Polarimetric Signatures of Bulk Comptonization from within the Plunging Region of Accreting Black Holes
Inverse Compton scattering by the thermal motions of electrons is believed to produce polarized hard X-rays in active galactic nuclei and black hole binaries. Meanwhile, plasma within the plunging region of the black hole free-falls into the event horizon with a bulk relativistic speed, which could...
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IOP Publishing
2025-01-01
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| Online Access: | https://doi.org/10.3847/1538-4357/adc9a0 |
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| author | Ho-Sang Chan Mitchell C. Begelman Jason Dexter |
| author_facet | Ho-Sang Chan Mitchell C. Begelman Jason Dexter |
| author_sort | Ho-Sang Chan |
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| description | Inverse Compton scattering by the thermal motions of electrons is believed to produce polarized hard X-rays in active galactic nuclei and black hole binaries. Meanwhile, plasma within the plunging region of the black hole free-falls into the event horizon with a bulk relativistic speed, which could also imprint polarization on upscattered photons but has not been discussed in detail. To examine this, we computed polarimetric signatures via general relativistic ray tracing of a toy model consisting of an accreting, geometrically thin plasma with moderate optical depth, falling into the black hole with a bulk relativistic speed within the plunging region. We show that the maximum spatially unresolved linear polarization could be as large as approximately 7%–8% when the black hole is viewed near edge-on, while the corresponding resolved linear polarization could be roughly 50%. The large discrepancy between the two is due to (1) dilution from the radiation outside the plunging region and (2) substantial cancellations of the Stokes Q and U fluxes. The resultant polarization contributed by bulk Comptonization could nevertheless exceed that of thermal electron scattering in a Novikov–Thorne disk. Our results thus suggest a new model for imprinting considerable polarization on the electromagnetic observables of accreting black holes. Measurements of X-ray polarization from black hole binaries and the central black hole of active galactic nuclei could provide direct detection of the plunging region and help constrain plasma properties in the immediate vicinity of the event horizon. |
| format | Article |
| id | doaj-art-07afec4e6fa24f19b89e41eff662a41c |
| institution | Kabale University |
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| language | English |
| publishDate | 2025-01-01 |
| publisher | IOP Publishing |
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| series | The Astrophysical Journal |
| spelling | doaj-art-07afec4e6fa24f19b89e41eff662a41c2025-08-20T03:47:44ZengIOP PublishingThe Astrophysical Journal1538-43572025-01-01985214910.3847/1538-4357/adc9a0Polarimetric Signatures of Bulk Comptonization from within the Plunging Region of Accreting Black HolesHo-Sang Chan0https://orcid.org/0000-0003-2776-082XMitchell C. Begelman1https://orcid.org/0000-0003-0936-8488Jason Dexter2https://orcid.org/0000-0003-3903-0373JILA, University of Colorado and National Institute of Standards and Technology , 440 UCB, Boulder, CO 80309-0440, USA ; hschanastrophy1997@gmail.com; Department of Astrophysical and Planetary Sciences, University of Colorado , 391 UCB, Boulder, CO 80309, USAJILA, University of Colorado and National Institute of Standards and Technology , 440 UCB, Boulder, CO 80309-0440, USA ; hschanastrophy1997@gmail.com; Department of Astrophysical and Planetary Sciences, University of Colorado , 391 UCB, Boulder, CO 80309, USAJILA, University of Colorado and National Institute of Standards and Technology , 440 UCB, Boulder, CO 80309-0440, USA ; hschanastrophy1997@gmail.com; Department of Astrophysical and Planetary Sciences, University of Colorado , 391 UCB, Boulder, CO 80309, USAInverse Compton scattering by the thermal motions of electrons is believed to produce polarized hard X-rays in active galactic nuclei and black hole binaries. Meanwhile, plasma within the plunging region of the black hole free-falls into the event horizon with a bulk relativistic speed, which could also imprint polarization on upscattered photons but has not been discussed in detail. To examine this, we computed polarimetric signatures via general relativistic ray tracing of a toy model consisting of an accreting, geometrically thin plasma with moderate optical depth, falling into the black hole with a bulk relativistic speed within the plunging region. We show that the maximum spatially unresolved linear polarization could be as large as approximately 7%–8% when the black hole is viewed near edge-on, while the corresponding resolved linear polarization could be roughly 50%. The large discrepancy between the two is due to (1) dilution from the radiation outside the plunging region and (2) substantial cancellations of the Stokes Q and U fluxes. The resultant polarization contributed by bulk Comptonization could nevertheless exceed that of thermal electron scattering in a Novikov–Thorne disk. Our results thus suggest a new model for imprinting considerable polarization on the electromagnetic observables of accreting black holes. Measurements of X-ray polarization from black hole binaries and the central black hole of active galactic nuclei could provide direct detection of the plunging region and help constrain plasma properties in the immediate vicinity of the event horizon.https://doi.org/10.3847/1538-4357/adc9a0High energy astrophysicsBlack hole physicsBlack holesAstrophysical black holesPolarimetryGeneral relativity |
| spellingShingle | Ho-Sang Chan Mitchell C. Begelman Jason Dexter Polarimetric Signatures of Bulk Comptonization from within the Plunging Region of Accreting Black Holes The Astrophysical Journal High energy astrophysics Black hole physics Black holes Astrophysical black holes Polarimetry General relativity |
| title | Polarimetric Signatures of Bulk Comptonization from within the Plunging Region of Accreting Black Holes |
| title_full | Polarimetric Signatures of Bulk Comptonization from within the Plunging Region of Accreting Black Holes |
| title_fullStr | Polarimetric Signatures of Bulk Comptonization from within the Plunging Region of Accreting Black Holes |
| title_full_unstemmed | Polarimetric Signatures of Bulk Comptonization from within the Plunging Region of Accreting Black Holes |
| title_short | Polarimetric Signatures of Bulk Comptonization from within the Plunging Region of Accreting Black Holes |
| title_sort | polarimetric signatures of bulk comptonization from within the plunging region of accreting black holes |
| topic | High energy astrophysics Black hole physics Black holes Astrophysical black holes Polarimetry General relativity |
| url | https://doi.org/10.3847/1538-4357/adc9a0 |
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