot noise character of Cygnus X-1 are not inconsistent with previous observations, but the short data set is unsuitable for such studies. A search for rapid bursts using the same analysis methods as previous workers was also inconclusive, the presence of the calibration source flux possibly being crucial in limiting the probability of a number of borderline features. The statistical interpretation of burst phenomena is discussed leading to the concept that the size of the features seen by SL-1306 are similar numerically to those reported by Rothschild et al. (1976) and therefore represent activity at approximately one-third the intensity. Such features occurring during the non-enhanced low states would not be detected with the customary analysis by previous smaller detectors. A frequency of ~ 1 per sec is suggested for these mini bursts, the duration being 0.5 mS, the bursts appearing to be stronger in the latter 0.25 mS. The author notes with considerable regret that two re-flight proposals were not approved.