New study proposes how to capture direct image of black hole photon ring

Supermassive black holes, those massive gravitational giants that can illuminate the cosmos with blazing quasars or discreetly hide amidst the stars in a galaxy’s core, have long intrigued astronomers. Our understanding of them primarily comes from indirect observations, like studying their bright accretion disks or the powerful jets of plasma they emit. However, glimpses of these enigmatic cosmic entities have become more direct, thanks to images of M87* and Sag A*.

Yet, one elusive aspect remains: capturing a direct image of the mysterious photon ring. A recent publication in Acta Astronautica presents a proposal for how we might finally achieve this feat.

In essence, black holes are not mere celestial objects but rather warped manifestations of spacetime. They’re characterized by an event horizon, a boundary where light can venture inward but can never escapeā€”a point of no return.

Close to a black hole lies another significant feature: the photon shell. It marks the inner limit of stable circular orbits for photons. In theory, light within the photon shell can endlessly orbit the black hole, though in reality, gravitational fluctuations render these orbits unstable over time. If the event horizon has a radius of R, the photon shell extends to a radius of 1.5R.

While the event horizon and photon shell elude direct observation, we can study the next best thing: the photon ring. This thin circle of light is formed by photons that graze the black hole so closely that their paths are deflected straight toward us. For a non-rotating black hole, the photon ring has a radius of approximately 2.6R.

In the case of a rotating black hole, the situation becomes more intricate because the black hole’s spin boosts a photon’s energy in the direction of rotation. Regardless, the photon ring represents the closest observable structure of a black hole from a distance. It holds immense potential to enhance our understanding of black holes and test Einstein’s gravitational theory.

The structures of a simple black hole. Credit: John F. Lindner, The College of Wooster
Proposed elliptical orbits for the VLBI. Credit: Hudson, et al

While the EHT images of M87* captured the photon ring, it remains indistinct. Some argue that extracting detailed data from the background is possible, while others dispute this claim. One significant challenge is that current technology is already at its resolution limit.

Obtaining the blurry images of M87* and Sag A* was a monumental achievement. Plans for a next-generation Event Horizon Telescope (ngEHT) are in motion, promising more observatories and sensitive detectors. Yet, even with these advancements, glimpsing the photon ring might remain elusive.

Enter the proposed solution: a constellation of space-based Very Long Baseline Interferometers (VLBI). These antennas could orbit Earth widely or be positioned at the L2 Lagrange point between Earth and the moon. Free from Earth’s atmospheric interference, this constellation’s receivers could capture radio light at shorter wavelengths than ground-based observatories.

By arranging the antennas in elliptical orbits, the array could create an effective baseline much wider than Earth’s diameter. Such a setup would empower astronomers to capture high-resolution images of both M87* and Sag A*, allowing them to observe the photon rings. Moreover, this telescope could provide lower-resolution images of other supermassive black holes, such as the one in the Andromeda galaxy.

To be clear, this study serves as a proof of concept. It will take decades to materialize such a telescope, and numerous engineering challenges must be overcome. Nevertheless, these ideas are worth contemplating. The photon ring represents the Holy Grail of black hole astronomy, a goal achievable only if we reach further than we have ever ventured before.

Source: Universe Today

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