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.
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The correlated data for RadioAstron experiments RAGS11AF, RAKS07AT and RAGS18H are available from the RadioAstron Data Archive at the Astrospace Centre of the PN Lebedev Physics Institute ASC LPI) in Moscow at http://opendata.asc.rssi.ru/index.php.
Data reduction has been performed with AIPS, which has been developed by the National Radio Astronomy Observatory and is documented and available at http://www.aips.nrao.edu/index.shtml. Plotting procedures have been used from the MATLAB Toolbox distributed by MathWorks, Inc. at https://www.mathworks.com/products/matlab.html. The PIMA software package for processing individual baselines can be found at http://astrogeo.org/pima/. A description of the ASC Software correlator can be found in the literature17.
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We dedicate this paper to the memory of our colleague and friend Nikolai Kardashev, a man of great vision, who persevered to realize the RadioAstron mission. We thank the observatory staff of the ground telescope stations Effelsberg, Green Bank, Torun, Yebes and the Kwazar stations Kalyazin, Svetloe and Badary for their participation in the observations. These observations have been correlated at the ASC DiFX correlator and we thank the Correlator Team members for their contributions, their repeated re-correlation efforts and their unfailing support of this project. We also thank the other members of the H2O MegaMaser Team for their support for this project: A. Alakoz, S. Ellingsen, I. Litovchenko, J. Moran and A. Tolmachev. We thank E. Vorobyov (University of Vienna) for valuable discussions about the stability criteria for the disc. The RadioAstron project has been led by the AstroSpace Centre of the Lebedev Physical Institute of the Russian Academy of Sciences and the Lavochkin Scientific and Production Association under a contract with the State Space Corporation ROSCOSMOS, in collaboration with partner organizations in Russia and other countries. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under a cooperative agreement by Associated Universities, Inc. The European VLBI Network is a joint facility of independent European, African, Asian and North American radio astronomy institutes. W.A.B. acknowledges the support from the National Natural Science Foundation of China under grant no. 11433008 and the Chinese Academy of Sciences President’s International Fellowship Initiative under grant nos. 2019VMA0040, 2021VMA0008 and 2022VMA0019. T.A. acknowledges the grant support from the Youth Innovation Promotion Association of CAS. AMS was supported by the Ministry of Education and Science of Russia (the basic part of the State assignment, K 1567 no. FEUZ-2020-0030).
The authors declare no competing interests.
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Baan, W.A., An, T., Henkel, C. et al. H2O MegaMaser emission in NGC 4258 indicative of a periodic disc instability. Nat Astron 6, 976–983 (2022). https://doi.org/10.1038/s41550-022-01706-y
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