Proton Injection & Acceleration at Weak Quasi-parallel ICM Shock

1. Proton Injection & Acceleration at Weak Quasi-parallel ICM shock HyesungKang (Pusan National University) DongsuRyu, Ji-Hoon Ha (UNIST, Korea) Supernova remnant shock Solar wind bow shock SN1006 Radio Relic shocks Galaxy clusters

2. ICM: IntraCluster Medium 0.3 - 1 50 - 100 : 2-3 20-30 Previous studies on collisionless shocks focus mainly on low b (<1) plasma (e.g. solar wind & ISM). Physics of weak shocks in b=100 ICM could be quite different (e.g. various microinstabilities).

3. DSA: Fermi first order process at shocks Shock front mean field B particle U1 U2 upstream downstream : Requirement for shock crossing Kinetic plasma processes are crucial for pre-acceleration/injection problem. Focus of this talk

4. Particle Energy Spectrum: Injection Problem Maxwellian DSA power-law spectrum Kinetic processes PIC/Hybrid simulations

5. Shocks in Structure Formation Simulations (Ryu et al 2003) • Weak internal shocks with Ms<3are dominant and energetically important inside hot ICM. • Cosmic-rays are accelerated via Fermi-I just like Earth’s bow shock &supernova remnants Accretion Shocks Q: Can weak ICM shocks accelerate CR protons/electrons ? 25h-1Mpc Shocks in ICM Quasi-parallel () shocks in the ICM  CR protons collision with ICM  p0decay g rays (not detected) Quasi-perpendicular () shocks in the ICM CR electrons radio synchrotron emiss (observed) Ji-Hoon’s talk ICM= Intracluster Medium (-K)

6. Low shocks subcritical 1D hybrid simulations = 1.5 : shock is steady & smooth. > 2.8 : shock is unsteady & undergoes self-reformation. • Subcritical weak shocks: • shock transition is smooth,lacking an overshoot. • Ion reflection is inefficient, • maybe no particle acceleration supercritical 1994

7. Earth Bow shock: Q-perp shock upstream -relative drift between reflected ions and incoming particles excites various waves via micro-instabilities in the shock foot. Shock criticality & ion reflection are crucial for the shock structure & particle acceleration. Collisionlessshock is not a simple MHD jump. Dissipation is mediated by collective EM interactions instead of inter-particle collisions.  complex kinetic processes are involved, e.g. microinstabilities.

8. B shock n QBn Two ways to reflect protons/electrons at the shock (1) magnetic mirror reflection due to compressed magnetic fields mirror force due to gradient of B dominant at shock Caprioli & Spitkovsky (2)Shock potential barrier: -decelerates ions but accelerates electrons -ions are reflected by overshoot dominant at shock Both magnetic field compression & shock potential drop depends on  Reflection fraction increases with increasing

9. Kinetic Plasma Simulations: PIC (Particle In Cell) -can follow kinetic plasma processes: e.g. wave-particle interactions -provide the most complete pictures, but very expensive Large mass ratio  large time scale ratio = /

10. 1D/2D PIC simulations for shocks (proton injection to DSA) Ha et al. 2018 upstream downstream B in x-y plane Simulation frame = downstream rest frame

11. Time evolution of shock structure: Stack plot Supercritical subcritical -Time-varying overshoot in eF& B -cyclic reformation of the shock -No overshoot, smooth transition First critical Mach number :

12. Ms=3.2 supercritical with a beam of reflected ions Ms=2.0 subcritical without reflected ions Overshoot ~ 200

13. Time Evolution of energy spectrum Injection to DSA DSA power-law spectrum extends to higher energy in time. Obliquity dependence Early stage of DSA in Q-par shocks q13 Only SDA (no DSA) in Q-perp shocks q=63

14. Mach number dependence Injection momentum Injection fraction Ms~2.25

15. SUMMARY 1. internal shocks driven by mergers and chaotic flows: , 2. In high b ICM, only supercriticalshocks with may inject suprathermal protons to DSA and accelerate CR protons. So most of ICM shocks do not accelerate CR protons. 3. The injection fraction, x(t), decreases with time. Long-term evolution of x(t) can be studied with other methods. Key words: Shock criticality, Ion Reflection, Ion Injection to Fermi-I process, Weak ICM Shock

17. Key elements for proton injection to DSA at shocks (1) Reflection at the shock & energy gain near the front via SDA (2) Backstreamingof ions upstream along B0 relative drift between reflected ions & incoming particles: free E source (3) Self-excitation of upstream waves: e.g. whistlers or Alfven waves  Scattering leads to Injection to Fermi I acceleration (3) self-excitation of waves (2) (1) after 1/2 gyro-orbit, advect downstream Scattering by upstream waves Backstreaming upstream

18. Particle Acceleration Processes operating at collisionless shocks Diffusive Shock Acceleration (DSA): Fermi 1st order process - effective at quasi-parallel () shocks - scattering off MHD waves in the upstream and downstream region (2) Shock Drift Acceleration (SDA) - effective at quasi-perpendicular () shocks - drifting along the convective E field (grad B) at the shock front (3) Shock Surfing Acceleration (SSA) - effective at quasi-perpendicular () shocks - reflected by shock potential, scattered by upstream waves - moving along the convective E field, while being trapped at the shock foot (4) Turbulent Acceleration: Fermi 2nd order process, stochastic acceleration - much less efficient than DSA - could be important only in turbulent plasma (5) Magnetic Reconnection

19. Shock Drift Acceleration (SDA) at shocks downstream upstream shock surface in x-y plane upstream Electrons drift anti-parallel to E field. • -perpendicular component of B is compressed at shock • -motional Electric field is induced • -grad(B) drift of electron in -z direction (anti-parallel to E) • protons in +z direction (parallel to E) • in the shock transition layer  gain energy • pre-acceleration of protons/electrons •  multi-cycles of SDA  injection to DSA SDA upstream drift along - z reflected Trajectory of an electron From Guo, Sironi, & Narayan 2014

20. CR proton acceleration efficiency from Hybrid simulations No proton acceleration at shocks High beta cases Caprioli & Spitkovsky 2014 Caprioli 2017 (KAW9)

21. 2016 Fermi LAT upper limits on gamma-ray flux from individual clusters To be compatible with Fermi upper limits, the CR proton acceleration efficiency should be h <0.1% for 2<M<5. according to hybrid simulations, b~1 Different DSA efficiency models

22. Signatures of shocks in ICM: X-ray shocks in merging clusters The Bullet Cluster Markevitch 2006 Shimwell et al. 2015 Cluster A665 shock Dasadia et al. 2016

23. Radio relics: diffuse radio sources in merging clusters 1RXS J060303.3 PLCKG287.0+32.9 Sausage relic ZwCl0008.8 shock shock Shock Mach numbers estimated from spectral index, based on DSA model,

24. Key Physical Processes in ICM: shocks, turbulence, magnetic fields, & particle acceleration nonthermal radiation Gravitational energy X-ray shocks: observed supersonic flows Relativistic particles Heat DSA (Fermi I) Shock Ji-Hoon’s talk Quasi-perpendicular shocks: CR electrons (Radio relics): observed Vorticity Turbulence turbulence dynamo Quasi-parallel shocks: CR protons pp collision p0decay g rays (not detected) turbulence decay Magnetic fields MHD/Plasma waves turbulent acceleration (Fermi II) CR 2ndary electrons (Radio halos) Heat CR protons  secondary ptls merging clusters dissipation scale

25. Hybrid simulation: Proton acceleration at shocks Caprioli & Sptikovsky 2014 upstream • At Q-par shocks: • stream of accelerated ions into upstream • self-generated waves • B amplification downstream n B n • At Q-perp shocks: • No backstreaming ions into upstream • No turbulent waves B

26. DSA beyond injection ? Amplitude of the DSA power-law should decrease in time. The injection fraction, x(t), decreases with time. Long-term evolution of x(t) can be studied with other methods such as hybrid simulations.

27. Ion reflection at super-critical shocks: Edmiston & Kennel (1984) High beta plasma (IPM or ICM) Low beta plasma (solar flare) For ICM shocks with b~50,