Mullard Space Science Laboratory

A. C. Fabian
The small-scale isotropy of the cosmic X-ray background

1972 (supervisor: P. W. Sanford)

Although the cosmic X-ray background was discovered ten years ago, no entirely satisfactory theory for its origin has been proposed. All agree that the origin is extragalactic and therefore is of cosmological significance. The extent of this significance may be revealed by detailed experimental analyses of the spectrum and isotropy. The work described in this thesis is an attempt to analyse the small-scale isotropy of the X-ray background, as measured by a rocket-borne detector, and prepare the groundwork for future experimental investigations.

A review of observational and theoretical studies of the cosmic X-ray background is presented, followed by a discussion of methods for reducing the environmental background in proportional counters. It becomes obvious in the first chapter that the environmental background (cosmic rays etc.) are a major limitation to the experimental investigation. A rocket payload was developed, launched, and the transmitted data of the cosmic X-ray background was analysed for fluctuations intrinsic to the nine areas of sky sampled. Using methods similar to those employed in the log N - log S plots of radio astronomy, a lower limit of 10^7 sources in the whole sky, of equal absolute brightness but distributed randomly in a sphere surrounding the observer, was deduced. A completely isotropic background is not ruled out, but theoretical considerations cause an expectation of granularity on some scale. A further deduction is that this scale may be of dimensions due to superclusters or just "ordinary" clusters of galaxies.


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