PSZ2 G181.06+48.47. I. X-Ray Exploration of a Low-mass Cluster with Exceptionally Distant Radio Relics

Abstract Relics are diffuse, highly polarized radio sources that trace merger-driven shocks at the periphery of merging galaxy clusters. The Low Frequency Array survey recently discovered a rare example of double relics in the low-mass cluster PSZ2 G181.06+48.47. Through a detailed exploration of new Chandra and XMM-Newton observations, we reveal that PSZ2 G181.06+48.47 has a lower mass ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>500,X</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>2.3</mml:mn> <mml:msubsup> <mml:mn>2</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.25</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.29</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mn>0</mml:mn> <mml:mn>14</mml:mn> </mml:msup> <mml:mspace width="0.33em"/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:math> ) than previously thought. Despite its cool global temperature of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>k</mml:mi> <mml:msub> <mml:mi>T</mml:mi> <mml:mn>500</mml:mn> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>3.6</mml:mn> <mml:msubsup> <mml:mn>2</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.07</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.15</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> keV, PSZ2 G181.06+48.47 is one of the most disturbed clusters in the Planck sample, with a complex morphological and thermodynamic structure. We discover a set of three discontinuities within &lt;500 kpc of the cluster center, and, from a surface brightness analysis, place 5 σ upper limits of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">M</mml:mi> <mml:mi>NE</mml:mi> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:mn>1.43</mml:mn> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">M</mml:mi> <mml:mi>SW</mml:mi> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:mn>1.57</mml:mn> </mml:math> for any shock associated with the relic locations. We also revise established scaling relations for double radio relics by adding 12 new systems not included in previous work. The PSZ2 G181.06+48.47 relics have the widest separation (scaled for r 500 ) of all known double relic systems. The exceptional distance from the cluster center (&gt; r 200 ) indicates the relics may be associated with shocks in the “runaway” phase. We propose that this late-stage, post-apocenter merger is captured as the two subclusters with a mass ratio of 1.2–1.4 fall back into each other. The outer relic shocks were likely produced at the first core passage, while the inner discontinuities are associated with the second infall.