2013/10/31 by Robert R. Lindner, Andrew J. Baker, John P. Hughes +8
Physics and Astronomy · #Astrophysics and Cosmic Phenomena #Cluster (spacecraft) #Faraday cage #Faraday effect #Fermi acceleration #Gamma-ray bursts and supernovae #Halo #Pulsars and Gravitational Waves Research #Radio halo #Radio spectrum #Shock wave #Spectral index #astro-ph.CO #astro-ph.HE
paper · pdf · doi:10.1088/0004-637x/786/1/49
22 pages, 19 figures, accepted in ApJ
arxiv created 2014/03/17 · openalex publication_date 2014/04/16 · arxiv updated 2015/06/17 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
We present 610 MHz and 2.1 GHz imaging of the massive Sunyaev–Zel'dovich Effect selected z = 0.870 cluster merger ACT-CL J0102−4915 ("El Gordo"), obtained with the Giant Metrewave Radio Telescope and the Australia Telescope Compact Array (ATCA), respectively. We detect two complexes of radio relics separated by 3 4 (1.6 Mpc) along the system's northwest-to-southeast collision axis that have high integrated polarization fractions (33%) and steep spectral indices (α between 1 and 2; S ν ∝ν −α ), consistent with creation via Fermi acceleration by shocks in the intracluster medium triggered by the cluster collision. From the spectral index of the relics, we compute a Mach number and shock speed of . With our wide-bandwidth, full-polarization ATCA data, we compute the Faraday depth ϕ across the northwest relic and find a range of values spanning Δϕ = 30 rad m −2 , with a mean value of 〈ϕ〉 = 11 rad m −2 and standard deviation σ ϕ = 6 rad m −2 . With the integrated line-of-sight gas density derived from new Chandra X-ray observations, our Faraday depth measurement implies B ∥ ∼ 0.01 μG in the cluster outskirts. The extremely narrow shock widths in the relics ( d shock ⩽ 23 kpc), caused by the short synchrotron cooling timescale of relativistic electrons at z = 0.870, prevent us from placing a meaningful constraint on the magnetic field strength B using cooling time arguments. In addition to the relics, we detect a large ( r H ≃ 1.1 Mpc radius), powerful (log ( L 1.4 /W Hz −1 ) = 25.66 ± 0.12) radio halo with a shape similar to El Gordo's "bullet"-like X-ray morphology. The spatially resolved spectral-index map of the halo shows the synchrotron spectrum is flattest near the relics, along the system's collision axis, and in regions of high T gas , all locations associated with recent energy injection. The spatial and spectral correlation between the halo emission and cluster X-ray properties supports primary-electron processes like turbulent reacceleration as the halo production mechanism. The halo's integrated 610 MHz to 2.1 GHz spectral index is a relatively flat α = 1.2 ± 0.1, consistent with the cluster's high T gas in view of previously established global scaling relations. El Gordo is the highest-redshift cluster known to host a radio halo and/or radio relics, and provides new constraints on the non-thermal physics in clusters at z > 0.6.