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In order to examine if the next generation space VLBI,such as VSOP-2 (VLBISpace Observatory Programme-2), will make it possible to obtain direct imagesof the accretion flow around the M87 black hole, we calculate the expectedobserved images by the relativistic ray-tracing simulations under theconsiderations of possible observational errors. We consider various cases ofelectron temperature profiles, as well as a variety of the distance, mass, andspin of the M87 black hole. We find it feasible to detect an asymmetricintensity profile around the black hole caused by rapid disk rotation, as longas the electron temperature does not steeply rises towards the black hole, aswas predicted by the accretion disk theory and the three dimensionalmagnetohydrodynamic simulations. Further, we can detect a deficit in theobserved intensity around the black hole when the apparent size of thegravitational radius is larger than $\gtrsim 1.5 \mu$ arcseconds. In the casesthat the inner edge of the disk is located at the radius of the innermoststable circular orbit (ISCO), moreover, even the black hole spin will bemeasured. We also estimate the required signal-to-noise ratio $\mathcal{R}_{\rmSN}$ for achieving the scientific goals mentioned above, finding that it shouldbe at least 10 at 22 GHz. To conclude, direct mapping observations by the nextgeneration space VLBI will provide us a unique opportunity to provide the bestevidence for the presence of a black hole and to test the accretion disktheory.

Constraining the size of the dark region around the M87 black hole by space-VLBI observations