EXPLORING THE ORIGIN OF THE SOFT-X-RAY BACKGROUND WITH ROSAT DEEP OBSERVATIONS

Images from the ROSA T X-ray observatory have been used to probe the origin of the soft X-ray background below a discrete source detection limit S(0.5-2 keV) approximately 1 X 10(-14) erg s-2 cm-2. By analyzing the energy-dependent autocorrelation function of the background in two ROSAT deep-pointing observations, we have statistically measured the spectrum of pointlike sources that are still embedded in the background. With this source spectrum, we have then decomposed the background into a pointlike source contribution characterized by a power law dI/dE almost-equal-to 9.5E(-0.7) keV s-1 cm-2 KeV-1 sr-1 and a diffuse contribution represented by a two-temperature plasma. In addition to the soft thermal component that arises primarily in the 10(6) K gas of the Local Bubble, a hard diffuse thermal component is present in the background spectrum, characterizing an important phase of the interstellar or intergalactic medium of a temperature 2-3 X 10(6) K.