Simultaneous Multi-Wavelength Spectrum of Sgr A* and Long Wavelength Cutoff at λ > 30 cm

Abstract

We reviewed the multiple-wavelength spectrum of Sgr A*. The recent continuum spectrum was determined using data obtained from the Very Large Array (VLA), the Giant Metrewave Radio Telescope (GMRT), the Submillimeter Array (SMA) and the Keck II 10m telescope on the same date (2003 June 17). The nearly simultaneous spectrum (S∝ λα) covers a broad wavelength (A) range from 90 cm to 3.8 micron. From about 2 cm to 1 mm, the source shows a rising spectrum with a spectral index of α = 0.43 ± 0.04. However, the IR, measurements suggested that the flux density of Sgr A* must drastically drop towards the shorter wavelengths and the turnover likely occurs in the submillimeter regime. The spectrum at wavelengths between 47 and 3.6 cm is flat with α = 0.11 ± 0.03. The flux density of Sgr A* is 0.22±0.06 Jy at 90 cm on this date in 2003. Compared with measurement at 47 cm, the flux density at 90 cm has decreased by a factor of two (or ∼ 4σ), suggesting a cutoff at wavelength longer than 47 cm. The fits using free-free absorption opacity to the spectrum suggest a cutoff wavelength at ∼ 100 cm. The cutoff wavelength appears to be three times longer than that of ∼ 30 cm determined by Davies, Walsh, & Booth [1] three decades ago based on observations in 1974 and 1975. A model interpreting the variation of the long wavelength cutoff using stellar winds (An et al. [2]) is summarized in this paper.