We present 610 MHz and 2.1 GHz imaging of the massive Sunyaev Zel ’ dovich Effect ( SZE ) -selected z = 0.870 cluster merger ACT-CL J0102-4915 ( “ El Gordo ” ) , obtained with the Giant Metre-wave Radio Telescope ( GMRT ) and the Australia Telescope Compact Array ( ATCA ) , respectively .
We detect two complexes of radio relics separated by 3.4 ^ { \prime } ( 1.6 Mpc ) along the system ’ s northwest-to-southeast collision axis that have high integrated polarization fractions ( 33 \% ) and steep spectral indices ( \alpha between 1–2 ; S _ { \nu } \propto \nu ^ { - \alpha } ) , 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 \mathcal { M } = 2.5 ^ { +0.7 } _ { -0.3 } and shock speed of 2500 ^ { +400 } _ { -300 } km s ^ { -1 } .
With our wide-bandwidth , full-polarization ATCA data , we compute the Faraday depth \phi across the northwest relic and find a range of values spanning \Delta \phi = 30 rad m ^ { -2 } , with a mean value of \left < \phi \right > = 11 rad m ^ { -2 } and standard deviation \sigma _ { \phi } = 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 _ { \parallel } \sim 0.01 \mu G in the cluster outskirts .
The extremely narrow shock widths in the relics ( d _ { shock } \leq 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 } \simeq 1.1 Mpc radius ) , powerful ( \log ( L _ { 1.4 } / W Hz ^ { -1 } ) = 25.66 \pm 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 \alpha = 1.2 \pm 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 .