The simultaneous VLA observations of Sgr A* from 90 to 0.7 cm

Details The simultaneous VLA observations of Sgr A* from 90 to 0.7 cm The simultaneous VLA observations of Sgr A* from 90 to 0.7 cm

2004-12-30

arxiv.org/abs/astro-ph/0412675v1

Astronomy and Astrophysics

Astrophysics

2 pages. 1 figures. Proceedings of the 7th European VLBI Network Symposium held in Toledo, Spain on October 12-15, 2004. Edito

Scientific paper

We present a spectrum of Sgr A* observed simultaneously on June 17, 2003 at wavelengths from 90 to 0.7 cm with the VLA. In the spectrum, we also include the measurements of Sgr A* observed on the same day with the GMRT at 49 cm, the SMA at 0.89 mm and the Keck II at 3.8 $\mu$m. The spectrum at the centimeter wavelengths suggests the presence of a break wavelength at 3.8 cm (8 GHz). The spectral index is alpha=0.43+-0.03 (propto nu(alpha}) at 3.8 cm and shorter wavelengths. The spectrum between 3.8 cm and 49 cm can be described by a power law with spectral index of alpha=0.10+-0.03$. We detected Sgr A* with 0.22+-0.06 Jy at 90 cm, suggesting a sharp decrease in flux density at the wavelengths longer than 49 cm. The best fit to the spectrum at the wavelengths longer than lambda_b appears to be consistent with free-free absorption by a screen of ionized gas with a turnover wavelength at nu(tau_ff}=1) = 100 cm (300 MHz). This turnover wavelength appears to be three times longer than that of 30 cm (1 GHz) as suggested by Davies et al. (1976) based on the observations in 1994 and 1995. Our analysis suggests that stellar winds from the massive stars near Sgr A* could modulate the flux density at the wavelengths longer than 30 cm (or frequencies below 1 GHz).

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