Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Radially polarized synchrotron from galaxy-cluster virial shocks

Uri Keshet

TL;DR

This work seeks to map virial shocks in galaxy clusters by exploiting polarization, exploiting the self-similar scaling by $R_{500}$ and focusing on the radial location $\tau\simeq 2.4$. By stacking GMIMS high-band polarization data around MCXC clusters and decomposing into radial and tangential components, the authors detect a radially polarized excess around $\tau\simeq 2.4$, with local $\sim 3$–$4\sigma$ significance and robust model comparisons ($TS\approx 5.8$ to $13.4$ depending on binning). The findings support synchrotron emission from virial-shock–accelerated electrons with a flat spectrum and high polarization fraction, consistent with a shock-driven transverse magnetic field and a minimal shock radius near $2.4R_{500}$ within a cylindrical geometry. These results provide directional, polarization-based confirmation of virial shocks and offer constraints on shock physics, magnetic-field amplification, and mass dependence in cluster accretion processes.

Abstract

Radio-to-$γ$-ray signals, recently found narrowly confined near the characteristic $2.4R_{500}$ scaled radii of galaxy clusters and groups, have been associated with their virial (structure-formation accretion) shocks based on spectro-spatial characteristics. By stacking high-latitude GMIMS radio data around MCXC galaxy clusters, we identify ($3σ$--$4σ$) excess radially polarized emission at the exact same scaled radius, providing directional support, and indicating tangential magnetic fields induced by the shocked inflow. The results suggest a strong mass dependence, a flat energy spectrum, and a high polarization fraction, consistent with synchrotron emission from electrons accelerated by strong virial shocks. The narrow radial range of such stacked virial-shock signals suggests that although the shocks are theorized to have diverse, irregular morphologies, they share similar $\sim2.4R_{500}$ minimal radii.

Radially polarized synchrotron from galaxy-cluster virial shocks

TL;DR

This work seeks to map virial shocks in galaxy clusters by exploiting polarization, exploiting the self-similar scaling by and focusing on the radial location . By stacking GMIMS high-band polarization data around MCXC clusters and decomposing into radial and tangential components, the authors detect a radially polarized excess around , with local – significance and robust model comparisons ( to depending on binning). The findings support synchrotron emission from virial-shock–accelerated electrons with a flat spectrum and high polarization fraction, consistent with a shock-driven transverse magnetic field and a minimal shock radius near within a cylindrical geometry. These results provide directional, polarization-based confirmation of virial shocks and offer constraints on shock physics, magnetic-field amplification, and mass dependence in cluster accretion processes.

Abstract

Radio-to--ray signals, recently found narrowly confined near the characteristic scaled radii of galaxy clusters and groups, have been associated with their virial (structure-formation accretion) shocks based on spectro-spatial characteristics. By stacking high-latitude GMIMS radio data around MCXC galaxy clusters, we identify (--) excess radially polarized emission at the exact same scaled radius, providing directional support, and indicating tangential magnetic fields induced by the shocked inflow. The results suggest a strong mass dependence, a flat energy spectrum, and a high polarization fraction, consistent with synchrotron emission from electrons accelerated by strong virial shocks. The narrow radial range of such stacked virial-shock signals suggests that although the shocks are theorized to have diverse, irregular morphologies, they share similar minimal radii.

Paper Structure

This paper contains 5 sections, 8 equations, 5 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Data and stacking method
  3. Radially polarized excess
  4. Discussion
  5. Modelling the stacked shock signature

Figures (5)

  • 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 connected by 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 (down triangles keshet2018evidence) and VERITAS (coincident with synchrotron emission and an SZ drop; up triangles KeshetEtAl17) data as a function of $\tau_b$, A2319 (right triangles) and A2142 (left triangles) in Fermi-LAT data (coincident with SZ drops keshet20coincident), and stacked MCXC clusters (labels specify their number) in Fermi-LAT (circles reiss2018detection) and LWA (diamonds HouEtAl23) data and in NVSS (five-pointed stars) and 2XRS (six-stars) source catalogs IlaniEtAl24a. Also shown are the $2.2<\tau<2.5$ radial range reiss2018detection of MCXC virial-shock signals (vertical yellow band), the co-added MCXC excess (squares), and a corresponding simple cylindrical shock model (dashed red curve; see text). See Table \ref{['tab:fig']} for more info.
  • Figure 2:
  • Figure 3: Nominal cluster sample stacked as in Fig. \ref{['fig:nuIh']}, but with different resolutions and frequencies. Top panel: Radial polarization in the $\nu_h$ channel for resolutions $\Delta \tau=1/4$ (nominal; blue up triangles with solid guide), $\Delta\tau=1/6$ (magenta diamonds; dashed guide), and $\Delta\tau=1/8$ (black down triangles; dash-dotted guide), each with its corresponding reference (large to small symbols) non-polarized LWA-extrapolated diffuse (cyan circle) and NVSS source (purple star) emission. Middle panel: Same as the bottom panel of Fig. \ref{['fig:nuIh']}, but for $\Delta\tau=1/8$ resolution. Bottom panel: Same as the top panel, but averaged over $[\nu_l,\nu_h]$ frequencies.
  • Figure 4: Significance of the nominal local excess of radial ($S_r$; blue up triangles) and tangential ($S_t$; green right triangles) polarizations in the virial-shock bin, and their ratio ($S_r/S_t$; red circles), after correcting for different putative Galactic Faraday RM values (abscissa; top axis shows the corresponding rotation angle) in the $\nu_h$ channel. Also shown is the measured TaylorEtAl09$\hbox{RM}=(3.0\pm0.5)\hbox{rad m}^{-2}$ range (for $b>75^{\circ}$; vertical yellow band). Sign conventions are standard: $\hbox{RM}>0$ implies that magnetic fields, preferentially oriented toward the observer, have rotated the polarization vector counter-clockwise on the sky.
  • Figure 5: Same as Fig. \ref{['fig:nuIh']} (top panel), but for a sample of lower, $10^{13}<M_{500}/M_\odot<10^{14}$ mass clusters and groups.