Parkes observations for project P916 semester 2016APRS

We propose to search for temporal variations in HI absorption spectra seen against background pulsars to characterise Tiny Scale Atomic Structure (TSAS) in the Milky Way interstellar medium (ISM). We will re-observe four previously studied bright pulsars, extending the experimental baseline to 16 years and providing sensitivity to poorly probed 1000 AU scales. These observations will test predictions that there is a minimum size scale set by the thermal properties of the ISM, below which TSAS is only sporadically seen, and above which it becomes ubiquitous. Due to their unique on-off state, and proper motions of 100 AU per year, our sample of bright pulsars provide the perfect tool to carry out this experiment. Furthermore, our project provides an excellent demonstration of recent developments in the spectral line capability of the CASPSR backend, made possible by modern advances in GPUs. In total we request 50 hours of Parkes time to achieve our science goal of detecting significant opacity variation in the atomic ISM on 1000 AU scales.本研究旨在搜寻以背景脉冲星为参照的中性氢（HI）吸收光谱的时间变化特征，以表征银河系星际介质（interstellar medium, ISM）中的微尺度原子结构（Tiny Scale Atomic Structure, TSAS）。我们将重新观测4颗此前已被研究过的明亮脉冲星，将实验基线延长至16年，从而获得对尚未被充分探测的1000天文单位（astronomical unit, AU）尺度的探测灵敏度。本次观测将验证一项理论预言：星际介质的热性质设定了一个最小尺寸尺度，当结构尺寸小于该尺度时，微尺度原子结构仅偶有出现；而当尺寸大于该尺度时，这类结构将无处不在。鉴于这些脉冲星独特的通断状态，以及每年100天文单位的自行运动，我们选取的明亮脉冲星样本为开展本实验提供了绝佳的研究工具。此外，本项目还将充分展示CASPSR后端系统在光谱线观测能力上的最新进展，这一进展得益于现代图形处理器（Graphics Processing Unit, GPU）的技术革新。我们总计申请50小时的帕克斯（Parkes）望远镜观测时长，以达成我们的科学目标：在1000天文单位尺度上探测到星际原子介质中显著的不透明度变化。