Observations of the Galactic Centre at 610 MHz with the GMRT

Subhashis Roy1, A. Pramesh Rao1

1 National Centre for Radio Astrophysics (TIFR),
Pune University Campus, Post Bag No.3, Ganeshkhind, Pune 411 007, India.

E-mail contact: roy@ncra.tifr.res.in

We have observed the central region of the Galaxy at 610 MHz with the Giant Metrewave Radio Telescope (GMRT). Emission from the central black hole candidate known as the Sgr-A* has been detected at a frequency of 610 MHz, which is the lowest frequency detection of this object. The estimated flux density of this object is 0.6 Jy. Compared to its flux density at 1.4 GHz, the estimated spectral index is quite flat. However, a comparison of the 610 MHz and 1.4 GHz map of this region indicates that most parts of the Sgr-A West HII region have optical depth 1. Therefore, the flat spectral index of sgr-A* between 1.4 GHz and 610 MHz, which is similar to its high freqency spectral index, indicates that the Sgr-A* is located in front of the Sgr-A West complex and is not embedded within this complex as was suggested before. Thermal free-free absorption by the Sgr-A West is unlikely to play an important role for it to be not detected at 330 MHz.