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

Images from the ROSAT X-ray observatory have been used to probe the origin of the soft X-ray background below a discrete source detetion limit S(0.5-2 keV)~1X10^-14 erg s^-2 cm^-2. By analyzing energy-dependent autocorrelation function of the background in two ROSAT deep-pointing observations, we have statistically measured the spectrum of point-like sources that are still embedded in the background. With this source spectrum, we have then decomposed the background into a point-like source contribution characterized by a power law dl/dE~~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 10⁶ 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 2-3X10⁶ K.