Evidence of periodic accretion disk instability found in a nearby spiral galaxy
Recently, an international team including Prof. Tao An from the Shanghai Astronomical Observatory of the Chinese Academy of Sciences (CAS) found evidence of periodic instability in the accretion disk of the nearby galaxy NGC4258 through the space VLBI observations of water masers. The observational results were obtained on the basis of unprecedented ultra-high resolution technique for detecting gas accretion disks surrounding black holes, and the results also advance our understanding of the dynamic properties of accretion disks by revealing the existence of magneto-rotational instability in gas disks using masers as probes. This work is published online in the June 30, 2022 issue of Nature Astronomy.
Scientists have observed laser-like sources of excited radiation from interstellar space in the Universe, and have termed them "celestial masers" to distinguish them from laboratory lasers. Astronomers have observed celestial masers such as water (H2O), hydroxyl (OH), methanol (CH-OH), formaldehyde (H2CO) and silicon monoxide (SiO) molecules. Astronomers believe that H2O MegaMaser may come from thin gas disks or jets around massive black holes at the centers of active galaxies like NGC 4258, but the physical conditions that lead to the amplification of the megaMaser radiation remain unclear. The structure of the maser-emitting region is very complex, with the most compact maser clumps embedded in the densest molecular clouds in the accretion disk. The study of masers, which can directly trace the dynamics of accretion disks, is extremely valuable for astrophysical studies of black holes, but requires very high resolution to detect their detailed structure.
Very long baseline interferometry (VLBI ) is the highest resolution observational technique in astronomy, and masers and black holes are the two traditional targets for VLBI astrophysical studies. The Event Horizon Telescope (EHT) has successfully observed the radiation region around black holes of the galaxy M87 and the Milky Way, but ground-based VLBI arrays still face the challenge of insufficient resolution when observing MegaMasers in galactic nuclei. Astronomers have developed space VLBI  to further improve the resolution.
An international collaborative team led by Prof. Willem Baan has carried out a series of observations of a spiral galaxy called NGC4258, about 21 million light-years from the Earth, over a period of more than 3 years starting in 2014, using a space-ground VLBI network consisting of the Russian RadioAstron Observatory launched into space and ground-based large radio telescopes, with the longest space-ground baseline up to 250,000 km (equivalent to about 20 times the diameter of the Earth), with an angular resolution of 11 micro-arcsec, the highest resolution ever achieved. With such a high resolution, it was possible to resolve dense clouds in the accretion disk with a size of about 60 astronomical units (1 astronomical unit is the distance from the Earth to the Sun, or 149,598,000 km). The clouds with maser radiation were found to be distributed within a thin gas disk surrounding the black hole with a radius of about 0.38 light-years and moving with the rotation of the gas disk, with several clouds about 170 astronomical units away from the black hole. When gas clouds containing water molecules in the disk drift in front of the radio jet, the jet provides input background photons for maser amplification, which causes the water molecules to be excited and amplified to produce powerful water maser radiation.
Interestingly, observations show that the velocities of several maser clumps exhibit regular differences, and their velocities and intensities varied with time. The team believes that the formation and dynamical nature of these compact maser clouds are related to the periodic magnetic rotational instability (MRI) occurring in the accretion disk. The shear instability is driven by the differential rotation in the accretion disk, and the disk regulates radial momentum transfer and viscosity through this turbulent instability. Dr. Tao An of the Shanghai Astronomical Observatory participated in the observational proposal, scientific analysis and manuscript improvement. These space VLBI observations not only provide unprecedented ultra-high resolution for detecting accretion disks around black holes, but also reveal the magneto-hydrodynamic instability in accretion disks by using masers as probes, advancing the understanding of the dynamic properties of accretion disks and the accretion process of black holes, Dr. An said. This observation suggests that there are regions in the NGC4258 accretion disk that satisfy the physical conditions for amplifying the spectral lines of water molecules, and the team think that magneto-rotational instability is the mechanism that generates these high-density regions. MRI processes are connected to viscous and turbulence that causes radial momentum transfer, and future studies will explore how this shear-driven instability promotes black hole accretion.
Top panel: Sketch of the magneto-rotational instability in an accretion disk; Bottom panel: Spectrum of the observed water masers, showing that several maser clouds are at regularly different velocities.
Very long baseline interferometry (VLBI), which combines radio telescopes distributed in different locations to mimic the effect of a super telescope, is the astronomical observation technique with the highest angular resolution to date and is widely used in astrophysics, astrometry, and deep space exploration. According to the basic principles of electromagnetism, the angular resolution of a telescope is approximately proportional to the observed wavelength divided by the antenna aperture. For the VLBI system, the effective aperture is equivalent to the baseline length (baseline refers to the distance between two radio telescopes) through the interference technique of radio waves, and the longest baseline of the VLBI array on the Earth can reach 10,000 km, which is 20,000 times higher than the largest single-dish telescope, and thus the observing resolution is four orders of magnitude higher. The current VLBI array with the highest angular resolution on the Earth is the Event Horizon Telescope (EHT), which has a maximum equivalent aperture comparable to the diameter of the Earth and a maximum angular resolution of 20 micro-arcsec, and has successfully observed the radiation around supermassive black holes at the center of the Milky Way and the center of galaxy M87.
 Space VLBI.
Soon after the successful use of VLBI technology for observations in the last century, astronomers became acutely aware that the longest baseline of ground-based VLBI cannot exceed the diameter of the Earth, and thus its maximum angular resolution is limited. To break this limitation, astronomers turned their attention to the vastness of space and came up with the idea of launching radio telescopes into space to form space-ground or space VLBI arrays with ground-based or other space-based radio telescopes, whose baseline lengths could then exceed the diameter of the Earth, thus obtaining higher angular resolution, which is the concept of space VLBI (SVLBI). In addition to the advantage of high resolution, space telescope observations are unaffected or less affected by the Earth's atmosphere, so the data quality is higher than that of ground-based VLBI; space telescopes are located in a superior electromagnetic environment, which makes them more suitable for low and high frequency radio observations that are impossible or difficult to perform on the ground. In the long run, space VLBI is the inevitable trend of VLBI development, and the VLBI community is actively developing this technology. Two representative space VLBI projects are VSOP led by Japan and RadioAstron led by Russia, both of which have achieved remarkable scientific breakthroughs in the field of VLBI astrophysics. In recent years, Shanghai Astronomical Observatory of Chinese Academy of Sciences has also proposed a space-based low-frequency radio astronomical observatory, in which two radio telescopes are to be launched into space to observe either in space-space baseline or to form a more powerful space-ground VLBI network together with large radio telescopes on the Earth, with higher resolution and better mapping capabilities while having high sensitivity in the low frequency band.
Link to the paper: https://www.nature.com/articles/s41550-022-01706-y
Willem Baan, Xinjiang Astronomical Observatory, Chinese Academy of Sciences, firstname.lastname@example.org
Tao An, Shanghai Astronomical Observatory, Chinese Academy of Sciences, email@example.com