Gamma-rays from SNRs and cosmic rays

V.N.Zirakashvili

Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation, Russian Academy of Sciences (IZMIRAN), 142190 Troitsk, Moscow Region, Russia

Outline

• Acceleration of particles at forward and reverse shocks in SNRs

• Amplification of magnetic fields

• Radioactivity and electron acceleration

• Modeling of DSA in SNRs

• Modeling of broad-band emission

Diffusive Shock Acceleration Krymsky 1977; Bell 1978; Axford et al.1977; Blandford & Ostriker 1978

Very attractive feature: power-law spectrum of particles accelerated, =(+2)/(-1), where is the shock compression ratio, for strong shocks =4 and =2

Maximum energy for SN: D0.1ushRsh 3·1027 cm2/s<Dgal

Diffusion coefficient should be small in the vicinity of SN shock

1

shsh14max skm3000pc3μG10

eV10uRB

ZE

In the Bohm limit D=DB=crg/3 and for interstellar magnetic field

Shock modification by the pressure of accelerated CRs.

Axford 1977, 1981

Eichler 1984

Higher compression ratio of the shock, concave spectrum of particles.

X-ray image of Tycho SNR (from Warren et al. 2005)

1. CD is close to the forward shock – evidence of the shock modification by CR pressure.

2. Thin non-thermal X-ray filaments at the periphery of the remnant – evidence of electron acceleration and of magnetic amplification.

Magnetic field amplification by non-resonant streaming instability

2 2 2 0

0

V k jB k

ca d

Fc

j BC R d 1Bell (2004) used Achterberg’s

results (1983) and found the regime of instability that was overlooked

saturated level of instability

Bck

j d4

k rg >>1, γmax=jdB0/2cρVa

Since the CR trajectories are weakly influenced by the small- scale field, the use of the mean jd is well justfied

MHD modeling in the shock transition region and downstream of the shock

0.02L

u1=3000 km/s Va=10 km/s

ηesc=0.14

Magnetic field is not damped and is perpendicular to the shock front downstream of the shock! Ratio=1.4 σB=33D 2562×512

B

Zirakashvili & Ptuskin 2008

Wood & Mufson 1992 Dickel et al. 1991

Radial magnetic fields were indeed observed in young SNRs

Schematic picture of the fast shock with accelerated particles

It is a great challenge - to perform the modeling of diffusive shock acceleration in such inhomogeneous and turbulent medium. Spectra of accelerated particles may differ from the spectra in the uniform medium.

Both further development of the DSA theory and the comparison with X-ray and gamma-ray observations are necessary

CR acceleration at the reverse shock (e.g.Ellison et al. 2005) ?

Probably presents in Cas A (Helder & Vink 2008)

Magnetic field of ejecta?

B~R-2, 104G at R=1013 cm -

10-8G at R=1019cm=3pc

Field may be amplified and become radial – enhanced ion injection at the reverse shock

+additional amplification by the non-resonant streaming instability (Bell 2004)

Radio-image of Cas A

Atoyan et al. 2000

Inner bright radio- and X-ray-ring is related with the reverse shock of Cas A while the diffuse radio-plateau and thin outer X-ray filaments are produced by electrons accelerated at the forward shock.

X-ray image of Cas A (Chandra)

X-rays: XMM-Newton, Acero et al. 2009

Radio-image of RX J1713.7-3946 (Lazendic et al. 2004)

Inner ring of X-ray and radio-emission is probably related with electrons accelerated at the reverse shock.

Radioactivity and electron acceleration in SNRs

(Zirakashvili & Aharonian (2011))

Forward and reverse shocks propagate in the medium with energetic electrons and positrons.

Cosmic ray positrons can be accelerated at reverse shocks of SNRs (Ellison et al. 1990).

44Ti t1/2=63 yr

1.6·10-4 Msol in Cas A, (Iyudin et al.

1994, Renaud et al. 2006)

1-5 ·10-5 Msol in G1.9+0.3 (Borkowski et

al. 2010)

“Radioactive” scenario in the youngest galactic SNR G1.9+0.3

Borkowski et al. 2010

X-ray image radio-image

Thermal X-rays and 4.1 keV Sc line (product of 44Ti) are observed from bright radio-regions (ejecta)

Numerical model of nonlinear diffusive shock acceleration

(Zirakashvili & Ptuskin 2011) (natural development of existing models of Berezhko et al. (1994-2006), Kang

& Jones 2006, see also half-analytical models of Blasi et al.(2005); Ellison et al. (2010) )

Spherically symmetric HD equations + CR transport equation

Minimal electron heating by Coulomb collisions with thermal ions

Acceleration at forward and reverse shocks

Numerical results

u

PCR

BS

nH= 0.1 cm-3

B0= 5 μG

T= 104 K

ESN=1051 erg

Mej=1.4Msol

η = 0.01

Protons and electrons are injected at the forward shock, ions and positrons are injected at the reverse shock.

Magnetic amplification in young SNRs is taken into account

Radial profiles at T=1000 years

u

ρ

PCR

Pg

Spectra of accelerated particles

p

p

Integrated spectra

Spectrum of ions is harder than the spectrum of protons because the ejecta density decreases in time.

The input of the forward shock was considered earlier (Berezhko & Völk 2007, Ptuskin et al. 2010, Kang 2011)

Integrated spectra

Alfven drift downstream of the forward shock results in the steeper spectra of particles accelerated at the forward shock.

Evolution of nonthermal emission from SNR

nH= 0.1 cm-3

TeV gamma-rays from young SNRs

RX J1713.7-3946 Vela Jr

Aharonian et al 2008 (HESS) Aharonian et al 2007 (HESS)Particles accelerated up to100 TeV in these SNRs. Gamma-rays can be produced in pp collisions (hadronic models) or via the Inverse Compton scattering of IR and MWBR photons on the electrons accelerated (leptonic models)

Fermi LAT results on RX J1713.7-3946 (Abdo et al. 2011)

RX J1713.7-3946

Although the leptonic model is preferable, the hadronic model is not excluded because of the possible energy dependent CR penetration in to the clouds. The main problem of the hadronic model is the absence of thermal X-rays.

hadronic

leptonic

Vela Jr.:Fermi results (Tanaka et al. 2011)

Hadronic model (Berezhko et al. 2009) is in agreement with the measured gamma-ray spectra. The forward shock is modified by CR pressure.

Leptonic model is excluded if the thin X-ray filaments observed in this remnant can be considered as the evidence of strong magnetic fields.

Chandra (Bamba et al. 2005)

Modeling of broad-band spectra of Cas A for the “radioactive” scenario of lepton injection

(Zirakashvili et al. 2012 in preparation)

ESN=1.7·1051 erg, Mej=2.1 Msol, t=330 yr, nH=0.8 cm-3, Vf=5700 km/s, Vb=3400 km/s, Bf=1.1 mG, Bb=0.24 mG

Emission is mainly produced at the reverse shock of Cas A

Broad-band modeling of Tycho SNR (Morlino & Caprioli 2011)

Good candidate to be a proton accelerator

Old SNRs (T>104 yr)

IC443 (Abdo et al. 2010) W44 (Giuliani et al. 2011)

Old SNRs show gamma-ray spectra with steeper parts or cut-offs. TeV protons accelerated earlier have already left the remnant.

Emax ~100 GeV in IC443 and Emax ~10 GeV in W44. Probably because of damping of MHD waves generated by accelerated particles.

The spectral shape at E<1 GeV favors a hadronic origin of gamma-emission in SNR W44 (AGILE coll.).

Summary1. Non-resonant streaming instability produced by

the electric current of run-away CR particles results in the significant magnetic amplification at fast SNR shocks.

2. The reverse shocks in SNRs can produce CR ions and positrons with spectrum that is harder in comparison with spectra of particles accelerated at the forward shock.

3. Both leptonic and hadronic origin of gamma-emission in SNRs RX J1713.7-3946, Vela Junior, Cas A are possible.

4. SNR Tycho and old SNRs are good candidates for a hadronic origin of gamma-emission.