标题： H$_2$O 水委缩星发射在NGC 4258中表明了盘不稳定性存在周期性 显示英文标题

标题： H$_2$O MegaMaser emission in NGC 4258 indicative of a periodic disc instability

摘要： H$_2$O 水 mega 火焰发射可能源自围绕星系中心大质量核的薄气体盘，例如 NGC 4258，但导致放大发射的物理条件尚不清楚。 通过空间甚长基线干涉测量（SVLBI）提供的非常高的角分辨率，可以详细观察这些区域。 这里我们报告了使用轨道上的 RadioAstron 观测站进行的 SVLBI 实验，该实验在 NGC 4258 中检测到了 H$_2$O 22 GHz 发射，具有 1.3、9.5 和 19.5 个地球直径的地-空基线。 在最高的角分辨率 11 和 23 $\mu$时的观测显示，在约 0.126 秒差距的轨道半径内，旋转盘中存在明显且有规律间隔的区域。 这三次后续观测也表明发射特征的时间演化，表现为幅度突然上升后缓慢衰减。 发射区域的形成、它们的规则间隔以及它们的时间依赖行为似乎与盘中周期性磁旋转不稳定性事件的发生一致。 这种由不同旋转盘内的剪切驱动的不稳定性被认为是控制吸积盘径向动量传递和粘度的机制。 H$_2$O mega 火焰活动与磁旋转不稳定性活动的联系将使其成为宿主星系核盘中吸积率的指示器。 显示英文摘要

摘要： H$_2$O MegaMaser emission may arise from thin gas discs surrounding the massive nuclei of galaxies such as NGC\,4258, but the physical conditions responsible for the amplified emission are unclear. A detailed view of these regions is possible using the very high angular resolution afforded by space very long baseline interferometry (SVLBI). Here we report SVLBI experiments conducted using the orbiting RadioAstron Observatory that have resulted in detections of the H$_2$O 22 GHz emission in NGC\,4258, with Earth-space baselines of 1.3, 9.5 and 19.5 Earth diameters. Observations at the highest angular resolution of 11 and 23 $\mu$as show distinct and regularly spaced regions within the rotating disc, at an orbital radius of about 0.126 pc. These observations at three subsequent epochs also indicate a time evolution of the emission features, with a sudden rise in amplitude followed by a slow decay. The formation of the emission regions, their regular spacing and their time-dependent behaviour appear consistent with the occurrence of a periodic magneto-rotational instability in the disc. This type of shear-driven instability within the differentially rotating disc has been suggested to be the mechanism governing the radial momentum transfer and viscosity within a mass-accreting disc. The connection of the H$_2$O MegaMaser activity with the magneto-rotational instability activity would make it an indicator of the mass-accretion rate in the nuclear disc of the host galaxy.