Oral Presentation
Measuring Black Hole Spin with Time-domain VLBI Observations of Infalling Gas Clouds
Presenter: Kotaro Moriyama (Massachusetts Institute of Technology Haystack Observatory)
Black holes are among the most important objects for testing general relativity. The black hole spacetime is described by two quantities: the black hole mass and spin. The mass can often be accurately estimated from astrometric observations of orbiting stars around the central black hole. Measuring the black hole spin requires information from the vicinity of the event horizon, which is spatially resolved for the Galactic center Sagittarius A* (Sgr A*) and nearby radio galaxy M 87 by means of very long baseline interferometry (VLBI) observations with the Event Horizon Telescope (EHT). However, it is not easy to extract the spin information from the horizon-scale emission, which depends on complexities of accretion flow properties and spacetime effects.
In this presentation, we simulate EHT observations of a gas cloud falling onto a black hole, and construct a method for spin measurement based on its relativistic flux variation. We investigate the signature of the spin dependency of relativistic flux variation by calculating the motion of an infalling gas cloud and photon trajectories in the Kerr spacetime using general relativistic ray tracing. The light curve of the infalling gas cloud is composed of peaks formed by photons which directly reach a distant observer and by secondary ones reaching the observer after more than one rotation around the black hole. The time interval between the peaks is determined by the period of photon rotation near the photon circular orbit, which uniquely depends on the spin.
We perform synthetic EHT observations of Sgr A* under a more realistic situation in which a number of gas clouds intermittently fall towards the black hole with various initial parameters. Even for this case, the signature of the black hole spin is detectable in correlated flux densities which are accurately calibrated by baselines between sites with more than one EHT observatory, such as Mauna Kea and Chajnantor. The synthetic observations indicate that our methodology can be applied to EHT observations of Sgr A* since April 2017. Importantly, horizon-scale movies of infalling gas clouds can be obtained by expanding the EHT ground array, providing a reliable measurement of the spin that is independent of the detailed properties of the accretion flow.
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