LBA observations for project V607 semester 2021OCTS

High-mass X-ray binaries hosting a neutron star accretor have only recently been detected as radio sources. While for transient systems, their radio emission is likely caused by jets launched from their accretion flow, the origin of this emission remains unconfirmed for the persistently accreting sources. Lacking large variations in accretion properties that can be tied to changes in radio, three options remain for these persistent sources: (i) thermal/free-free stellar wind emission, or non-thermal/synchrotron emission from (ii) jets or (iii) accretion wake shocks. The stellar wind emission is expected to have a low brightness temperature (1e4-1e5 K) and large extent (100-200 binary orbits), especially compared to higher brightness temperatures (up to 1e12 K) possible for the synchrotron processes. Therefore, we propose a 12-hour single-run LBA observation of the radio-brightest neutron star high-mass X-ray binary, 4U 1700-37. If the source is unresolved at a 2 mas resolution, we can rule out the thermal emission scenario from both brightness temperature and size arguments. If the emission is instead resolved, its morphology will distinguish between a wind or jet scenario. Then, we will independently measure the wind mass loss rates and velocity, or the jet emission height, depending on the emission origin, while we place limits on the other mechanism.拥有中子星吸积体（neutron star accretor）的大质量X射线双星（High-mass X-ray binaries, HMXBs）直到近年才被探测为射电源。对于暂现源系统，其射电辐射大概率由吸积流驱动的喷流（jets）产生，但对于持续吸积的源，该辐射的起源仍未明确。由于缺乏可与射电辐射变化关联的吸积特性大幅变动，这类持续吸积源的辐射起源尚存三种候选机制：(i) 热辐射/自由-自由星风辐射，或(ii) 源自喷流或(iii) 吸积尾激波（accretion wake shocks）的非热辐射/同步辐射（synchrotron emission）。星风辐射的亮温度（brightness temperature）预计较低（10^4~10^5 K）且空间延展范围较大（可达100~200倍双星轨道半径），相较之下同步辐射过程可实现的亮温度上限可达10^12 K，二者差异显著。据此，我们提议对射电最亮的中子星大质量X射线双星——4U 1700-37开展单次12小时的甚长基线阵列（Long Baseline Array, LBA）观测。若该源在2毫角秒（milliarcsecond, mas）的分辨率下未被分辨，则可通过亮温度与空间尺度两个维度排除热辐射假说。若辐射可被分辨，则其形态学特征可区分星风与喷流两种起源假说。随后，我们将根据辐射起源分别独立测量星风的质量损失率与速度，或喷流的辐射高度，同时对另一类机制设置约束上限。