Figure 6: The x-ray spectrum is a measure of the number of events detected by the CCDs as a function of energy. The red line is the x-ray spectra detected by one of the highest-performing nodes in the REXIS detector array before the cover opened. There is a peak 5.9 keV (the blue shaded region) from the cover-mounted internal Fe-55 x-ray calibration source. The low energy spectrum is due to only internal noise. The spectrum after the cover opened for the same node (black line) shows detection of a low-energy continuum consistent with predictions of the cosmic x-ray background (blue dashed line) and a weakening of the Fe-55 line indicating that the instrument is now looking at cosmic x-rays from space instead of the underside of the cover.
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