ON THE ORIGIN OF THE SOFT-X-RAY BACKGROUND

Using two deep images from the ROSAT X-ray satellite, we have studied the angular autocorrelation function and spectrum of the soft X-ray background below a discrete source detection limit S(0.5-2 keV) almost-equal-to 1 x 10(-14) ergs s-2 cm-2. The autocorrelation function is used to determine the average spectral shape of pointlike sources that are still embedded in the background. We find that pointlike sources can account for 40%-60% of the observed background intensity in the 0.5-0.9 keV band (M band). The background spectrum is then decomposed into a pointlike source component characterized by a power law dI/dE almost-equal-to 9.5 E-0.7 keV s-1 cm-2 keV-1 sr-1 and a diffuse component represented by a two-temperature plasma. We find that whereas the 10(6) K gas in the Local Bubble dominates the background below about 0.3 keV, about 40% of the background radiation in the M band arises in a diffuse thermal component with a temperature of approximately 2.2 x 10(6) K. This component may represent an important constituent of the interstellar or intergalactic medium.