Measurements of fluctuations in the hard X-ray background with RXTE

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. We have measured the fluctuations in the X-ray background (XRB) from 15-40 keV using differences between adjacent HEXTE background fields. The fluctuations as a fraction of the XRB flux are : 0.092 +/- 0.014 % from 15 to 20 keV, 0.11 +/- 0.02 % from 20 to 25 keV and 0.23 +/- 0.08 % from 34 to 41 keV for a 1.1 square degree field of view. The measured fractional fluctuations increase with energy, which implies that the effective number of contributing sources is decreasing with energy. The fluctuation level and energy dependence are consistent with HEAO-1 results.