Simultaneous spectrum of Sgr A * and low frequency cutoff

We present a simultaneous spectrum of Sgr A* observed on June 17, 2003. 1.3 and 0.7 cm were measured using the VLA and at 47 cm using the GMRT. On the same date, the SMA and Keck II observed Sgr A* at 0.89 mm and at 3.8 µm, respectively. In agreement with Falcke et al (1998), the spectrum at centimeter wavelengths suggests the presence of a break near λ b ∼3.6 cm (8.3 GHz). The spectrum between 0.89 mm and 3.6 cm can be fitted with a power law with a spectral index of α 0.89mm 3.6cm = 0.43 ± 0.04, while the spectrum between 3.6 cm and 47 cm can be described by a flat power law with a spectral index of α 3.6cm 47cm = 0.11 ± 0.03. Sgr A* is measured at 90 cm with a flux density of 0.22 ± 0.06 Jy, suggesting a sharp decrease in flux density at wavelengths longer than 47 cm. The spectrum below 640 MHz (λ > 47cm) appears to be consistent with free-free absorption by a screen of ionized gas with a cutoff wavelength at λ(τ ff = 1) near 100 cm. However, this cutoff wavelength appears to be three times longer than that of 30 cm (1 GHz) as suggested by Davies et al. (1976) based on observations in 1974 and 1975. Our analysis suggests that the flux density at wavelengths longer than 30 cm could be attenuated and modulated by stellar winds from massive stars in the vicinity of Sgr A*.