High-resolution imaging of a black hole shadow with Millimetron orbit around lagrange point l2

Imaging of the shadow around supermassive black hole (SMBH) horizon with a very long baseline interferometry (VLBI) is recognised recently as a powerful tool for experimental testing of Einstein's General relativity. The Event Horizon Telescope (EHT) has demonstrated that an Earth-extended VLBI with the maximum long base (D = 10 700 km) can provide a sufficient angular resolution θ ~ 20 μas at λ = 1.3 mm (ν = 230 GHz) for imaging the shadow around SMBH located in the galaxy M87*. However, the accuracy of critically important characteristics, such as the asymmetry of the crescent-shaped bright structure around the shadow and the sharpness of a transition zone between the shadow floor and the bright crescent silhouette, both of order Δθ ~ 4 μas, is still to be improved. In our previous paper, we have shown that Space-Earth VLBI observation within a joint Millimetron and EHT configuration at the near-Earth high elliptical orbit can considerably improve the image quality. Even more solid grounds for firm experimental validation of General relativity can be obtained with a higher resolution available within the joint Millimetron and EHT program at the Lagrangian point L2 in the Sun-Earth system with an expected imaging resolution at 230 GHz of Δθ ~ 5 μas. In this paper, we argue that in spite of limitations of L2 orbit, an adequate sparse (u, v) coverage can be achieved and the imaging of the shadows around Sgr A* and M87* can be performed with a reasonable quality.