What comes after photographing a black gap’s occasion horizon? May we see the photon ring?

In 2019, the Event Horizon Telescope (EHT) provided us with the first direct image of a black hole. On the one hand, the resulting picture was rather inconspicuous. Just a circular blur of light that surrounds a dark central area. On the other hand, subtle properties of the image contain enormous information about the size and rotation of the black hole. Most of the details of the black hole image are blurred by the boundaries of the EHT. But the next generation EHT should provide a sharper view and could reveal the dark edge of a black hole’s event horizon.

How strongly bundled light creates a ring of photons. Source: Center for Astrophysics, Harvard & Smithsonian

A black hole itself does not emit light. Any light that crosses its event horizon is forever trapped. The glowing ring we see in the EHT image of M87 * is caused by the background radio glow from gas and dust surrounding the black hole. Some of this light happens very close to the black hole and is directed gravitationally in our direction. The next limit at which light can graze the black hole and reach us is known as the photon ring.

If we could observe the black hole perfectly, the photon ring would be a thin bright line. Some of the light from the photon ring is scattered before it reaches us, and when combined with the resolution limits of the EHT, this creates the blurry image we see. However, the next generation EHT will have higher resolution and be able to capture images in less time. This enables detailed images not only of M87 *, but also of the supermassive black hole in our galaxy.

Different photon paths create layers of light. Photo credit: George Wong (UIUC) and Michael Johnson (CfA)

One of the things the ngEHT could reveal is multiple layers of lens light. Most of the light we see around the black hole is that of the photon ring. That is, the light with a strong lens that grazed the black hole. But some light will make a full loop around the black hole before it sets off on our way, and a small amount will make multiple loops. Each type of photon path creates a different ring of light around the black hole. If we can pull these layers apart, we will better understand the nature of gravity near a black hole.

And as a recent paper on the arXiv shows, the ngEHT could also help us to examine the event horizon of a black hole itself. The dark central area of ​​the M87 * image is not that of the event horizon. It’s just a shadow of the black hole caused by the photon ring. But within the central region there should be an inner shadow. A shadow of the event horizon. As this current work shows, this inner shadow would not be a simple circle. Its shape depends on the size and rotation of the black hole.

The gravitational field near a black hole is so strong that it distorts not only the glow of the photon ring, but also the shadow of the event horizon. So while the event horizon is truly spherical, our view of the event horizon can be skewed by the gravity of the black hole. In this latest work, the team shows how we can observe both the photon ring and the inner shadow. By comparing the two, we would gain a deep understanding of black hole dynamics, including information on how light and matter are captured by a black hole.

In time, we may finally be able to see the dark shadow of gravity for ourselves.

Reference: Andrew Chael et al. “Observing the Inner Shadow of a Black Hole: A Direct View of the Event Horizon.” ArXiv-Preprint arXiv: 2106.00683 (2021).

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