= Cosmic X-ray background
The <cosmic X-ray background> is the aggregate <X-ray> radiation received from the sky after accounting for foreground and instrumental contributions. In an instrument with limited angular resolution, many distant sources contribute an apparently diffuse signal. Resolving more sources changes the resolved fraction without making their summed radiation cease to be a background.
A measurement therefore requires both an energy spectrum and control of detector backgrounds and sky coverage. The <HEAO 1> A2 experiment measured the cosmic background in the 3–50 keV band, providing a benchmark for separating a broad background spectrum from individual sources.<The High Energy Astrophysics Observatory-1 by NASA>[Scientific highlights, A2 cosmic X-ray background measurement]{ref}
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