A Kinematic Measurement of the Effective Viscosity of the Intracluster Medium: The Cold Front of A3667 Seen by XRISM
Abstract
Constraints on the effective viscosity of the intracluster medium (ICM) come almost exclusively from imaging: the power spectrum of X-ray surface brightness fluctuations, and the morphology of Kelvin-Helmholtz rolls at cold fronts. Both indicate a viscosity below the isotropic Spitzer value. XRISM/Resolve opens a complementary, purely kinematic route, in which the viscosity is inferred from the thickness of the viscous shear layer that develops at a sliding contact discontinuity. We apply it to the prototypical cold front of A3667. The interface is the classical Rayleigh problem, in which momentum diffuses to a depth d in a time T, giving a kinematic viscosity ν~ d^2/(4T). The XRISM velocity map shows a line-of-sight velocity jump of 535 km/s across the front, completed within the 100 kpc-wide region immediately inside it, the only region with an anomalously broad line. Hence d ~< 100 kpc. Adopting interaction times set by the sizes of the interacting regions divided by this velocity, we obtain dynamic viscosities of 5.2 x 10^3 and 3.3 x 10^3 g/cm/s for the cool and the hot side of the front, or 6.4 and 0.72 times the Spitzer value. Because d is limited by the angular resolution of XRISM, and the viscosity scales as its square, these are upper limits. The kinematic constraint is consistent with, and independent of, the imaging results.
Related
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BibTeX
@misc{fujita2026kinematic,
title = {{A Kinematic Measurement of the Effective Viscosity of the Intracluster Medium: The Cold Front of A3667 Seen by XRISM}},
author = {Yutaka Fujita and Yuki Omiya},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2609.04316},
eprint = {2609.04316},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.04316},
}
RIS
TY - UNPB
TI - A Kinematic Measurement of the Effective Viscosity of the Intracluster Medium: The Cold Front of A3667 Seen by XRISM
AU - Yutaka Fujita
AU - Yuki Omiya
PY - 2026
DA - 2026/10/24
PB - arXiv
DO - 10.48550/arxiv.2609.04316
UR - https://arxiv.org/abs/2609.04316
AB - Constraints on the effective viscosity of the intracluster medium (ICM) come almost exclusively from imaging: the power spectrum of X-ray surface brightness fluctuations, and the morphology of Kelvin-Helmholtz rolls at cold fronts. Both indicate a viscosity below the isotropic Spitzer value. XRISM/Resolve opens a complementary, purely kinematic route, in which the viscosity is inferred from the thickness of the viscous shear layer that develops at a sliding contact discontinuity. We apply it to the prototypical cold front of A3667. The interface is the classical Rayleigh problem, in which momentum diffuses to a depth d in a time T, giving a kinematic viscosity ν~ d^2/(4T). The XRISM velocity map shows a line-of-sight velocity jump of 535 km/s across the front, completed within the 100 kpc-wide region immediately inside it, the only region with an anomalously broad line. Hence d ~< 100 kpc. Adopting interaction times set by the sizes of the interacting regions divided by this velocity, we obtain dynamic viscosities of 5.2 x 10^3 and 3.3 x 10^3 g/cm/s for the cool and the hot side of the front, or 6.4 and 0.72 times the Spitzer value. Because d is limited by the angular resolution of XRISM, and the viscosity scales as its square, these are upper limits. The kinematic constraint is consistent with, and independent of, the imaging results.
ER -