H₂O 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 H₂O 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 resolutions of 11 and 23 μas 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 these 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 H₂O 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.

H ₂ O MegaMaser 发射可能来自围绕星系大质量核的薄气体盘，例如 NGC 4258，但造成放大发射的物理条件尚不清楚。使用空间超长基线干涉仪 (SVLBI) 提供的非常高的角分辨率，可以查看这些区域的详细视图。在这里，我们报告了使用轨道 RadioAstron 天文台进行的 SVLBI 实验，这些实验导致了 H ₂O NGC 4258 中的 22 GHz 发射，地球-空间基线为 1.3、9.5 和 19.5 地球直径。在 11 和 23 μas 的最高角分辨率下的观测显示旋转盘内的明显且规则间隔的区域，轨道半径约为 0.126 pc。在随后的三个时期的这些观察也表明了发射特征的时间演变，幅度突然上升，然后缓慢衰减。这些发射区域的形成、它们的规则间距和它们的时间相关行为似乎与圆盘中周期性磁旋转不稳定性的发生一致。这种类型的差动旋转盘内的剪切驱动不稳定性被认为是控制质量积累盘内径向动量传递和粘度的机制。H的连接₂ O MegaMaser 活动与磁旋转不稳定性活动将使其成为宿主星系核盘中质量吸积率的指标。