LBA observations for project V630 semester 2023OCTS

White dwarfs (WDs) are the final evolutionary stage of stars not massive enough to become a neutron star or black hole. Non-accreting white dwarfs are almost never detected at radio frequencies. Currently, the only known radio-bright white dwarf system (excluding cataclysmic variables) is AR Scorpii (AR Sco), which is a WD-star close binary. Its radio signal pulses with the WD's spin period, earning AR Sco the moniker ``white dwarf pulsar''. However, the exact radio emission mechanism remains unknown in the source, causing scepticism about whether such a moniker is warranted. Localising and resolving the emission region within a WD-star stellar system would determine the emission mechanism and validate current theoretical models for such systems. Unfortunately, the angular separation in AR Sco is too small to resolve with current VLBI facilities. Here, we propose LBA observations of a newly discovered radio-bright non-accreting WD-star system with properties that suggest it is similar to AR Sco. This system is significantly closer to us and possesses a larger separation (around 3.5 mas) between the star and the WD than AR Sco. The position of this system in the southern sky implies LBA is the only VLBI array that can resolve it. With our proposed observations, we will determine if the emitter remains unresolved on few-mas scales and search for the anticipated orbital motion of the emitter. This, when combined with Gaia data or followup radial velocity observations, will allow us to confidently localise the emitter within the system and determine its powering mechanism.

白矮星（White Dwarfs, WDs）是质量不足以形成中子星或黑洞的恒星演化最终阶段。不吸积的白矮星几乎从未在射电波段被探测到。目前已知的唯一一类射电明亮的白矮星系统（不含激变变星）是天蝎AR（AR Scorpii, AR Sco），它是一颗白矮星与另一恒星组成的密近双星。其射电信号随白矮星的自转周期发生脉动，因此AR Sco获得了"白矮星脉冲星"的绰号。然而，该系统的确切射电辐射机制仍未明确，这引发了对这一称谓是否恰当的质疑。对该白矮星-恒星系统内的辐射区域进行定位与分辨，将有助于明确其辐射机制并验证当前针对此类系统的理论模型。遗憾的是，AR Sco的角间距过小，无法通过当前的甚长基线干涉测量（Very Long Baseline Interferometry, VLBI）设备进行分辨。本文提出对一颗新发现的射电明亮非吸积白矮星系统开展低频阵列（Low Frequency Array, LBA）观测——该系统的各项性质表明其与AR Sco类似。相较于AR Sco，该系统距离地球更近，且恒星与白矮星间的角间距更大（约3.5毫角秒, mas）。该系统位于南天天区，这意味着LBA是唯一能够对其进行分辨的VLBI阵列。通过本次计划的观测，我们将确认辐射源在数毫角秒尺度下是否仍无法被分辨，并搜寻预期存在的辐射源轨道运动。结合盖亚（Gaia）卫星数据或后续视向速度观测，我们将能够精准定位该系统内的辐射源，并明确其驱动机制。