H2O MegaMaser emission in sources such as the prominent NGC 4258 arise from a thin gas disc surrounding the supermassive nucleus. Space Very Long Baseline Interferometry (SVLBI) experiments with the Russian-built RadioAstron Observatory in Earth orbit and the large Green Bank and Effelsberg telescopes have resulted in the detection of the regularly-spaced emitting clouds inside the disc. The above figure shows a spectrum with a series of H2O emission components obtained with an Earth-space baseline of 19.5 Earth Diameters (about 250.000 km). The upper and lower frames of the above figure show the broad ground-based spectrum and the cross-correlated spectrum of one experiment. The angular resolution of this observation is 11 micro-arcsecs, which corresponds to a footprint of only 62 AU at the host galaxy. The observed molecular emission regions are found to be orbiting inside the rotating disc at a radius of about 0.126 parsec (0.38 lightyears) from the black hole nucleus of the galaxy.
The H2O emission in these regions results from maser-amplification by clouds with excited/pumped water molecules in the foreground as they drift in front of the radio continuum in the nucleus of NGC 4258. The formation of these regions, their regular velocity separation and their time-dependent emission appear consistent with the occurrence of a periodic magneto-rotational instability in the disc. This shear-instability is driven by differential rotation in the disc and is thought to regulate the momentum transfer and viscosity within an accretion disc.
The paper 'Space VLBI Observations of the H2O Megamaser in NGC 4258: evidence for periodic disc instabilities' will be published in Nature Astronomy on 30 June, 2022.
The authors are Willem Baan (XinJiang Astronomical Observatory, CHN, and ASTRON, NL), Tao An (Shanghai Astronomical Observatory, CHN), Christian Henkel (MPIfR, GER), Hiroshi Imai (Kagoshima Univ., JAP), Vladimir Kostenko (AstroSpace Center, RUS), and Andrej Sobolev (Ural Federal Univ., RUS).