Kotaro Moriyama at Goethe Univeirsity Frankfurt

Research Interest

The black hole is the most extreme object in the Universe predicted by general relativity. Observational measurement of the black-hole spacetime is one of the most important topics in modern physics and astrophysics, since it will lead to a critical test of the theory of general relativity. My research has focused on measuring the black-hole spacetime by utilizing ultra high-angular resolution observations of supermassive black holes with the Event Horizon Telescope (EHT). The EHT is a global very long baseline interferometric (VLBI) array at a wavelength of 1.3mm, capable of resolving the electromagnetic emission around the event horizon.

Black Hole Spin Measurement with Echoes of Gravitationally Lensed Radiation

General relativity predicts that the spacetime around a black hole is uniquely described by its mass and spin parameter. The black hole mass can be accurately measured by orbits of stars or gas dynamics inside the sphere of its gravitational influence extending up to hundred thousands gravitational radius. On the other hand, the spin measurement requires capturing horizon-scale emission from the black hole, since general relativistic effects of the spin significantly appear only at the immediate vicinity of the black hole (a few gravitational radius). EHT observations of the horizon-scale emission provide the spatial and temporal information of the accretion flow in the black hole’s vicinity. However, it is not easy to extract the spin information from the emission because it depends on both the complexity of accretion properties and spacetime effects.
To overcome these challenges in spin measurements, I have studied general relativistic echoes of radiation from gas clouds intermittently falling into the black hole. This echo is constructed near the photon circular orbit where emitted photons are temporarily captured in orbit due to the strong gravity (Figure 1a). We consider a situation shown in some recent three-dimensional general relativistic magnetohydrodynamics (3D-GRMHD) simulations in which gas clouds are tidally stripped off from the innermost parts of an accretion disk and intermittently fall toward the black hole.   I calculated motions of infalling gas clouds with various model parameters, and investigated the spin dependence of relativistic flux variation using our general relativistic ray-tracing scheme. The light curve of the infalling gas cloud is composed of two characteristic peaks. The first peak (blue curve in Figure 1b) is formed by photons that directly reach a distant observer (blue curve in Figure 1a). The secondary one (red curve in Figure 1b) is composed of photons reaching the observer after more than one rotation around the black hole (red curve in Figure 1a). The time interval of the echo is determined by the period of photon rotation near the photon circular orbit, whose radius uniquely depends on the spin (Figure 1c).

Importantly, I found that the characteristic time interval uniquely depends on the spin and is not significantly affected by other model parameters of the accretion flow. This advantage enables us to overcome the difficulty of spin measurement; we can estimate the spin value without considering the detailed properties of infalling gas clouds. Furthermore, I performed synthetic EHT observations for Sgr A* under a situation that a number of gas clouds intermittently fall towards the black hole with various initial parameters (Figure 2a). Even for this more complicated case, the synthetic observations indicate that the black hole spin is detectable without the uncertainty of the other parameters, and our methodology can be applied to EHT observations of Sgr A* since 2017 (Figure 2b, c, Moriyama et al. 2019).

Event Horizon Telescope (EHT) Imaging of Supermassive Black Holes

Since April 2018, I have been an active member of several EHT Working Groups, most notably the Imaging Working Group, in which I have produced images of our primary targets M87*. Among many projects I have been involved, my contribution is particularly substantial in the imaging projects of its primary targets M87* and Sgr A*.
In our first EHT M87* results (EHT Collaboration 2019d, e, f), I was a key member of the imaging group and a core developer of SMILI (Akiyama & Moriyama et al. 2019; Moriyama in prep), one of three software imaging packages used to create the first-ever images of a black hole taken for M87*. I particularly co-led the imaging parameter survey with SMILI for the first M87* results, assessing the uncertainties in our images by exploring a wide range of parameter sets on both the data and training synthetic data tests. It demonstrated the essential result for M87* imaging challenges; all acceptable images have ring features with the consistent diameter and asymmetry illuminating the central shadow of the supermassive black hole. 

After the release of the first-ever images of a black hole, my roles in the key imaging projects have been rapidly expanded. I currently lead the entire SMILI group in the EHT Collaboration. Furthermore, for the on-going imaging project of the other, quickly varying target Sgr A*, I lead the development of computational imaging techniques for video reconstructions with SMILI (Moriyama et al. in prep.), enabling dynamical imaging of infalling gas clouds in the vicinity of the black hole (e.g. Johnson et al. 2017). Beyond the SMILI group, I coordinating the work of subgroups in the Sgr A* imaging project, including designing the imaging parameter survey for Sgr A*.