ROSAT EXTENDED MEDIUM-DEEP SENSITIVITY SURVEY - AVERAGE SOURCE SPECTRA

We present a spectral analysis of the serendipitous sources detected i n 130 high Galactic latitude, long-exposure ROSAT PSPC observations. B y application of an efficient source detection technique and analyzing a large number of fields, we have detected 2678 sources above our lim iting flux of 1.2 x 10(-15) ergs s(-1) cm(-2) (see Papers I and II). I n this paper, we investigate the spectral properties of the detected s ources. Analysis of hardness ratios as a function of flux confirms the suggestion in the recent work of Hasinger et al. that the source spec tra show a hardening as the source fluxes decrease. Quantitatively, th e hardness ratios are equivalent to a power-law energy index of 0.7 at a flux of 6 x 10(-15) ergs s(-1) cm(-2). In addition to analyzing mea n hardness ratios, we performed detailed spectral fits to the average pulse-height data of the sources as a function of flux. We find that a t fluxes above 2 x 10(-14) ergs s(-1) cm(-2), the average spectrum has a best-fit power-law energy index of 1.4 (after accounting for a Gala ctic stellar component). The best-fit power-law exponent decreases smo othly to 0.4 below a flux of 10(-14) ergs s(-1) cm(-2). This average s pectrum of the faintest sources approaches that observed for the 2-10 keV X-ray background. We discuss the implications of measurements of t he source counts and average spectra for models of the origin of the X -ray background.