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Abstract Low-collisionality plasma in a magnetic field generically develops anisotropy in its distribution function with respect to the magnetic field direction. Motivated by the application to radiation from accretion flows and jets, we explore the effect of temperature anisotropy on synchrotron emission. We derive analytically and provide numerical fits for the polarized synchrotron emission and absorption coefficients for a relativistic bi-Maxwellian plasma (we do not consider Faraday conversion/rotation). Temperature anisotropy can significantly change how the synchrotron emission and absorption coefficients depend on observing angle with respect to the magnetic field. The emitted linear polarization fraction does not depend strongly on anisotropy, while the emitted circular polarization does. We apply our results to black hole imaging of Sgr A* and M87* by ray tracing a GRMHD simulation and assuming that the plasma temperature anisotropy is set by the thresholds of kinetic-scale anisotropy-driven instabilities. We find that the azimuthal asymmetry of the 230 GHz images can change by up to a factor of 3, accentuating (T⊥>T∥) or counteracting (T⊥<T∥) the image asymmetry produced by Doppler beaming. This can change the physical inferences from observations relative to models with an isotropic distribution function, e.g., by allowing for larger inclination between the line of sight and spin direction in Sgr A*. The observed image diameter and the size of the black hole shadow can also vary significantly due to plasma temperature anisotropy. We describe how the anisotropy of the plasma can affect future multifrequency and photon ring observations. We also calculate kinetic anisotropy-driven instabilities (mirror, whistler, and firehose) for relativistically hot plasmas.
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This content will become publicly available on May 21, 2026
The 230 GHz Variability of Numerical Models of Sagittarius A*. II. The Physical Origins of the Variability
Abstract We continue our previous work, H.-S. Chan et al., to investigate how variations in the electron temperature prescription parameter,RLow, influence the 3 hr variability at 230 GHz,MΔT, in magnetically arrested disk (MAD) models of Sagittarius A* (Sgr A*), through analyzing a series of general-relativistic magnetohydrodynamics and ray-tracing simulations. For models with a black hole spina > 0, we discovered that increasingRLowrenders the photon ring more optically thick, obscuring the varying accretion flows that contribute to the variability. However, asRLowincreases further, MAD flux eruptions become more pronounced, compensating for the decrease inMΔT. For models with spina < 0, although a higherRLowalso increases the optical thickness of the fluid, voids within the optically thick gas fail to cover the entire photon ring. Similarly, flux eruptions become more prominent asRLowincreases further, contributing to the observed rise inMΔTrelative toRLow. For black holes with spina= 0, although the effect of increasing optical depth is still present, their 230 GHz light curves, and henceMΔT, are insensitive to changes inRLow. Furthermore, we found that the variability of the 230 GHz light curves atRLow = 1 might correlate with fluctuations in the internal energy of the gas near the black hole, and we listed potential causes and solutions to the over-variability problem. Our findings highlight potential approaches for refiningMΔTto better align with observations when modeling Sgr A*.
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- Award ID(s):
- 2034306
- PAR ID:
- 10660239
- Publisher / Repository:
- IOP
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 985
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 164
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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