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 ~ deg2. The PCA spectrum of CXB in 3–20 keV energy band is adequately approximated by a single power law with photon index and normalization at 1 keV phot/s/cm2/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 ~7% on angular scales of ~1 square deg.