A search for the 55-MHz OH line

Abstract

The OH molecule, found abundantly in the Milky Way, has four transitions at the ground-state rotational level (J= 3/2) at cm wavelengths. These are E1 transitions between the F⁺ and F⁻ hyperfine levels of the Λ doublet of the ²π_3/2, J = 3/2 state. There are also forbidden M1 transitions between the hyperfine levels within each of the doublet states, occurring at frequencies of 53.171 and 55.128 MHz. These are extremely weak and hence difficult to detect. However, there is a possibility that the level populations giving rise to these lines are inverted under special conditions, in which case it may be possible to detect them through their maser emission. We describe the observational diagnostics for determining when the hyperfine levels are inverted and identify a region around W44 where these conditions are satisfied. A high-velocity-resolution search for these hyperfine OH lines using the low-frequency feeds on four antennas of the Giant Metrewave Radio Telescope (GMRT) and the new GMRT Software Backend was performed on a target surrounding W44. We place a 3σ upper limit of ^~17.3 Jy (at 1 km s⁻¹ velocity resolution) for the 55-MHz line from this region. This corresponds to an upper limit of 3×10⁸ for the amplification of the Galactic synchrotron emission providing the background.