Roles of Cosmic Rays in Galaxy Clusters Yutaka Fujita (Osaka U)

Content CRs may be playing more important roles in clusters than you imagine Cosmic ray (CR) heating of cluster cores  Fujita & Ohira (2011, 2012, 2013), Fujita, Kimura, & Ohira (2013) ▫AGNs accelerate CRs ▫Heating by CR streaming  Stability ▫Non-thermal emission  Radio mini-halos Entropy deficit at the outskirts of clusters  Fujita, Ohira, & Yamazaki (2013) ▫Energy consumption by CR acceleration

Core heating by cosmic-ray (CR) streaming

AGN AGN activities are often found in cool cores ▫AGNs create energy ▫They are heating cores Energy transfer from the AGNs to the intracluster medium (ICM) ▫Mechanical heating?  Shocks, sound waves…  Often unstable (Fujita & Suzuki 2005) ▫We consider another solution  CR streaming  Rephaeli 1979; Böhringer & Morfill 1988; Loewenstein et al. 1991; Guo & Oh 2008 Color: X-ray Contours: Radio (Blanton et al. 2001) AGN Core of A2052

CR streaming ICM contains Cosmic-Rays (CRs) ▫CRs stream in the ICM CRs are well scattered by Alfvén waves (Alfvén velocity = v A  c) in the ICM ▫CRs effectively move along with the waves with a streaming (Alfvén wave) velocity of v A  c. Waves CR Particles vAvA CRs → balls Waves → pins Pinball

Our model of heating CRs are accelerated by the central AGN ▫We mainly consider CR protons ▫CR injection rate ∝ Mass inflow rate into the central black hole Energy transfer ▫AGN → CRs → (streaming instability) → Alfvén waves → (non-linear damping) → ICM Net heating rate ▫v A : streaming ( Alfvén ) velocity, P c : CR pressure Balanced → pdV work by CR pressure A

Models Basic equations (Fujita & Ohira 2011, 2012) ↓ conduction ↑ radiation ↑ CR streaming ← CR collision ↓ ↑ diffusion CR source ↑ gas CR gas P ↓ mag. P ↓ CR P ↑

Steady state solutions Steady state solutions can be obtained by solving a boundary value problem  Kim & Narayan 2003, Guo et al  Heating = cooling ▫Solutions (dashed) ▫Dots (observations; Ettori et al. 2002) A1795

Lagrangian perturbation analysis We linearize the equations to study the stability of the steady state solutions ▫We solve the equations as an eigenvalue problem There is no unstable mode ▫The solutions must be very stable (Fujita+ 2013) ▫Numerical simulations confirm this result

Heating by mechanical heating (sound waves, shocks...) is often unstable ▫Heating tends to be localized (e.g. Fujita & Suzuki 2005) Heating by cosmic-ray (CR) streamin g ▫Heating is not localized Why stable? Cluster AGN Cool Core Heated ICM AGN Cool Core Heated ICM Cluster Heating by sound waves Heating by CR streaming CRs

Radio Mini-Halos Our model predicts that radio mini- halos are created by CRs that are heating cool cores ▫Synchrotron emission from secondary electrons Radio surface brightness should have a jump at a cold front ▫Density contrast of target protons ▫Change of the direction of magnetic fields Radio intensity profiles (Fujita & Ohira 2013) Giacintucci et al. (2014) Lines: predictions Dots: observations Front B

Astro-H, Hard X-ray, γ-ray Our model does not require strong turbulence to heat cores and produce mini-halos ▫There may be cool core clusters with mini-halos that do not have strong turbulence → Astro-H It is difficult to detect hard X-rays (inverse Compton) and γ-rays (π 0 -decay) from cores ▫If they are detected, the origin must be different Fujita & Ohira (2013)

Entropy deficit at the outskirts of clusters

K ∝ r 1.1 Suzaku observations of outskirts Entropy: K = T/n 2/3 Observed entropy profiles are not consistent with the prediction of theoretical simulations (K ∝ r 1.1 ) ▫Universal phenomena (Okabe, YF+ 2014) Suzaku (Sato, YF+ 2012) Simulation (Voit et al. 2005) K ∝ r 1.1 r/r 200 Deficit

Cosmic acceleration at outskirts Infalling gas is thermalized at shocks at cluster outskirts CRs are supposed to be accelerated at the shocks If CRs are effectively accelerated at the shock, part of the kinetic energy is consumed by the acceleration ▫Less efficient thermalization → lower entropy Cluster (Thermal energy) Gas infall (Kinetic energy) Shock ~ Virial radius Kinetic energy Thermal energy ShockCRs acceleration CR acceleration

CR acceleration at a shock We consider boundary conditions at a shock including the effect of CR acceleration ▫We compare the results with observed entropy profiles ▫Acceleration efficiency of  7% ▫CR pressure of  40% ▫Observed entropy deficit around clusters can be explained Groups should not have entropy deficit ▫Less efficient acceleration CR acceleration efficiency Fraction of CR pressure Fujita, Ohira, & Yamazaki 2013

Summary We have studied the heating of cool cores via CR streaming ▫CR heating is remarkably stable ▫Radio mini-halos can be explained by our model Entropy deficit at the outskirts of clusters can also be caused by CRs ▫CR acceleration consumes kinetic energy of infalling gas → inefficient thermalization