Simultaneous Multiwavelength Observations of Sagittarius A*

We observed Sgr A* using the Very Large Array (VLA) and the Giant MetrewaveRadio Telescope (GMRT) at multiple cm and mm wavelengths on 17 June 2003. Themeasured flux densities of Sgr A*, together with those obtained from theSubmillimeter Array (SMA) and the Keck II 10 m telescope on the same date, areused to construct a simultaneous spectrum of Sgr A* from 90 cm to 3.8 $\mu$m.The simultaneous spectrum shows a spectral break at about 3.6 cm, a possiblesignature of synchrotron self-absorption of the strong radio outburst whichoccurred near epoch 17 July 2003. At 90 cm, the flux density of Sgr A* is$0.22\pm0.06$ Jy, suggesting a sharp decrease in flux density at wavelengthslonger than 47 cm. The spectrum at long cm wavelengths appears to be consistentwith free-free absorption by a screen of ionized gas with a cutoff $\sim$100cm. This cutoff wavelength appears to be three times longer than that of$\sim$30 cm suggested by Davies, Walsh & Booth based on observations in 1974and 1975. Our analysis suggests that the flux densities of Sgr A* atwavelengths longer than 30 cm could be attenuated and modulated by stellarwinds from massive stars close to Sgr A*.Comment: 15 pages, 4 figures, accepted by ApJ Lette