A search for the 55 MHz OH line

TL;DR

This study searches for the elusive 55 MHz hyperfine OH line near W44 using GMRT, setting upper limits on its emission and exploring conditions for maser activity in the interstellar medium.

Contribution

First observational search for the 55 MHz OH hyperfine line using GMRT, providing upper limits and diagnostic insights into level inversion conditions.

Findings

No detection of the 55 MHz line, with a 3-sigma upper limit of ~17.3 Jy.

Established constraints on maser amplification of background synchrotron emission.

Identified conditions where hyperfine level inversion could occur in the interstellar medium.

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 Lambda doublet of the J=3/2 state. There are also forbidden M1 transitions between the hyperfine levels within each of the doublet states occuring at frequencies 53.171 MHz 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 near 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 GMRT and the new GMRT Software Backend(GSB) was performed on this target near W44. We place a 3-sigma upper limit of ~17.3 Jy (at 1 km/s velocity resolution) for the 55 MHz line from this region. This corresponds to an upper limit of 3 X 10^8 for the amplification of the Galactic synchrotron emission providing the background.