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| Main Authors: | , , , |
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| Format: | Preprint |
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2025
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| Subjects: | |
| Online Access: | https://arxiv.org/abs/2509.23628 |
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| _version_ | 1866914061586792448 |
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| author | Farah, Joseph R. Lupsasca, Alexandru Quataert, Eliot Johnson, Michael D. |
| author_facet | Farah, Joseph R. Lupsasca, Alexandru Quataert, Eliot Johnson, Michael D. |
| contents | The $n=1$ photon ring is a full image of the astrophysical source around a black hole, produced by photons that execute $n\approx1$ half-orbit around the event horizon on their way to an observer. The Black Hole Explorer (BHEX) is a proposed extension of the Event Horizon Telescope to space that will target the $n=1$ photon rings of the supermassive black holes M87${}^\ast$ and Sgr\,A${}^\ast$. In this paper, we introduce a new interferometric observable that will be directly measurable on BHEX baselines and which admits a clear image-domain interpretation in terms of the photon ring brightness profile. Across a wide range of semi-analytic equatorial emission models, we find that the azimuthal intensity profile of the ring can change depending on the astrophysics of the source, but its width $w_b$ is weakly sensitive to these details -- much like the ring shape, which has previously been identified as a probe of the spacetime geometry. Our survey suggests that interferometric measurements of the photon ring diameter and $w_b$ can place constraints (to $\lesssim\!20\%$) on the spin and inclination of a black hole with a known mass-to-distance ratio, such as Sgr\,A${}^\ast$. State-of-the-art numerical simulations support this finding, paving the way to a precise photon-ring-based spin measurement for Sgr\,A${}^\ast$ with BHEX. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2509_23628 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Interferometric inference of black hole spin from photon ring size and brightness Farah, Joseph R. Lupsasca, Alexandru Quataert, Eliot Johnson, Michael D. High Energy Astrophysical Phenomena The $n=1$ photon ring is a full image of the astrophysical source around a black hole, produced by photons that execute $n\approx1$ half-orbit around the event horizon on their way to an observer. The Black Hole Explorer (BHEX) is a proposed extension of the Event Horizon Telescope to space that will target the $n=1$ photon rings of the supermassive black holes M87${}^\ast$ and Sgr\,A${}^\ast$. In this paper, we introduce a new interferometric observable that will be directly measurable on BHEX baselines and which admits a clear image-domain interpretation in terms of the photon ring brightness profile. Across a wide range of semi-analytic equatorial emission models, we find that the azimuthal intensity profile of the ring can change depending on the astrophysics of the source, but its width $w_b$ is weakly sensitive to these details -- much like the ring shape, which has previously been identified as a probe of the spacetime geometry. Our survey suggests that interferometric measurements of the photon ring diameter and $w_b$ can place constraints (to $\lesssim\!20\%$) on the spin and inclination of a black hole with a known mass-to-distance ratio, such as Sgr\,A${}^\ast$. State-of-the-art numerical simulations support this finding, paving the way to a precise photon-ring-based spin measurement for Sgr\,A${}^\ast$ with BHEX. |
| title | Interferometric inference of black hole spin from photon ring size and brightness |
| topic | High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2509.23628 |