Gamma-quanta from SNRs

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

• Modeling of broad-band emission

• Radioactivity and electron acceleration

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.

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, 100G at R=1012 cm -

10-12G 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.

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

Rayleigh-Taylor instability of contact discontinuitymay also produce amplified radial fields (e.g. Gull 1973)

Blondin & Ellison 2001

Magnetic amplification at the shock moving in the non-uniform medium Density perturbtions

produce vortex motions downstream the shock (Kontorovich 1959, McKenzie & Westphal 1968, Bykov 1982). These motions amplify magnetic fields (Giacalone & Jokipii 2007).

Inoue et al. 2009

Mean amplification factor ~ 20 for perp. shock even without CRs

observationsradio emissionνMHz = 4.6 BμGEe,GeV

2

E = 50 MeV – 30 GeV

(100 GeV for IR)

γ = 1.9 – 2.5

We = 1048 – 1049 erg

Ginzburg &

Syrovatskii 1964

Shklovsky 1976

nonthermal X-raysεkeV = 1 BμG(Ee/120 TeV)2

εmax ~ 100 TeV

SN1006 Koyama et al. 1995Cas A Allen et al. 1997RX J1713.7-3946 Koyama et al. 1997RX J0852-46 (“Vela jr”) Slane et 2001

γ-rays (π0)Ε = 30-3000 MeVγ Cygni, IC443Esposito et al. 1996Sturner & Dermer 1996Cas A, Abdo et al. 2010RX J1713.7-3946 (Fermi LAT)

TeV γ – rayselectrons/protonsεmax ~ 100 TeV

SN1006 Tanimori et al 1998RX J1713.7-3946 Muraishi et al. 2000 Aharonian et al. 2004Cas A Aharonian et al. 2001RX J0852-46 (“Vela jr”)G338.3-0.0; G23.3-0.3; G8.7-0.1… Aharonian et al. 2005 Tycho (VERITAS 2010)

e

γsynchrotron

e

γinverse Comptonεγ = ε0(Ee/mec2)2

pπ0

γ

SNR

confirmedby HESS (2008) !

TeV gamma-rays from SNRs

RX J1713.7-3946 Vela Jr

Aharonian et al 2008 (HESS) Aharonian et al 2007 (HESS)

Particles effectively accelerated up to100 TeV in these SNRs

Numerical model of nonlinear diffusive shock acceleration

(Zirakashvili & Ptuskin 2011 in preparation) (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 for RX J1713.7-3946 (Zirakashvili & Aharonian 2010)

u Te

PCR

Pg

ρ

BS FSCD

Spectra of accelerated particles

p

p

e

Integrated spectra of RX J1713.7-3946

Emax=800 TeV for this SNR in the hadronic model

Spectral modeling of RXJ1713.7-3946

The main problem of hadronic origin of the gamma-rays of this supernova – absence of thermal X-ray emission (Katz & Waxman 2007). This gives an upper limit of circumstellar density only 0.02-0.05 cm-3

(Cassam-Chenai et al. 2004, Ellison et al. 2010) .

synchrotron

thermal bremsstrahlung

IC

pp

Leptonic model with a non-modified forward shock

Comparison of hadronic and leptonic radial

gamma-ray profiles

Correlation with molecular gasFukui 2008

cloud C - 400 solar masses

cloud D -300 solar masses

Clouds C and D are probably swept up by the forward shock

Distance: D=6 kpc (Slane et al. 1999)

D=1 kpc (Fukui et al. 2003), D=1.3±0.4 kpc (Cassam-Chenai et al. 2004)

AD393 in ancient Chinese records (Wang et al.1997)

age: 1600 yr

Composite modelFactor of 120 enhancement for clouds swept up by the forward shock

Recent Fermi LAT results (Abdo et al. 2011)

RX J1713.7-3946

Although leptonic model is preferable, hadronic model is not excluded because of the possible energy dependent CR penetration in to the clouds

Radioactivity and electron acceleration in SNRs

(Zirakashvili & Aharonian (2010), astro-ph:1011.4775)

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

Cosmic ray positrons can be accelerated at reverse shocks of SNRs.

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 are observed from bright radio-regions (ejecta)

Numerical results for Cas A ( Zirakashvili et al. 2011 in preparation)

Radial dependences of hydrodynamic quantities

It is assumed that forward shock is not modified by CRs. If this is not the case, the gamma-ray flux would be a factor of 10 above the flux measured by Fermi LAT.

The most probable reason – the azimuthal magnetic field of the stellar wind where the forward shock propagates.

Spectra of particles in Cas A

Х 10

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

(Zirakashvili et al. 2011 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

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 perpendicular to the shock front component of the amplified magnetic field is larger than the parallel components downstream of the shock. This naturally explains the preferable radial orientation of magnetic fields in young SNRs.

3. Further development of the DSA theory at the corrugated shock (spectra) and the comparison with X-ray and gamma-ray observations are necessary.

4. CRs can be also accelerated at reverse shocks of young SNRs.

5. Both leptonic and hadronic origin of gamma-emission in SNR RX J1713.7-3946 are possible.

6. CR positrons can be accelerated in SNRs.