An EHT image of M87* compared to a simulated image of its photon ring. Source: EHT Collaboration (left), AE Broderick (right)
Understanding the physics of supermassive black holes requires studying data from accretion disks and the jets they eject, as well as observing objects like M87* and Sag A*. However, scientists still failed to capture the ring of photons around the black hole. A new study offers a new approach to this task. Black holes are structures in space and time that bend them. A black hole’s event horizon is the surface that light can only pass through once: anything that crosses the horizon stays inside the black hole. Additionally, next to the black hole is the photon shell, which represents the inner limit of the photon’s stable circular orbit. This boundary is located at a distance of 1.5 radii from the event horizon.
The event horizon and photon envelope cannot be directly observed, but it is possible to detect their closest equivalent – the photon ring, which forms when photons make several semi-circles around the black hole, their orbits. curved toward the observer. For a typical black hole, the radius of the photon ring is about 2.6 times the radius of the event horizon. In the case of a rotating black hole, the radius of the photon ring can be different because rotation increases the energy of the photon in the direction of rotation. The photon ring is the closest black hole structure to us, and observing it could provide a wealth of information about black holes and Einstein’s theory of gravity.
To date, the photon ring from the M87* black hole has been identified in data obtained with the Event Horizon Telescope (EHT), but with low resolution. Current resolving power has reached its limit; Astronomers cannot clearly separate the photon ring from the background.
So, in a new study, astrophysicists proposed using a Very Long Baseline Interferometer (VLBI) array to obtain high-resolution images of the M87* photon ring and holes another supermassive black, like the image of the Andromeda galaxy (M31). To do this, scientists propose placing radiation receivers in orbit around the Earth at the Lagrange point L2. Such a telescope would be able to acquire data with a field of view larger than the diameter of the Earth. This research is conceptual and implementing such a telescope would require significant effort and time. However, the ideas presented in this work deserve attention, since the photon ring is the main target of astronomical research on black holes. The ability to observe and study photon rings will help overcome current limitations and improve our understanding of the physics and gravity of black holes.