ANATOMY OF THE SAGITTARIUS-A COMPLEX .5. INTERPRETATION OF THE SGR-A-ASTERISK SPECTRUM

The spectrum of Sgr A, the compact synchrotron source supposedly loca ted at or near the dynamical center of the Galaxy, has an upper cut-of f and a lower turnover (Zylka et al. 1995; Davies, Walsh and Booth 197 6) and a radio/IR luminosity L(radio,IR)[1 - 10(4) GHz] similar to 3 1 0(2) L.. In this paper we fit the high-frequency (nu greater than or s imilar to 1.5 GHz) part of the spectrum with optically thin synchrotro n emission of quasi-monoenergetic relativistic electrons and discuss f ree-free absorption due to the HII region Sgr A West and synchrotron s elf-absorption in Sgr A as two possible physical processes that could explain the low-frequency (nu less than or similar to 1.5 GHz) turnov er in the radio spectrum of Sgr A. We show that both processes must o ccur, that the corresponding models can fit the time-averaged Sgr A r adio spectrum and that the best-fit model parameters are in accordance with currently available observations. It is found that Sgr A must b e located close to the center of Sgr A West and in front of the Bar of the Minispiral as seen from the Sun. In principle one can discriminat e between the two absorption processes if the shape of the low-frequen cy spectrum nu < 1.5 GHz is more accurately determined. We discuss X-r ay emission from Sgr A based on recent ROSAT and ASCA observations of a weak soft X-ray source that coincides within 10 '' with the radio p osition of Sgr A. Since our radio observations exclude a large intrin sic X-ray absorption, Sgr A is - if at all - only a low-luminosity X- ray emitter (L(x)[0.5 - 10 keV] less than or similar to 2 10(2) L.). A bsorption by Interstellar Matter between Sun and Galactic Center preve nts any direct observations of Sgr A from the NIR (lambda < 1 mu m) t o the soft X-ray (E less than or similar to 1 keV) domain. Indirect ob servations suggest an upper limit for the optical/UV luminosity of Sgr A of less than or similar to 10(4) ... 5 L..