An article published in the journal “Science Advances” reports a study on the possibility of obtaining sharp images of the subrings that form the photon ring around a black hole. A team of researchers led by Michael Johnson of of the Center for Astrophysics – Harvard and Smithsonian (CfA) worked on the feat announced almost a year ago with the publication of the photo of the area around the supermassive black hole at the center of the galaxy M87. The researchers think that adding a space telescope to the ones used by the Event Horizon Telescope (EHT) collaboration would allow to obtain the sharpness needed to distinguish one of the subrings, whose characteristics would allow to obtain more precise measurements of the mass of the black hole and more.
On April 10, 2019, the presentation by the Event Horizon Telescope (EHT) collaboration of the photo of the area around the supermassive black hole at the center of the galaxy M87 was a great event that went beyond the world of astronomy. The images published in the media show a limited amount of detail but the data collected by the various radio telescopes that participated in the research contain a far greater amount of them. Those data include those about a series of nested photon rings surrounding a sort of shadow cast by the black hole. They can be considered subrings, which are stacked on top of each other with increasing sharpness, which correspond to the number of orbits that the photons took around the black hole before being observed. That division is illustrated in the image (Courtesy George Wong (UIUC) and Michael Johnson (CfA). All rights reserved).
This ring with its subrings is in the area around the black hole’s event horizon, therefore at the edge of the area beyond which even light gets captured. Even on the outside, the gravity is so strong that it bends the light trajectory so photons can take a variable number of orbits around the black hole forming the various nested subrings. Each subring is very similar to the previous one but becomes progressively sharper because its light took a greater number of orbits.
Even looking at the best images of the ring around the supermassive black hole at the center of the galaxy M87, the subrings are almost imperceptible to the naked eye. Their sharpness grows with the distance between the telescopes used for the observations, so two telescopes could be enough, as long as they’re very distant from each other. According to the researchers, adding a space telescope in low Earth orbit, the one where the Hubble space telescope orbits to be clear, would allow to distinguish the subring that in the image is indicated with n=1. To distinguish the subring indicated as with = 2, a telescope on the Moon would be needed.
The ability to distinguish the subrings around a black hole would help measure their characteristics such as mass, size and more with greater precision. Near the event horizon, space-time is strongly distorted and that allows to test the predictions deriving from the theory of general relativity, so precise measurements of the black hole’s characteristics would help the research in this field. In essence, the great international campaign involved in the EHT project doesn’t represent the conclusion, but a phase of research that in the future may lead to further discoveries regarding the secrets of the universe.
