Parkes observations for project P990 semester 2018OCTS_01

The only published instance of natural, fast-switching, stimulated OH emission was observed toward PSR B1641-45. Without having to move the beam, the natural ON and OFF signals in the time-domain from a pulsar were successfully utilized as a unique tool for probing microphysics as well as ISM structures. The amplification of pulsar photons by intervening gas, presumably with non-LTE excitation, also demonstrate a feasible principle for interstellar communication by extraterrestrial intelligence. Recently, we have carried out systematic studies of OH and CH ?-doubling transitions, showing that they both well trace the so-called “dark-molecular-gas”. The similarity in their astrochemical conditions, distribution, and energy level structures, motivate us to try to detect stimulated emission in CH. If successful, this will be the 2nd such detection as well as the first in a species other than OH. A combined analysis of stimulated emission (or non-detection) with multiple other emission/absorption components in the Parkes beam, will significant improve our understanding of ISM toward PSR B1641-45 and G339 regions. The new UWL currently under commission at Parkes promises critical new capability of carrying out the proposed observations much more efficiently than before. We thus propose to obtain emission, absorption, and stimulated emission (high-cadence spectra in ‘gated’ mode or baseband recording) of HI, OH, and CH, simultaneously. We also propose to map out the OH/CH emission components in the L-band beam to further resolve the origin of those ISM structures.