High Resolution Imaging of a Black Hole Shadow with Millimetron Orbit around Lagrange Point L2

TL;DR

This paper discusses high-resolution imaging of black hole shadows using the Millimetron space telescope at Lagrange Point L2, aiming to improve tests of General relativity beyond Earth-based VLBI capabilities.

Contribution

It proposes that joint Millimetron and EHT observations at L2 can achieve sufficient resolution and coverage for detailed black hole shadow imaging, surpassing previous Earth-based methods.

Findings

Imaging resolution at 230 GHz can reach ~5 μas at L2.

Adequate sparse (u,v) coverage is achievable from L2.

Black hole shadow imaging quality is sufficient for scientific analysis.

Abstract

Imaging of the shadow around supermassive black hole (SMBH) horizon with a very long baseline interferometry (VLBI) is recognized 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 $\theta\sim 20~\mu$as at $\lambda=1.3$ mm ($\nu=230$ GHz) for imaging the shadow around SMBH located in the galaxy M87$^\ast$. 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 $\Delta\theta\sim 4~\mu$as, 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 (HEO) 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 $\Delta\theta\sim 5~\mu$as. 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$^\ast$ and M87$^\ast$ can be performed with a reasonable quality.