SIMBAD references

The spectrum of Sgr A*, the compact synchrotron source supposedly located at or near the dynamical center of the Galaxy, has an upper cut-off and a lower turnover (Zylka et al. 1995; Davies, Walsh and Booth 1976) and a radio/IR luminosity L_radio,IR[1-10⁴GHz]∼310²L_☉. In this paper we fit the high-frequency (ν>1.5GHz) part of the spectrum with optically thin synchrotron emission of quasi-monoenergetic relativistic electrons and discuss free-free absorption due to the HII region Sgr A West and synchrotron self-absorption in Sgr A* as two possible physical processes that could explain the low-frequency (ν≲1.5 GHz) turnover in the radio spectrum of Sgr A*. We show that both processes must occur, that the corresponding models can fit the time-averaged Sgr A* radio spectrum and that the best-fit model parameters are in accordance with currently available observations. It is found that Sgr A* must be 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 discriminate between the two absorption processes if the shape of the low-frequency spectrum ν< 1.5GHz is more accurately determined. We discuss X-ray 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 position of Sgr A*. Since our radio observations exclude a large intrinsic X-ray absorption, Sgr A* is - if at all - only a low-luminosity X-ray emitter (L_X[0.5-10keV]≲210²L_☉). Absorption by Interstellar Matter between Sun and Galactic Center prevents any direct observations of Sgr A* from the NIR (λ< 1µm) to the soft X-ray (E≲1keV) domain. Indirect observations suggest an upper limit for the optical/UV luminosity of Sgr A* of ≲10^4...5L_☉.