Parkes observations for project P990 semester 2021OCTS_02

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, toward a small sample of sources showing OH absorption against pulsars. If successful, this will be the 2nd such detection as well as the first in a species other than OH. We have successfully processed UWL baseband data to confirm Weisberg et al. (2005) results with potential new results, and thus confident of the feasibility of the proposed observations. We thus propose to obtain emission, absorption, and stimulated emission of HI, OH, and CH, simultaneously. 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.

目前已发表的唯一一例天然快速切换受激羟基（OH）辐射实例，是在脉冲星PSR B1641-45方向观测到的。无需移动观测波束，我们可直接利用脉冲星时域下的天然开、关信号，作为探测微观物理与星际介质（ISM）结构的独特工具。脉冲星光子被居间星际气体（推测其处于非局部热动平衡(non-LTE)激发态）所放大的现象，同时也为地外文明开展星际通信提供了可行原理。近年来，我们针对羟基（OH）与碳氢（CH）的λ双重跃迁（λ-doubling transitions）开展了系统性研究，结果表明二者均可有效示踪所谓的“暗分子气体（dark-molecular-gas）”。二者在天体化学条件、空间分布与能级结构上的相似性，促使我们尝试在以脉冲星为背景、显示羟基吸收的小样本源方向，探测碳氢的受激辐射。若探测成功，这将是继羟基之后，第二种被观测到的此类受激辐射，同时也是首次在非羟基分子中发现该现象。我们已成功处理UWL基带数据（UWL baseband data），在验证Weisberg等人（2005）的研究结果的同时获得了潜在新成果，因此确信本次拟开展观测的可行性。据此我们提出同时获取中性氢（HI）、羟基（OH）与碳氢（CH）的辐射、吸收与受激辐射观测数据。对帕克斯波束（Parkes beam）内受激辐射（或未探测到该辐射的情况）与多种其他辐射/吸收成分开展联合分析，将显著提升我们对星际介质（ISM）的认知水平。