Observations of the Galactic centre at 610 MHz with the GMRT

Abstract

We have observed the central region of the Galaxy at 610 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 610 MHz to be 0.5 ± 0.1 Jy. This is the lowest frequency at which Sgr A has been detected. Comparison of the 610 MHz and 1.4 GHz map of this region indicates that most parts of the Sgr-A∗ West HII region are optically thick at 610 MHz, having optical depth of ˜2. However, Sgr A∗, which is seen in the same region in projection, shows a flat spectral index between 1.4 GHz and 610 MHz. This is consistent with its high frequency spectral index, which indicates that Sgr A∗ is located in front of the Sgr-A∗ West complex. An intrinsic turnover in its spectrum between 580 and 330 MHz is likely to be the cause of its non detection at 408 and 330 MHz.