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
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<Dgal
D0.1ushRsh
Diffusion coefficient should be small in
the vicinity of SN shock
In the Bohm limit D=DB=crg/3 and for
interstellar magnetic field
Emax
 B  Rsh 

ush



 Z 10 eV 
1 
 10μG  3 pc  3000kms 
14
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 Xray filaments at the
periphery of the
remnant – evidence
of electron
acceleration and of
magnetic
amplification.
Magnetic field amplification by nonresonant streaming instability
Bell (2004) used Achterberg’s
results (1983) and found the
regime of instability that was
overlooked
FCR

1
  jd  B
c

B0 k
  V k  jd
c0
2
2
a
2
k rg >>1, γmax=jdB0/2cρVa
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
ηesc=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 σB=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
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
+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.
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
X-ray image
Thermal X-rays and 4.1 keV Sc line
(product of 44Ti) are observed from bright
radio-regions (ejecta)
radio-image
Borkowski et al. 2010
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
Acceleration at
forward and
reverse shocks
Minimal
electron heating
by Coulomb
collisions with
thermal ions
nH= 0.1 cm-3
Numerical results
B0= 5 μG
T= 104 K
ESN=1051 erg
u
Mej=1.4Msol
η = 0.01
PCR
BS
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
PCR
ρ
Pg
Spectra of accelerated particles
p
p
Integrated spectra
The input of the
forward shock was
considered earlier
(Berezhko & Völk
2007, Ptuskin et al.
2010, Kang 2011)
Spectrum of
ions is harder
than the
spectrum of
protons
because the
ejecta density
decreases in
time.
Integrated spectra
Alfven drift
downstream of
the forward
shock results in
the steeper
spectra of
particles
accelerated at
the forward
shock.
nH= 0.1 cm-3
Evolution of
nonthermal
emission
from SNR
TeV gamma-rays from young SNRs
Aharonian et al 2008 (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)
RX J1713.7-3946
Aharonian et al 2007 (HESS)
Vela Jr
hadronic
Fermi LAT results on RX J1713.73946 (Abdo et al. 2011)
RX J1713.7-3946
leptonic
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.
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.).
Summary
1. 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 gammaemission 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.