# To see invisible: image of the event horizon within the black hole shadow

## TL;DR

This paper proposes a method to reconstruct a gravitationally lensed image of the entire black hole event horizon on the celestial sphere, enabling a view of black holes from both front and back sides.

## Contribution

It introduces the concept of the 'lensed event horizon image' as a way to visualize the entire event horizon surface through gravitational lensing effects.

## Key findings

- The event horizon image is a lensed projection of the whole horizon surface.
- Black holes can be observed from both front and back sides via this method.
- The event horizon image appears within the black hole shadow on the celestial sphere.

## Abstract

How the supermassive black hole SgrA* in the Milky Way Center looks like for a distant observer? It depends on the black hole highlighting by the surrounding hot matter. The black hole shadow (the photon capture cross-section) would be viewed if there is a stationary luminous background. The black hole event horizon is invisible directly (per se). Nevertheless, a more compact (with respect to black hole shadow) projection of the black hole event horizon on the celestial sphere may be reconstructed by detecting the highly red-shifted photons emitted by the non-stationary luminous matter plunging into the black hole and approaching to the event horizon. It is appropriate to call this reconstructed projection of the event horizon on the celestial sphere for a distant observer as the ``lensed event horizon image'', or simply the ``event horizon image''. This event horizon image is placed on the celestial sphere within the position of black hole shadow. Amazingly, the event horizon image is a gravitationally lensed projection on the celestial sphere of the whole surface of the event horizon globe. In result, the black holes may be viewed at once from both the front and back sides. The lensed event horizon image may be considered as a genuine silhouette of the black hole. For example, a dark northern hemisphere of the event horizon image is the simplest model for a black hole silhouette in the presence of a thin accretion disk.