Journal
ASTROPHYSICAL JOURNAL
Volume 925, Issue 1, Pages -Publisher
IOP Publishing Ltd
DOI: 10.3847/1538-4357/ac3bc0
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Funding
- National Research Foundation (NRF) of Korea [2016R1A5A1013277, 2020R1A2C2102800, 2020R1F1A1048189, 2020R1C1C 1012112]
- Global PhD Fellowship of the NRF [2017H1A2A 1042370]
- National Research Foundation of Korea [2020R1F1A1048189] Funding Source: Korea Institute of Science & Technology Information (KISTI), National Science & Technology Information Service (NTIS)
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Radio relics in galaxy clusters suggest the diffusive shock acceleration of electrons, but the electron preacceleration problem at weak shocks remains unresolved.
Radio relics in the outskirts of galaxy clusters imply the diffusive shock acceleration (DSA) of electrons at merger-driven shocks with Mach number M ( s ) less than or similar to 3-4 in the intracluster medium (ICM). Recent studies have suggested that electron preacceleration and injection, prerequisite steps for DSA, could occur at supercritical shocks with M ( s ) greater than or similar to 2.3 in the ICM, thanks to the generation of multiscale waves by microinstabilities such as the Alfven ion cyclotron (AIC) instability, the electron firehose instability (EFI), and the whistler instability (WI). On the other hand, some relics are observed to have subcritical shocks with M ( s ) less than or similar to 2.3, leaving DSA at such weak shocks as an outstanding problem. Reacceleration of preexisting nonthermal electrons has been contemplated as one of possible solutions for that puzzle. To explore this idea, we perform particle-in-cell simulations for weak quasi-perpendicular shocks in high-beta (beta = P (gas)/P ( B )) plasmas with power-law suprathermal electrons in addition to Maxwellian thermal electrons. We find that suprathermal electrons enhance the excitation of electron-scale waves via the EFI and WI. However, they do not affect the ion reflection and the ensuing generation of ion-scale waves via the AIC instability. The presence of ion-scale waves is the key for the preacceleration of electrons up to the injection momentum; thus, the shock criticality condition for electron injection to DSA is preserved. Based on the results, we conclude that preexisting nonthermal electrons in the preshock region alone would not resolve the issue of electron preacceleration at subcritical ICM shocks.
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