Giant radio relics in galaxy clusters: reacceleration of fossil relativistic electrons?

Abstract

Many bright radio relics in the outskirts of galaxy clusters have low inferred Mach numbers, defying expectations from shock acceleration theory and heliospheric observations that the injection efficiency of relativistic particles plummets at low Mach numbers. With a suite of cosmological simulations, we follow the diffusive shock acceleration as well as radiative and Coulomb cooling of cosmic ray electrons during the assembly of a cluster. We find a substantial population of fossil electrons. When reaccelerated at a shock (through diffusive shock acceleration), they are competitive with direct injection at strong shocks and overwhelmingly dominate by many orders of magnitude at weak shocks, M < ∼ 3, which are the vast majority at the cluster periphery. Their relative importance depends on cooling physics and is robust to the shock acceleration model used. While the abundance of fossils can vary by a factor of ∼ 10, the typical reaccelerated fossil population has radio brightness in excellent agreement with observations. Fossil electrons with 1 < ∼ γ < ∼ 100 (10 < ∼ γ < ∼ 10 4 ) provide the main seeds for reacceleration at strong (weak) shocks; we show that these are well-resolved by our simulation. We construct a simple self-similar analytic model which assumes steady recent injection and cooling. It agrees well with our simulations, allowing rapid estimates and physical insight into the shape of the distribution function. We predict that LOFAR should find many more bright steep-spectrum radio relics, which are inconsistent with direct injection. A failure to take fossil cosmic ray electrons into account will lead to erroneous conclusions about the nature of particle acceleration at weak shocks; they arise from well-understood physical processes and cannot be ignored.

Chandra and JVLA Observations of HST Frontier Fields Cluster MACS J0717.5+3745

“…Another model to solve the low acceleration efficiency of standard DSA is that of re-acceleration of fossil electrons (e.g., Markevitch et al 2005;Giacintucci et al 2008;Kang & Ryu 2011;Kang et al 2012;Pinzke et al 2013). These fossil electrons could, for example, originate from the (old) lobes of radio galaxies.…”

“…A problem with this interpretation is that shock Mach numbers in clusters are typically low (  3), in which case DSA is thought to be inefficient. For that reason, several alternative models have been proposed including shock re-acceleration (e.g., Markevitch et al 2005;Giacintucci et al 2008;Kang & Ryu 2011;Kang et al 2012;Pinzke et al 2013) and turbulent re-acceleration (Fujita et al 2015). Recent work from particle-in-cell (PIC) simulations indicates that cluster shocks can inject electrons from the thermal pool (Guo et al 2014a(Guo et al , 2014band that these electrons gain energy via the shock drift acceleration (SDA) mechanism.…”

SuzakuX-ray study of the double radio relic galaxy cluster CIZA J2242.8+5301

“…Using the measured integrated spectral index and an assumption for the cooling effect, a similar trend has been seen in several radio relics and edges of the halo (Markevitch et al 2005;). There are several mechanism proposed to explain the particle acceleration in clusters (Markevitch et al 2005;Kang et al 2012;Pinzke et al 2013;Skillman et al 2013). A plausible possibility is re-acceleration of pre-existing nonthermal (low-energy cosmic-ray) particles in the ICM.…”

Lofar, Vla, and Chandra Observations of the Toothbrush Galaxy Cluster

“…In the secondary (or hadronic) model, the CR electrons are secondary products of collisions between thermal ions and relativistic protons in the ICM (e.g., Dennison 1980;Blasi & Colafrancesco 1999;Dolag & Enßlin 2000;Miniati et al 2001;Pfrommer & Enßlin 2004;Pfrommer et al 2008;Keshet & Loeb 2010;Enßlin et al 2011;Zandanel et al 2014). In the turbulent re-acceleration model, electrons are re-accelerated by merger induced magnetohydrodynamical turbulence (e.g., Schlickeiser et al 1987;Brunetti et al 2001;Petrosian 2001;Pinzke et al 2015). Secondary models are challenged by the large energy content of cosmic ray protons needed to explain radio halos with very steep spectra (Brunetti 2004;Brunetti et al 2008) and by the non-detection of γ-rays (e.g., Jeltema & Profumo 2011;Brunetti et al 2012;Ackermann et al 2014).…”