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

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

Maximum energy for SN: 3·1027 cm2/s
D0.1ushR

sh

Diffusion coefficient should be small in the vicinity of SN shock In the Bohm limit D=DB=crg/3 and for interstellar magnetic field

Em

ax

B Z 10 eV 10G
14

Rsh ush 3 pc 3000kms1


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
Warren et al. 2005)

(from

1. CD is close to the forward shock ­ evidence of the shock modification by CR pressure. 2. Thin non-thermal Xray 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

+additional amplification by the nonresonant streaming instability (Bell 2004)

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


Radio-image of Cas A
Atoyan et al. 2000

X-ray image of Cas A (Chandra)

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


Radio-image of RX J1713.73946 (Lazendic et al. 2004)

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

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


Magnetic field amplification by nonresonant streaming instability
Bell (2004) used Achterberg's results (1983) and found the regime of instability that was overlooked

B0 k V k jd c0
2 2 a 2

1 FCR jd B c





k rg >>1, max=jdB0/2cVa
Since the CR trajectories are weakly influenced by the smallscale field, the use of the mean jd is well justfied saturated level of instability

4 B jd ck

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

Zirakashvili & Ptuskin 2008

B

u1=3000 km/s Va=10 km/s esc=0.14 0.02L

3D 2562 512

Magnetic field is not damped and is perpendicular to the shock front downstream of the shock! Ratio=1.4 B=3


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 discontinuity may also produce amplified radial fields (e.g. Gull 1973)

Blondin & Ellison 2001


Inoue et al. 2009

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).

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


observations
radio emission
MHz = 4.6 B E = 50 MeV ­ 30 GeV (100 GeV for IR) = 1.9 ­ 2.5 We = 1048 ­ 1049 erg Ginzburg & Syrovatskii 1964 Shklovsky 1976
2 GEe,GeV

nonthermal X-rays
keV = 1 BG(Ee/120 TeV) max ~ 100 TeV
2

synchrotron



e

SNR

0

inverse Compton = 0(Ee/mec2)2

SN1006 Koyama et al. Cas A Allen et al. RX J1713.7-3946 Koyama et al. RX J0852-46 ("Vela jr") Slane et

1995 1997 1997 2001



p

TeV ­ rays
confirmed by HESS (2008) !

e

electrons/protons max ~ 100 TeV

-rays (0)
= 30-3000 MeV Cygni, IC443 Esposito et al. 1996 Sturner & Dermer 1996 Cas A, Abdo et al. 2010 RX J1713.7-3946 (Fermi LAT)

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


TeV gamma-rays from SNRs
Aharonian et al 2008 (HESS) Aharonian et al 2007 (HESS)

Particles effectively accelerated up to100 TeV in these SNRs

RX J1713.7-3946

Vela Jr


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 Acceleration at forward and reverse shocks

Minimal electron heating by Coulomb collisions with thermal ions


Numerical results for RX J1713.73946 (Zirakashvili & Aharonian 2010) u Te

PCR Pg BS CD FS


Spectra of accelerated particles

p e

p


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 0.020.05 cm-3 (Cassam-Chenai et al. 2004, Ellison et al. 2010) .


Leptonic model with a non-modified forward shock
synchrotron pp IC thermal bremsstrahlung


Comparison of hadronic and leptonic radial gamma-ray profiles


Correlation with molecular gas
Fukui 2008
cloud D -300 solar masses cloud C - 400 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 model

Factor 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.
44

Ti

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
X-ray image radio-image

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

Borkowski et al. 2010


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



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


Summary
1. 2. Non-resonant streaming instability produced by the electric current of run-away CR particles results in the significant magnetic amplification at fast SNR shocks. 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. Further development of the DSA theory at the corrugated shock (spectra) and the comparison with Xray and gamma-ray observations are necessary. CRs can be also accelerated at reverse shocks of young SNRs.

3. 4.

5.
6.

Both leptonic and hadronic origin of gamma-emission in SNR RX J1713.7-3946 are possible. CR positrons can be accelerated in SNRs.