Russian astrophysicists have found a way to observe photon rings around supermassive black holes using a space-ground interferometer.

A team of scientists from the Astro Space Center of P. N. Lebedev Physical Institute's Astrospace Center has demonstrated two novel methods for observing photon rings in the vicinity of a supermassive black hole's shadow.

Astrophysicists have shown that studies of flares in photon rings provide a unique opportunity to test the theory of relativity under extreme conditions, and determine the basic properties of black holes, such as mass and rotation speed. Both the Millimetron space telescope and the ngEHT (next generation EHT) ground-based telescope network will have access to these observation methods. The study is published in the Physical Review.

The black hole's gravity captures photons emitted by the hot plasma of the accretion disk as it orbits. After several revolutions in a quasi-stable orbit, they are either forced to fall under the event horizon, or can fly away infinitely. As these photons reach the observer, they form a complex picture of bright rings nestled together, with brightness decreasing from outer to inner. Photons coming directly from the disk form its direct image. This is the outermost ring, denoted by the number n=0, while photons that have made half a turn correspond to the next inner ring with number n=1, full turns correspond to the next inner ring with number n=2, and so on. A black hole's parameters, such as mass and rotation speed, will determine how these photon rings appear and how bright they are. The properties of space itself near powerful gravitational fields will also affect the parameters of the rings. In this way, scientists can test relativity and search for possible manifestations of new physics.


A simplified (not to scale) model of the geometrically thin disk around a Kerr black hole and the image of photon rings in the picture plane. A radius of 8RG is set for the inner disk, and a radius of 45RG is set for the outer disk. A picture plane image will consist of an accretion disk (n=0) and photon rings (n=1,2,3).

The Event Horizon Telescope (EHT) published images of two supermassive black holes in 2019 and 2022. The objects are located in the center of our Galaxy and in the M87 galaxy. These observations had a 25 microsecond angular resolution. Despite the fact that this resolution is adequate for seeing the "shadow of a black hole", higher angular resolution is required to examine individual details within the accretion disk. The capabilities of the Event Horizon Telescope are limited by the size of our planet, and by the properties of the atmosphere, which prevent observations at shorter wavelengths. Ground-based VLBI networks, particularly joint EHT observations with the Millimetron observatory, will result in an increase in angular resolution of 6–10 times in reconstructed images. Nevertheless, even greater resolution is needed to reliably observe rings created by photons that have completed more than one orbit.

It is able to achieve an angular resolution of up to 40 nanoarcseconds using a special observation configuration of the Millimetron space telescope. This telescope will be located about 1.5 million kilometers from the Earth. In this case, the entire resulting pattern degenerates into a practically one-dimensional slice along the longest baseline projection of the interferometer. Theoretically, such a regime of observations would allow detection of photon rings signs from characteristic changes in brightness near the black hole shadow. According to scientists at the Astro Space Center, each new ring only reaches 4-13% of the brightness of the previous one. Therefore, high ring orders may be too dim for the EHT-Millimetron tandem's capabilities.

However, in a new work, Russian astrophysicists have shown that despite this limitation, the Millimetron can be used to observe rings with at least n = 2 and 3. X-ray, near-infrared, and submillimeter flares periodically occur in the inner regions of the accretion disk. During the reconnection of magnetic fields in the innermost regions of the disk around the black hole, energy may be released, resulting in their formation.

As a result of modeling flares in an accretion disk of black holes of various masses and rotation speeds, researchers have demonstrated that each photon ring will be illuminated in turn, resulting in successive subflares of decreasing intensity. A flash can double the brightness of the source. The photon rings, however, become brighter by almost two orders of magnitude during the flare, due to the compact nature of the flare region on the disk and observations with a ground-space interferometer. This flash, in fact, makes the rings very visible to interferometer observations! The time interval between the bursts roughly corresponds to half the orbit of photons around the black hole. Scientists will have the opportunity to directly analyze the parameters of subflares, such as their brightness, time delays, and spatial distribution.


The left image shows the flare in Sgr A* accretion disk and its echo in the first and second photon rings. The model image is of the ngEHT network, and the observation time is 30 minutes. On the right, spin dependence on angle between images of a black hole.