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Research / Projects / Shadows of a black hole
Link to our paper on arXiv: https://arxiv.org/abs/2301.04967
This work has been published in the journal Classical and Quantum Gravity.
Black holes are one of the most mysterious objects in our cosmos, being regions of such strong gravitational fields that even light cannot escape. Our most direct visual evidence of black holes came in 2019 when the Event Horizon Telescope (an international astrophysics collaboration) published the first image of the black hole lying at the center of galaxy Messier 87 followed by the image of the black hole at the center of the Milky Way galaxy in 2022. The images were compared against many simulations based on Einstein’s theory of relativity, and from them, we can also deduce properties of the black holes such as their masses and angular momenta.
The figures below showed the images produced by the EHT team : the bright ring is an emission ring which contained heated gas and particles in the accretion process, whereas the dark ‘shadow’ is bounded by a closed curve representing the borderline between light rays that will eventually be captured by the black hole and those that escape to infinity.
(left) Diagram showing bright X-ray flare emerging from Sagittarius A* – the black hole at the center of our galaxy. (center) Black hole shadow image of Sgr A*. (right) Black hole shadow of M87*. Image Credits NASA/CXC/Stanford/Zhuravleva et al.
In a work published in Classical and Quantum Gravity, we derived the shape of the shadow region of a class of black hole solutions which are rotating and accreting mass at the same time – both features being physical properties of the real black holes being imaged. Our theoretical model enables us to develop an analytic expression for the shadow shape and we found a simple power law describing how the average radius of the shadow decreases with the accretion rate. We also generated a set of predictions for the Milky Way galaxy’s black hole which can be useful once the circularity measurements are improved in the future.
A couple of black hole shadow portraits showing how geometry changes from the outer to inner shapes as we increase the accretion rates of the black holes. Kerr-Vaidya geometry is a model for rotating black holes which are also accreting mass. See our paper for details of derivation using General Relativity. Our results can be used for analyzing future observations of the black holes, in particular, in the aspect of how accretion rate affects shadow geometry.
Soon after we released our preprint, we received a kind email from Dr. S. Vagnozzi sharing with us a few related works he and his team has completed on analyzing what the EHT observations can tell us about our current theories in fundamental particle physics and cosmology. We also noted that his work has been cited by the EHT team in their official papers.
We hope that our results contributed to this growing body of works that lie at the forefront of discovering constraints on new physics from astrophysical observations of black holes!