Black Hole Images as Tests of General Relativity: Effects of Spacetime Geometry
The images of supermassive black holes surrounded by optically thin, radiatively inefficient accretion flows, like those observed with the Event Horizon Telescope, are characterized by a bright ring of emission surrounding the black hole shadow. In the Kerr spacetime, this bright ring, when narrow,...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
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IOP Publishing
2023-01-01
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| Series: | The Astrophysical Journal |
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| Online Access: | https://doi.org/10.3847/1538-4357/aca58a |
| _version_ | 1797697206248013824 |
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| author | Ziri Younsi Dimitrios Psaltis Feryal Özel |
| author_facet | Ziri Younsi Dimitrios Psaltis Feryal Özel |
| author_sort | Ziri Younsi |
| collection | DOAJ |
| description | The images of supermassive black holes surrounded by optically thin, radiatively inefficient accretion flows, like those observed with the Event Horizon Telescope, are characterized by a bright ring of emission surrounding the black hole shadow. In the Kerr spacetime, this bright ring, when narrow, closely traces the boundary of the shadow and can, with appropriate calibration, serve as its proxy. The present paper expands the validity of this statement by considering two particular spacetime geometries: a solution to the field equations of a modified gravity theory and another that parametrically deviates from Kerr but recovers the Kerr spacetime when its deviation parameters vanish. A covariant, axisymmetric analytic model of the accretion flow based on conservation laws and spanning a broad range of plasma conditions is utilized to calculate synthetic non-Kerr black hole images, which are then analyzed and characterized. We find that in all spacetimes: (i) it is the gravitationally lensed unstable photon orbit that plays the critical role in establishing the diameter of the rings observed in black hole images, not the event horizon or the innermost stable circular orbit, (ii) bright rings in these images scale in size with, and encompass, the boundaries of the black hole shadows, even when deviating significantly from Kerr, and (iii) uncertainties in the physical properties of the accreting plasma introduce subdominant corrections to the relation between the diameter of the image and the diameter of the black hole shadow. These results provide important new theoretical justification for using black hole images to probe and test the spacetimes of supermassive black holes. |
| first_indexed | 2024-03-12T03:36:57Z |
| format | Article |
| id | doaj.art-a31aaec8228646f186d61cf4e4510154 |
| institution | Directory Open Access Journal |
| issn | 1538-4357 |
| language | English |
| last_indexed | 2024-03-12T03:36:57Z |
| publishDate | 2023-01-01 |
| publisher | IOP Publishing |
| record_format | Article |
| series | The Astrophysical Journal |
| spelling | doaj.art-a31aaec8228646f186d61cf4e45101542023-09-03T13:08:34ZengIOP PublishingThe Astrophysical Journal1538-43572023-01-0194214710.3847/1538-4357/aca58aBlack Hole Images as Tests of General Relativity: Effects of Spacetime GeometryZiri Younsi0https://orcid.org/0000-0001-9283-1191Dimitrios Psaltis1https://orcid.org/0000-0003-1035-3240Feryal Özel2https://orcid.org/0000-0003-4413-1523Mullard Space Science Laboratory, University College London , Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UKDepartment of Astronomy and Steward Observatory, University of Arizona , 933 N. Cherry Avenue, Tucson, AZ 85721, USADepartment of Astronomy and Steward Observatory, University of Arizona , 933 N. Cherry Avenue, Tucson, AZ 85721, USAThe images of supermassive black holes surrounded by optically thin, radiatively inefficient accretion flows, like those observed with the Event Horizon Telescope, are characterized by a bright ring of emission surrounding the black hole shadow. In the Kerr spacetime, this bright ring, when narrow, closely traces the boundary of the shadow and can, with appropriate calibration, serve as its proxy. The present paper expands the validity of this statement by considering two particular spacetime geometries: a solution to the field equations of a modified gravity theory and another that parametrically deviates from Kerr but recovers the Kerr spacetime when its deviation parameters vanish. A covariant, axisymmetric analytic model of the accretion flow based on conservation laws and spanning a broad range of plasma conditions is utilized to calculate synthetic non-Kerr black hole images, which are then analyzed and characterized. We find that in all spacetimes: (i) it is the gravitationally lensed unstable photon orbit that plays the critical role in establishing the diameter of the rings observed in black hole images, not the event horizon or the innermost stable circular orbit, (ii) bright rings in these images scale in size with, and encompass, the boundaries of the black hole shadows, even when deviating significantly from Kerr, and (iii) uncertainties in the physical properties of the accreting plasma introduce subdominant corrections to the relation between the diameter of the image and the diameter of the black hole shadow. These results provide important new theoretical justification for using black hole images to probe and test the spacetimes of supermassive black holes.https://doi.org/10.3847/1538-4357/aca58aHigh energy astrophysics |
| spellingShingle | Ziri Younsi Dimitrios Psaltis Feryal Özel Black Hole Images as Tests of General Relativity: Effects of Spacetime Geometry The Astrophysical Journal High energy astrophysics |
| title | Black Hole Images as Tests of General Relativity: Effects of Spacetime Geometry |
| title_full | Black Hole Images as Tests of General Relativity: Effects of Spacetime Geometry |
| title_fullStr | Black Hole Images as Tests of General Relativity: Effects of Spacetime Geometry |
| title_full_unstemmed | Black Hole Images as Tests of General Relativity: Effects of Spacetime Geometry |
| title_short | Black Hole Images as Tests of General Relativity: Effects of Spacetime Geometry |
| title_sort | black hole images as tests of general relativity effects of spacetime geometry |
| topic | High energy astrophysics |
| url | https://doi.org/10.3847/1538-4357/aca58a |
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