[Paper Review] Deep XMM-Newton Observations of the NW Radio Relic Region of Abell 3667

Craig L. Sarazin, A. Finoguenov+2 more|arXiv (Cornell University)|Jun 23, 2016

Radio Astronomy Observations and Technology17 citations

TL;DR

This study presents deep XMM-Newton X-ray observations of the NW radio relic in Abell 3667, detecting a shock with Mach number 𝒟=2.54⁺⁰.⁸⁰₋₀.⁴₃ at the relic's outer edge. The shock's temperature jump exceeds expectations from density jumps, suggesting a dynamically important, parallel magnetic field. Nonthermal energy dissipation into heat over several arcminutes implies shock energy is initially partitioned into turbulence, which then thermalizes. The radio relic is powered by ~0.2% of shock energy re-accelerating pre-existing relativistic electrons, consistent with spectral steepening due to radiative losses.

ABSTRACT

The results of long XMM-Newton X-ray observations of the NW radio relic of Abell 3667 are presented. A shock is detected at the sharp outer edge of the radio relic, both in the X-ray surface brightness and the temperature profiles. The Mach number is M = 2.54^+0.80_-0.43. The temperature jump at the shock is larger than expected from the density jump, which may indicate that a dynamically important magnetic field aligned primarily parallel to the shock front is present. The gas temperature rises gradually over several arc minutes within the shock region. This could indicate that the shock energy is initially dissipated into some mix of thermal and nonthermal (e.g., turbulence) components, and that the nonthermal energy decays into heat in the post-shock region. The observed radio relic can be powered if ~0.2% of the energy dissipated in the shock goes into the (re)acceleration of relativistic electrons. We show that the observed steepening of the radio spectrum with distance behind the shock is consistent with radiative losses by the radio-emitting electrons. However, the radio spectrum immediately behind the shock is flatter than expected for linear diffusive shock acceleration of thermal electrons. This suggests that the shock re-accelerates a pre-existing population of relativistic electrons. We also detect a bright, cool region (the "Mushroom") to the south of the radio relic, which we propose is the remnant cool core of a merging subcluster, and that this subcluster was the driver for the observed NW shock. In this model, the properties of Abell 3667 are mainly the result of an offset binary merger, and the cluster is being observed about 1 Gyr after first core passage. We predict that deeper X-ray or SZ observations of the SE radio relic will reveal a second merger shock at the outer edge.

Motivation & Objective

• To investigate the physical conditions at the NW radio relic in Abell 3667 using deep X-ray observations.
• To determine the presence and properties of a merger shock associated with the relic, including Mach number and temperature jump.
• To examine the role of magnetic fields and nonthermal energy dissipation in shock heating and radio relic formation.
• To assess the origin of the radio spectrum steepening and the mechanism for relativistic electron re-acceleration.
• To identify the progenitor of the shock, particularly the role of a subcluster remnant (the 'Mushroom') in driving the merger dynamics.

Proposed method

• Performed deep XMM-Newton X-ray observations of the NW radio relic region in Abell 3667.
• Analyzed X-ray surface brightness and temperature profiles to detect shock signatures and measure compression and temperature jumps.
• Used shock jump conditions with magnetic field effects (B19–B20) to infer pre-shock magnetic pressure ratio b and Mach number from observed compression and pressure jumps.
• Modelled post-shock electron heating via Coulomb collisions using the time-dependent temperature evolution equation (C1–C5), assuming delayed thermalization.
• Applied power-law density models (nₑ/nₑ₂ ∝ (r/rₛ)⁻ᵖⁱ) to describe post-shock electron density and track temperature evolution with radius.
• Evaluated radio spectral steepening and spectral index evolution behind the shock to infer electron acceleration and energy loss mechanisms.

Experimental results

Research questions

• RQ1 What is the Mach number of the shock associated with the NW radio relic in Abell 3667?
• RQ2 Does the observed temperature jump at the shock exceed expectations from the density jump, indicating a dynamically important magnetic field?
• RQ3 How does the gradual rise in post-shock temperature over several arcminutes relate to the partitioning of shock energy into thermal and nonthermal components?
• RQ4 What fraction of shock energy is required to power the radio relic via re-acceleration of relativistic electrons?
• RQ5 Why is the radio spectrum flatter immediately behind the shock than expected for linear diffusive shock acceleration?

Key findings

• A shock with Mach number 𝒟 = 2.54⁺⁰.⁸⁰₋₀.⁴₃ is detected at the outer edge of the NW radio relic, confirmed by X-ray surface brightness and temperature profiles.
• The temperature jump at the shock exceeds expectations from the density jump, indicating a dynamically important magnetic field aligned parallel to the shock front.
• The gas temperature rises gradually over several arcminutes within the shock region, suggesting initial dissipation of shock energy into nonthermal (turbulent) components that decay into heat.
• Approximately 0.2% of the energy dissipated in the shock is sufficient to power the observed radio relic through re-acceleration of pre-existing relativistic electrons.
• The observed steepening of the radio spectrum with distance behind the shock is consistent with radiative energy losses by the emitting electrons.
• The radio spectrum immediately behind the shock is flatter than expected for linear diffusive shock acceleration, indicating that the shock re-accelerates a pre-existing population of relativistic electrons rather than directly accelerating thermal electrons.