Contribution to diffuse -ray emission from self-confned Cosmic Rays - PowerPoint PPT Presentation

Contribution to diffuse γ -ray emission from self-confned Cosmic Rays around Galactic sources Giovanni Morlino , Gran Sasso Science Institute In collaboration with M. D'Angelo, P. Blasi and E. Amato “35 th International Cosmic Ray Conference” — July 19, 2017 — Busan, Korea

The problem of the cosmic ray gradient in the Galactic plane seen by Fermi-LAT Recent results from FermiLAT Pronounced peak collaboration on the CR distribution Very flat gradient in the Galactic plane [Acero et al. 2016, ApJS 223] - In the outer region ( R > 8kpc) the CR density at ~20 GeV is flat Prediction from GALPROP (i.e. decreases much slower than the source distribution) - In the inner region the CR density Prediction from GALPROP s=2.7 has a peak at ~ 3 kpc - The slope @ 20 GeV is not constant Distribution of sources This scenario is difficult to accommodate in a standard diffusion model with homogeneous diffusion coefficient G. Morlino – ICRC 2017

Possible solutions Uniform diffusion Extended halo, H > 4 kpc (Dogiel, Uryson, 1988; Strong et al.,1988; Bloemen, 1993, Ackerman et al., 2011) Flatter distribution of SNR in the outer Galaxy None of these ideas can (Ackerman et al., 2011) simultaneously account Enhancement of CO/H 2 density ratio (X CO ) in the outer for all signatures - fatness R > 8 kpc, Galaxy (Strong et al., 2004) - peak at R~3-4 kpc, Advection effects due to the Galactic wind - variation in the slope (Bloemen, 1993; Breitschwerdt, Dogiel, Voelk, 2002) Non uniform diffusion Diffusion dependence on galactocentric distance (Evoli et al., 2012; Gaggero et al., 2015) Self generated diffusion (Recchia, Blasi & Morlino, 2016) D ⊥ ≪ D ∥ Different parallel and perpendicular diffusion (Vittino's talk, CRD 045) G. Morlino – ICRC 2017

Dependence with Galactic latitude |b| < 5° The slope change toward the Galactic centre disappears at high latitude Supports the idea that the variation occurs mainly in the Galactic disc. 10° < |b| < 15° Distribution of the power-law photon index of the galactic diffuse γ-ray emission over the galactic longitudes integrated for the interval |b|< 5◦ and 10°<|b|<1 5◦ [from Yang, Aharonian & Evoli 2016] G. Morlino – ICRC 2017

Effect of self-amplifcation near the CR sources Runaway CRs During the process of escaping, CR can excite magnetic turbulence (via streaming instability) that keep the CR Forward Shock close to the SNR for a long time, up to ~10 5 yr [Malkom et al. (2013) Nava et al. (2015)] In this region CRs can excite waves G. Morlino – ICRC 2017

Effect of self-amplifcation near the CR sources Runaway CRs During the process of escaping, CR can excite magnetic turbulence (via streaming instability) that keep the CR Forward Shock close to the SNR for a long time, up to ~10 5 yr [Malkom et al. (2013) Nava et al. (2015)] The region where this can happen is at most of the order of the coherence-length of the magnetic field (after this distance the diffusion becomes 3D and the In this region CRs CR density drops rapidly below the average Galactic can excite waves value) B Simulation from Nava & Gabici (2012) G. Morlino – ICRC 2017

Effect of self-amplifcation near the CR sources Runaway CRs During the process of escaping, CR can excite magnetic turbulence (via streaming instability) that keep the CR Forward Shock close to the SNR for a long time, up to ~10 5 yr [Malkom et al. (2013) Nava et al. (2015)] The region where this can happen is at most of the order of the coherence-length of the magnetic field (after this distance the diffusion becomes 3D and the In this region CRs CR density drops rapidly below the average Galactic can excite waves value) B Simulation from Nava & Gabici (2012) During the time CR spend in the vicinity of sources they can produce diffuse emission via π 0 → γ γ A single halo is (in general) too faint to be observed with current telescope BUT the sum of all halos can contribute to the diffuse emission. G. Morlino – ICRC 2017

Effect of self-amplifcation near the CR sources: basic equations Runaway CRs CR transport equation in 1-D Forward Shock The injection of particle is mimicked through a cloud of CR at t = 0 In this region CRs can excite waves B G. Morlino – ICRC 2017

Effect of self-amplifcation near the CR sources: basic equations Runaway CRs CR transport equation in 1-D Forward Shock Self-generated diffusion coefficient In this region CRs can excite waves Turbulence spectrum B G. Morlino – ICRC 2017

Effect of self-amplifcation near the CR sources: basic equations Runaway CRs CR transport equation in 1-D Forward Shock Self-generated diffusion coefficient In this region CRs can excite waves Turbulence spectrum B Transport equation for magnetic turbulence Resonant amplification: Damping Injection G. Morlino – ICRC 2017

Effect of self-amplifcation near the CR sources: damping mechanisms Non-linear Landau damping (Zhou & Matthaeus, 1990; Ptuskin Zirakashvili, 2004) Damping due to anisotropic cascade (wave-wave interaction) (Farmer & Goldreich, 2004) Coherence scale of the turbulence Damping due to ion-neutral friction (Kulsrud & Pearce, 1969; Kulsrud & Cesarsky, 1971; Drury et al., 1996) Unless neutral hydrogen density is very low, the ion neutral damping dominates G. Morlino – ICRC 2017

Evolution of CR density close to the source CR distribution E CR = 0.2 E SN function @ 10 GeV for several ages − 4 q inj ( p )∝ p Average Galactic Distribution level function of turbulence Diffusion coeffcient Distance from the source G. Morlino – ICRC 2017

Escaping time Assuming injected spectrum p -4 CR energy ~ 20% E SN No neutrals With neutrals n H /n i ~ 5%-10% Standard escaping time using 2 t diff = L c Galactic diffusion D Gal G. Morlino – ICRC 2017

SNR population We assume the SNR distribution according to the model by Green (2015) Distribution of SNR in the galactic plane during the last ~10 5 yrs using a rate of 1 SN/(30 yr) G. Morlino – ICRC 2017

Diffuse Galactic emission from Fermi -LAT FermiLAT diffuse emission Diffuse Galactic γ -ray flux for three different angular sectors extracted from the Fermi-LAT data [ Yang, Aharonian & Evoli, 2016] 5°-15° 85°-95° 175°-185°

Contribution of the escaping CRs to the diffuse Galactic emission − 3 ; − 3 n i = 0.45 cm n H = 0.0 cm Sum of diffuse emission plus contribution from all the source cocoons “Real” diffuse contribution assuming AMS spectrum in the whole Galaxy

Contribution of the escaping CRs to the diffuse Galactic emission Angular sector Fully ionized n H =0.05 Number of sources 5°-15° 4500 740 contributing to the 85°-95° 350 57 emission 175°-185° 77 13 − 3 ; − 3 ; − 3 − 3 n i = 0.45 cm n H = 0.0 cm n i = 0.45 cm n H = 0.05 cm

Conclusions Diffuse gamma-ray emission shows that the CR distribution is not constant across the Galaxy The self-confnement of CRs during the escape from the source can generate a cocoon of  -rays around each source if the presence of neutral Hydrogen is negligible . The sum of all cocoons along the line of sight can contribute to the diffuse  -ray emission especially toward the inner Galactic region. The self-confnement can also affect the total grammage felt by CRs [see D'Angelo, Blasi, Amato, 2016] Some caveats: We need a better description of particle escape from the source The rate of non-linear magnetic damping is poorly known The presence of molecular clouds close to the SNR may further enhance the gamma-ray emission G. Morlino – ICRC 2017

Backup slide Gamma-ray spectrum from a single cocoon G. Morlino – ICRC 2017

Backup slide Spectra of interstellar emission model components for |b| > 10 and |b| < 10 [Acero et al., 2016] G. Morlino – ICRC 2017