Measuring the granularity of the diffuse hard X-ray background

The diffuse X-ray background is widely held to originate largely from distant active galaxies. This view is supported by both counts of sources in survey missions and fluctuation studies in the soft x-ray energy band. However, most of the x-ray background emission occurs in the 10--60 keV hard x-ray band and sensitive studies in this energy band are only now possible.;The fluctuations in the x-ray background for three wide energy bands have been determined. They are 13.0 +/- 2.1% of XRB flux from 15.5 to 20.2 keV, 15.8 +/- 2.7% from 20.2 to 24.9 keV and 32 +/- 11% from 34 to 41 keV for a 1.1 square degree field of view.;During normal observations, the High Energy X-ray Timing Experiment instrument (HEXTE) on the Rossi X-ray Timing Explorer (RXTE) Spacecraft makes nearly simultaneous observations of two pairs of background fields separated by 3.0 degrees. Differences in fluxes from these pairs of background fields can be used to detect a source population much deeper than can be studied from individual detections. A measurement of the fluctuations in the X-ray background in wide energy bands above 15 keV can be made using the ensemble of all long RXTE pointings, a total of ~36.7 million seconds of observations. A distribution of the sky flux differences can be made from a data set of these long pointings. Tails in the distribution of the differences are dominated by sources that occur at a spatial density of one source per field of view. The nominal HEXTE instrument field of view is one square degree, but this field can be smeared out by seasonal rotation of the spacecraft pointing. An excess variation can be found by comparing the fluctuation distribution to a distribution characterized by a data set that only contains statistical counting errors. This excess variation in wide energy bands is used to create the fluctuation spectrum.;In a field created by a distribution of discrete sources, the fractional fluctuations---the smoothness of the field---is determined by the number of sources which create the field. Knowing the fractional fluctuations, the source density can be calculated.;The spectrum of the source density, when combined with data from the HEAO-1 A4 instrument, results in an energy power law spectral index to the number of sources of --0.64 +/- 0.20 from 15 to 80 keV. This result is in direct disagreement with models of the XRB in which the emission in this energy range is dominated by a class of sources more numerous than Seyfert 1s.