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Macroscopic magnetization of primordial plasma by virial shocks

Uri Keshet, Kuan-Chou Hou

Abstract

Galaxy-cluster virial (structure-formation accretion) shock observations are shown to imply $\gtrsim1\%$ magnetization of a layer extending $\gtrsim10^{16}$ Debye lengths downstream, challenging the modelling of high Alfvén-Mach collisionless shocks. Unlike similar shocks in supernova remnants or relativistic shocks in $γ$-ray burst afterglows, where macroscopic magnetized layers were detected but purportedly attributed to preexisting or non-resonant cosmic-ray streaming-seeded substructure, the upstream of strong virial shocks is both weakly magnetized and pristine. Hence, some mechanism must generate large-scale and possibly self-similar magnetic sub-structure out of the accreted primordial plasma; such a mechanism may dominate other high-Mach shock systems, too.

Macroscopic magnetization of primordial plasma by virial shocks

Abstract

Galaxy-cluster virial (structure-formation accretion) shock observations are shown to imply magnetization of a layer extending Debye lengths downstream, challenging the modelling of high Alfvén-Mach collisionless shocks. Unlike similar shocks in supernova remnants or relativistic shocks in -ray burst afterglows, where macroscopic magnetized layers were detected but purportedly attributed to preexisting or non-resonant cosmic-ray streaming-seeded substructure, the upstream of strong virial shocks is both weakly magnetized and pristine. Hence, some mechanism must generate large-scale and possibly self-similar magnetic sub-structure out of the accreted primordial plasma; such a mechanism may dominate other high-Mach shock systems, too.

Paper Structure

This paper contains 5 sections, 30 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Observations of virial shocks and their environment
  3. Virial-shock magnetization
  4. Discussion
  5. Virial-shock synchrotron model

Figures (1)

  • Figure 1: Virial shock signals identified in individual (triangles) or stacked (other symbols) clusters, in diffuse emission (empty symbols), discrete sources (filled), or without separating the two (intermittent empty and filled symbols). The significance $S$ (symbols with lines to guide the eye, in standard-error units) of the excess above the background $S_0$ (labelled horizontal lines, shifted vertically for visibility) is plotted as a function of the normalized radius $\tau$, for (bottom to top) clusters Coma, in Fermi-LAT keshet2018evidence and VERITAS KeshetEtAl17 data as a function of $\tau_b$, A2319 (right triangles) and A2142 (left triangles) in Fermi-LAT data keshet20coincident, and for stacked MCXC clusters (labels specify sample sizes) in LWA HouEtAl23, polarized GMIMS Keshet24GMIMS, and Fermi-LAT ReissKeshet18 data, and in NVSS (five-stars) and 2XRS (six-stars) source catalogs IlaniEtAl24a. Also shown are the $2.2<\tau<2.5$ radial range ReissKeshet18 of MCXC virial-shock signals (vertical yellow band), the co-added MCXC excess (black squares), and a corresponding cylindrical shock model Keshet24GMIMS.