Sgr A* at low radio frequencies: Giant Metrewave Radio Telescope observations

Abstract

The central region of the Galaxy has been observed at 580, 620 and 1010 MHz with the Giant Metrewave Radio Telescope (GMRT). We detect emission from Sgr A*, the compact object at the dynamical centre of the Galaxy, and estimate its flux density at 620 MHz to be 0.5 ± 0.1 Jy. This is the first detection of Sgr A* below 1 GHz, which along with a possible detection at 330 MHz provides its spectrum below 1 GHz. Comparison of the 620-MHz map with maps made at other frequencies indicates that most parts of the Sgr A West H ii region have optical depth ∼2. However, Sgr A*, which is seen in the same region in projection, shows a slightly inverted spectral index between 1010 and 620 MHz. This is consistent with its high-frequency spectral index, and indicates that Sgr A* is located in front of the Sgr A West complex, and rules out any low-frequency turnover around 1 GHz, as suggested by Davies, Walsh & Booth.