Plunging region

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In astrophysics, a plunging region is a region near a black hole in which matter can no longer follow circular orbits and will instead rapidly "plunge" towards the event horizon at nearly the speed of light. [1] [2] [3] This region exists between the innermost stable circular orbit and the event horizon of the black hole. [4] [5] Unlike inside the event horizon, light and radiation can still escape the black hole, but matter is doomed to fall in. [6] The existence of such a region is predicted by Einstein's general theory of relativity. [6] [7]

Contents

Dynamics

Upon crossing the innermost stable circular orbit (ISCO), matter can no longer stably orbit the black hole and sharply plunges inwards towards the event horizon. [5] [7] Particles within this plunging region experience rapid acceleration both angularly and towards the horizon. [5] Due to the rapid increase in velocity, for the first half of the matter's inspiral, the density of the accretion disk decreases, but it stabilizes afterwards. [5] [7]

While inside the plunging region, disk material becomes hotter, aided by magnetic reconnection converting magnetic energy into heat, and can contribute to observed x-ray emissions. [5] [8] [7] [9] Self-irradiation by the disk due to extreme gravitational lensing near the horizon may also contribute to the heating. [10] Plunging region dynamics may also be responsible for stronger-than-expected relativistic jets on many black holes, since they induce the magnetic extraction of spin energy, causing plasma to be accelerated away from the disk. [11] [12]

Disk luminosity in the plunging region is dramatically decreased compared to the luminosity of the disk outside the ISCO. Accretion flow inside the ISCO is also smoother and less turbulent than accretion flow far from the black hole. [10] For nonspinning black holes without strong magnetic fields, the accretion disk remains optically thick all the way down to the event horizon. [13] [14] Some sub-Eddington "puffy" accretion disks may even be stable within the ISCO. [15] [16]

Implications

The plunging region affects light given off by a black hole's accretion disk. From this, scientists can deduce the location of the ISCO and the spin of the black hole, since ISCOs closer to the event horizon correspond to faster-spinning black holes. [7] [9] [14] Additionally, magnetosonic waves can escape from inside the plunging region, minorly affecting the luminosity of the disk even outside the ISCO. [10]

Astronomical observations of high-energy nonthermal radiation emitted by x-ray binaries may be explained by electrons escaping from inside the plunging region. [17]

References

  1. ↑ "First proof that "plunging regions" exist around black holes in space". University of Oxford. 2024-05-17. Retrieved 2025-08-12.
  2. ↑ Parshall, Allison (2024-05-23). Billings, Lee (ed.). "We've Finally Seen Matter Plunge into a Black Hole". Scientific American. Retrieved 2025-08-12.
  3. ↑ "First Proof that "Plunging Regions" Exist Around Black Holes". NuSTAR. 2024-05-16. Retrieved 2025-08-12.
  4. ↑ Crane, Leah (2024-05-16). "Einstein was right about the way matter plunges into black holes". NewScientist.
  5. 1 2 3 4 5 Mummery, Andrew; Stone, James M. (2024). "The three-dimensional structure of black hole accretion flows within the plunging region". Monthly Notices of the Royal Astronomical Society. 532 (3): 3395–3416. doi: 10.1093/mnras/stae1643 .
  6. 1 2 Prisco, Jacopo (2024-05-17). "Study proves black holes have a 'plunging region,' just as Einstein predicted". CNN .
  7. 1 2 3 4 5 Wilkins, D. R.; Reynolds, C. S.; Fabian, A. C. (2020). "Venturing beyond the ISCO: Detecting X-ray emission from the plunging regions around black holes". Monthly Notices of the Royal Astronomical Society. 493 (4): 5532–5550. arXiv: 2003.00019 . doi: 10.1093/mnras/staa628 .
  8. ↑ Machida, Mami; Matsumoto, Ryoji (2003). "Global Three-dimensional Magnetohydrodynamic Simulations of Black Hole Accretion Disks: X-Ray Flares in the Plunging Region". The Astrophysical Journal. 585 (1): 429–442. arXiv: astro-ph/0211240 . Bibcode:2003ApJ...585..429M. doi:10.1086/346070.
  9. 1 2 Zhu, Yucong; Davis, Shane W.; Narayan, Ramesh; Kulkarni, Akshay K.; Penna, Robert F.; McClintock, Jeffrey E. (2012). "The eye of the storm: Light from the inner plunging region of black hole accretion discs". Monthly Notices of the Royal Astronomical Society. 424 (4): 2504–2521. arXiv: 1202.1530 . Bibcode:2012MNRAS.424.2504Z. doi: 10.1111/j.1365-2966.2012.21181.x .
  10. 1 2 3 Shafee, Rebecca; McKinney, Jonathan C.; Narayan, Ramesh; Tchekhovskoy, Alexander; Gammie, Charles F.; McClintock, Jeffrey E. (2008). "Three-Dimensional Simulations of Magnetized Thin Accretion Disks around Black Holes: Stress in the Plunging Region". The Astrophysical Journal. 687 (1): L25 –L28. arXiv: 0808.2860 . Bibcode:2008ApJ...687L..25S. doi:10.1086/593148.
  11. ↑ Reynolds, Christopher S.; Garofalo, David; Begelman, Mitchell C. (2006). "Trapping of Magnetic Flux by the Plunge Region of a Black Hole Accretion Disk". The Astrophysical Journal. 651 (2): 1023–1030. arXiv: astro-ph/0607381 . Bibcode:2006ApJ...651.1023R. doi:10.1086/507691.
  12. ↑ Chen, Bin; Hou, Yehui; Li, Junyi; Shen, Ye (2024). "Energy extraction from a Kerr black hole via magnetic reconnection within the plunging region". Physical Review D. 110 (6) 063003. arXiv: 2405.11488 . Bibcode:2024PhRvD.110f3003C. doi:10.1103/PhysRevD.110.063003.
  13. ↑ Reynolds, Christopher S.; Begelman, Mitchell C. (1997). "Iron Fluorescence from within the Innermost Stable Orbit of Black Hole Accretion Disks". The Astrophysical Journal. 488 (1): 109–118. arXiv: astro-ph/9705136 . Bibcode:1997ApJ...488..109R. doi:10.1086/304703.
  14. 1 2 Kopáček, Ondřej; Karas, Vladimír (2024). "On Innermost Stable Spherical Orbits near a Rotating Black Hole: A Numerical Study of the Particle Motion near the Plunging Region". The Astrophysical Journal. 966 (2): 226. arXiv: 2404.04501 . Bibcode:2024ApJ...966..226K. doi: 10.3847/1538-4357/ad3932 .
  15. ↑ Lančová, Debora; Abarca, David; Kluźniak, Włodek; Wielgus, Maciek; Sa̧Dowski, Aleksander; Narayan, Ramesh; Schee, Jan; Török, Gabriel; Abramowicz, Marek (2019). "Puffy Accretion Disks: Sub-Eddington, Optically Thick, and Stable". The Astrophysical Journal Letters. 884 (2): L37. arXiv: 1908.08396 . Bibcode:2019ApJ...884L..37L. doi: 10.3847/2041-8213/ab48f5 .
  16. ↑ Wielgus, Maciek; Lančová, Debora; Straub, Odele; Kluźniak, Włodek; Narayan, Ramesh; Abarca, David; Różańska, Agata; Vincent, Frederic; Török, Gabriel; Abramowicz, Marek (2022). "Observational properties of puffy discs: Radiative GRMHD spectra of mildly sub-Eddington accretion". Monthly Notices of the Royal Astronomical Society. 514: 780–789. arXiv: 2202.08831 . doi: 10.1093/mnras/stac1317 .
  17. ↑ Hankla, Amelia M.; Scepi, Nicolas; Dexter, Jason (2022). "Non-thermal emission from the plunging region: A model for the high-energy tail of black hole X-ray binary soft states". Monthly Notices of the Royal Astronomical Society. 515: 775–784. arXiv: 2206.12018 . doi: 10.1093/mnras/stac1785 .
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