Dynamical properties and detectability of the magneto-thermal instability in the intracluster medium

Institut de Recherche en Astrophysique et Planétologie (IRAP), Université de Toulouse, CNRS, UPS, 31400 Toulouse, France
e-mail: jean.kempf@irap.omp.eu

Received: 8 June 2023
Accepted: 15 September 2023

Abstract

Context. Amongst the many plasma processes potentially relevant to the dynamics of the intracluster medium (ICM), turbulence driven at observable scales by internal magnetised buoyancy instabilities such as the magneto-thermal instability (MTI) stands out in the outskirts of the ICM, where the background temperature decreases with the radius.

Aims. We characterise the statistical properties of MTI turbulence in the ICM and assess whether such large-scale magnetised plasma dynamics would be detectable with the future X-ray calorimeter X-IFU on board Athena.

Methods. We made use of scaling laws previously derived to phenomenologically estimate the observable turbulent saturation levels and injection length of MTI turbulence for different ICM thermodynamic profiles, and performed a numerical magnetohydrodynamic simulation of the dynamics with Braginskii heat and momentum diffusion. As a prospective exercise, we used the simulation to virtually observe MTI turbulence through the X-IFU.

Results. In bright enough regions amenable to X-ray observations, the MTI drives mild turbulence up to ∼5% and ∼100 km s⁻¹ (root-mean square temperature fluctuation and velocity). However, the measurable integrated temperature fluctuation and line-of-sight velocity fields, the latter being essentially the azimuthal velocity component in cluster haloes, hardly exceed 1% and 10 km s⁻¹, respectively (root-mean square). We show that such moderate signals would be difficult to detect with upcoming X-ray telescopes. We also find that MTI turbulence is anisotropic in the direction of gravity and develops at scales ≳0.2 Mpc. If the fluctuation intensities were to be stronger than the current theoretical estimates, MTI fluctuations would be detectable and their anisotropy discernible with the X-IFU.

Conclusions. Finding direct signatures of magnetised plasma dynamics in the ICM, even at observable scales typical of the fluid MTI, remains challenging. This study only marks a first step in this direction. Several numerical and observational strategies are discussed to make further progress in the future.