- Award ID(s):
- 2034306
- PAR ID:
- 10390971
- Date Published:
- Journal Name:
- Galaxies
- Volume:
- 10
- Issue:
- 5
- ISSN:
- 2075-4434
- Page Range / eLocation ID:
- 103
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract With unprecedented angular resolution, the Event Horizon Telescope (EHT) has opened a new era of black hole studies. We have previously calculated the expected polarization images of M 87* with EHT observations in mind. There, we demonstrated that circular polarization (CP) images, as well as linear polarization (LP) maps, can convey quite useful information, such as the flow structure and magnetic field configuration around the black hole. In this paper, we make new predictions for the cases in which disk emission dominates over jet emission, bearing Sgr A* in mind. Here we set the proton-to-electron temperature ratio of the disk component to be Tp/Te ∼ 2 so as to suppress jet emission relative to emission from accretion flow. As a result, we obtain ring-like images and triple-forked images around the black hole for face-on and edge-on cases, respectively. We also find significant CP components in the images (≳10% in fraction), with both positive and negative signs, amplified through the Faraday conversion, not depending sensitively on the inclination angles. Furthermore, we find a “separatrix” in the CP images, across which the sign of CP is reversed and on which the LP flux is brightest, that can be attributed to the helical magnetic field structure in the disk. These results indicate that future full polarization EHT images are a quite useful tracer of the magnetic field structure. We also discuss to what extent we will be able to extract information regarding magnetic field configurations under the scattering in the interstellar plasma, in future EHT polarimetric observations of Sgr A*.more » « less
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Abstract For testing different electron temperature (
T e) prescriptions in general relativistic magnetohydrodynamics (GRMHD) simulations through observations, we propose to utilize linear polarization (LP) and circular polarization (CP) images. We calculate the polarization images based on a semi-magnetically arrested disk GRMHD model for variousT eparameters, bearing M87 in mind. We find an LP–CP separation in the images of the low-T edisk cases at 230GHz; namely, the LP flux mainly originates from downstream of the jet, and the CP flux comes from the counter-side jet, while the total intensity is maximum at the jet base. This can be understood as follows: although the LP flux is generated through synchrotron emission widely around the black hole, most of the LP flux from the jet base does not reach the observer, since it undergoes Faraday rotation ( ) when passing through the outer cold disk and is thus depolarized. Hence, only the LP flux from the downstream (not passing the cold dense plasmas) can survive. Meanwhile, the CP flux is generated from the LP flux by Faraday conversion ( ∝T e) in the inner hot region. Stronger CP flux is thus observed from the counter-side jet. Moreover, the LP–CP separation is more enhanced at a lower frequency, such as 86 GHz, but is rather weak at 43 GHz, since the media in the latter case is optically thick for synchrotron self-absorption so that all of the fluxes should come from the photosphere. The same is true for cases with higher mass accretion rates and/or larger inclination angles. -
ABSTRACT Horizon-scale observations of the jetted active galactic nucleus M87 are compared with simulations spanning a broad range of dissipation mechanisms and plasma content in three-dimensional general relativistic flows around spinning black holes. Observations of synchrotron radiation from radio to X-ray frequencies can be compared with simulations by adding prescriptions specifying the relativistic electron-plus-positron distribution function and associated radiative transfer coefficients. A suite of time-varying simulations with various spins, plasma magnetizations and turbulent heating and equipartition-based emission prescriptions (and piecewise combinations thereof) is chosen to represent distinct possibilities for the M87 jet/accretion flow/black hole system. Simulation jet morphology, polarization, and variation are then ‘observed’ and compared with real observations to infer the rules that govern the polarized emissivity. Our models support several possible spin/emission model/plasma composition combinations supplying the jet in M87, whose black hole shadow has been observed down to the photon ring at 230 GHz by the Event Horizon Telescope (EHT). Net linear polarization and circular polarization constraints favour magnetically arrested disc (MAD) models whereas resolved linear polarization favours standard and normal evolution (SANE) in our parameter space. We also show that some MAD cases dominated by intrinsic circular polarization have near-linear V/I dependence on un-paired electron or positron content while SANE polarization exhibits markedly greater positron-dependent Faraday effects – future probes of the SANE/MAD dichotomy and plasma content with the EHT. This is the second work in a series also applying the ‘observing’ simulations methodology to near-horizon regions of supermassive black holes in Sgr A* and 3C 279.
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ABSTRACT The circular polarization of black hole accretion flows can encode properties of the underlying magnetic field structure. Using general relativistic magnetohydrodynamic (GRMHD) simulations, we study the imprint of magnetic field geometry on circular polarization images potentially observable by the Event Horizon Telescope (EHT). We decompose images into the different mechanisms that generate circular polarization in these models that are sensitive to both the line-of-sight direction and twist of the magnetic field. In these models, a stable sign of the circular polarization over time, as observed for several sources, can be attributed to a stability of these properties. We illustrate how different aspects of a generic helical magnetic field geometry become imprinted on a circular polarization image. We also identify novel effects of light bending that affect the circular polarization image on event horizon scales. One consequence is the sign flipping of successive photon rings in face-on systems, which if observable and uncorrupted by Faraday rotation, can directly encode the handedness of the approaching magnetic field.
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Abstract Event Horizon Telescope (EHT) observations have revealed a bright ring of emission around the supermassive black hole at the center of the M87 galaxy. EHT images in linear polarization have further identified a coherent spiral pattern around the black hole, produced from ordered magnetic fields threading the emitting plasma. Here we present the first analysis of circular polarization using EHT data, acquired in 2017, which can potentially provide additional insights into the magnetic fields and plasma composition near the black hole. Interferometric closure quantities provide convincing evidence for the presence of circularly polarized emission on event-horizon scales. We produce images of the circular polarization using both traditional and newly developed methods. All methods find a moderate level of resolved circular polarization across the image (〈∣
v ∣〉 < 3.7%), consistent with the low image-integrated circular polarization fraction measured by the Atacama Large Millimeter/submillimeter Array (∣v int∣ < 1%). Despite this broad agreement, the methods show substantial variation in the morphology of the circularly polarized emission, indicating that our conclusions are strongly dependent on the imaging assumptions because of the limited baseline coverage, uncertain telescope gain calibration, and weakly polarized signal. We include this upper limit in an updated comparison to general relativistic magnetohydrodynamic simulation models. This analysis reinforces the previously reported preference for magnetically arrested accretion flow models. We find that most simulations naturally produce a low level of circular polarization consistent with our upper limit and that Faraday conversion is likely the dominant production mechanism for circular polarization at 230 GHz in M87*.