The spectrum of the cosmic X-ray background observed by RTXE/PCA

We have analyzed a large set of RXTE/PCA scanning and slewing observations performed between April 1996 and March 1999. We obtained the 3-20 keV spectrum of the cosmic X-ray background (CXB) by subtracting Earth-occulted observations from observations of the X-ray sky at high galactic latitude and far away from sources. The sky coverage is approximately similar to22.6 x 10(3) deg(2). The PCA spectrum of CXB in 3-20 keV energy band is adequately approximated by a single power law with photon index Gamma similar to 1.4 and normalization at 1 keV similar to 9.5 phot/s/cm(2)/keV/sr. Instrumental background uncertainty precludes accurate RXTE/PCA measurements of the spectrum of cosmic X-ray background at energies above 15 keV and therefore we cannot detect the high energy cutoff observed by the HEAO-1 A2 experiment. Deep observations of the 6 high latitude points used to model the PCA background provide a coarse measure of the spatial variation of the CXB. The CXB variations are consistent with a fixed spectral shape and variable normalization characterized by a fractional rms amplitude of similar to7% on angular scales of similar to1 square deg.