Gamma-Ray Emission from Core-Collapse SNRs
Don Ellison, NCSU
Co-workers for cr-hydro-NEI code:
Andrei Bykov, Herman Lee (poster), Hiro Nagataki, Dan
Patnaude, John Raymond, Pat Slane
SNR RX J1713
SNR RX J1713
Fukui et al 2012
Abdo et al 2011
HESS
Fermi LAT
Discuss Nonlinear Diffusive Shock Acceleration (DSA)
Origin of GeV-TeV emission:
Pion-decay from protons or inverse-Compton (IC) from electrons ?
 We discuss a uniform wind model with no non-spherical density
inhomogeneities.
Our fit to broad-band spectrum from radio to TeV gives strong
indication that IC dominates the high-energy emission
 In contrast, Fukui & Sano et al (2012) investigate clumpy structure
of gas density and photon emission
They conclude that pion-decay dominates the GeV-TeV emission.
Homogeneous Model of Thermal & Non-thermal Emission in SNR RX J1713
Look at X-ray energies
► Suzaku X-ray
observations  smooth
continuum well fit by
synchrotron from TeV
electrons
► No discernable line
emission from shockheated heavy elements
► Strong constraint on
Non-thermal emission at
GeV-TeV energies
Important to consider broad-band emission including thermal X-rays 
In nonlinear Diffusive Shock Acceleration (DSA) the production of gamma rays
is coupled to thermal X-ray emission
Nonlinear Diffusive Shock Acceleration (DSA) with cr-hydro-NEI code
(Ellison et al. 2012 and earlier papers) :
Generalized cr-hydro-NEI code (Lee et al. 2012) couples together:
1) 1-D, spherically symmetric hydro simulation of SNR based on VH-1
2) NL DSA model largely following published, semi-analytic solutions of Blasi,
Amato, Caprioli, Gabici & co-workers  some differences and additions
3) Non-equilibrium ionization (NEI) calculation of X-ray line emission
4) Broad-band continuum emission from trapped CRs within SNR and forward
shock precursor
5) Continuum emission from escaping CRs
 Explicit calculation of upstream, cosmic-ray precursor
 Momentum and space dependent CR diffusion coefficient
 Explicit calculation of resonant, quasi-linear magnetic field amplification (MFA)
 Calculation of Emax using amplified B-field
 Finite Alfven speed for CR scattering centers
 Line-of-sight projections of individual X-ray lines
Shocked ISM
material :
Calculate
X-ray
emission from
this region
CD
1-D core-collapse model: Pre-SN wind density:  ∝ 1/r2
Forward Shock
Reverse
Shock
Escaping
CRs
Shocked Ejecta
material :
Here, we ignore
emission from reverse
shock and ejecta
material
Extent of
shock
precursor
Spherically symmetric: We do not
model clumpy structure
1) CR electrons and ions
accelerated at FS
a) Protons give pion-decay
-rays
b) Electrons give
synchrotron, IC, &
non-thermal brems.
c) High-energy CRs escape
from shock precursor &
interact with external
mass
2) Follow evolution of shockheated plasma between FS
and contact discontinuity (CD)
a) Calculate electron
temperature, density,
charge states of heavy
elements, and X-ray line
emission
b) Include adiabatic losses
& radiation losses
5
Radial structure for core-collapse model for J1713

340 yr

990 yr
1630 yr
1/r2 pre-SN wind density profile
FS
Flow Speed
Log Pres.
Pressure precursor from shock
accelerated CRs
FS
Modified shock speed from CR
pressure
B [G]
Low B-field in pre-SN wind:
<10-7 G at forward shock at 1600 yr
Can include shell
Radius [pc]
SNR evolution
FS radius
FS compression
4
Frac. of ESN
in CRs
B [G]
B-field compression
at subshock
Magnetic field
amplification (MFA)
in precursor x40
Total CRs
escaping CRs
~17% of SN explosion energy
put into CRs at 1630 yr
proton pmax
proton pmax nearly
constant for J1713
SNR Age [yr]
Forward shock of SNR produces 3 particle distributions that will
contribute to the photon emission
1) Ions accelerated and trapped within SNR and precursor
2) Electrons accelerated and trapped within SNR and precursor
3) CRs escaping upstream (mainly ions)
Ellison & Bykov 2011
Escaping CRs
FS protons
trapped
FS electrons
See T. Bell’s & M. Malkov’s talks concerning escaping CRs
Results from generalized cr-hydro-NEI code (Lee, Ellison & Nagataki
2012, also Ellison, Slane, Patnaude, Bykov 2012 & previous work)
Core-collapse SN explodes in a
1/r2 pre-SN wind.
SNR RX J1713
Excellent fit to broad-band
(integrated) emission
synch
IC
 IC dominates GeV-TeV
emission
p-p
Most CR energy is still in ions
even with IC dominating the
radiation
brems
Note: p-p from escaping CRs small
unless external mass > 104 Msun
Pre-SN wind magnetic field much lower than ISM  Strong Magnetic field amplification
occurs but still have B-field low enough to have high electron energy to match HESS points
For J1713, we predict average shocked B ~ 10 µG ! (Amplification factor ~40)
CRs accelerated by FS and trapped within SNR
Log p4 f(p)
protons
CR spectra calculated between
forward shock (FS) and contact
discontinuity (CD) over 1630 yr
age of SNR RX J1713
Photons produced by CR spectra
with different evolutionary
histories and in different B-fields
electrons
Parameters for J1713 at 1630 yr:
 e/p = Kep =0.01
 EffDSA ~ 40%,
 ESN put into CRs ~17%
 B2 ~ 10 G
Shape of turnover is important. Controls X-ray synch. & TeV match. Turnover
from escaping CRs not yet modeled self-consistently. Warning: Cannot take
simple power law prediction from DSA and match only GeV-TeV emission.
(see M. Malkov’s talk for more on problem of escape)
Before Fermi data, possible to get good fit to J1713 continuum with piondecay dominating GeV-TeV. For example: Zirakashvili & Aharonian 2010
My apologies
to the many
papers on
modeling
young SNRs
that I don’t
mention
Even with Fermi data, shape of spectrum at GeV-TeV may not be enough
to discriminate between IC and pion-decay (Fukui & Sano et al. 2012;
Zirakashvili & Aharonian 2010 )
 Essential to consider X-ray line emission
NEI calculation of Thermal X-ray emission
p-p
Don Ellison, NCSU
ne [cm-3]
NEI calculation of heavy element ionization and X-ray line emission
np = 1 cm-3
Ionization fraction
Ionization fraction
Te [K]
Electrons reach X-ray emitting
temperatures rapidly even if DSA
highly efficient and shocked
temperature is reduced
Not easy to suppress thermal X-rays
np = 0.1 cm-3
Forward shock overtakes this gas at 100 yr
General calculations: Patnaude et al. , ApJ 2009
Applied to J1713: Ellison et al. ApJ 2010
R [arcsec]
Forward shock
Hadron
► Compare Hadronic & Leptonic fits
Coulomb Eq.
► Range of electron temperature
equilibration models
Suzaku
Hadron
► The high ambient densities needed for
pion-decay to dominate at TeV energies
result in strong X-ray lines
Instant equilibration
To be consistent with Suzaku observations
with lines weaker than synchrotron
continuum,
must have low density and high
accelerated e/p ratio (Kep ~ 0.01)
Lepton model
Coulomb Eq.
This impacts GeV-TeV emission
Best fit models have low shocked B-field
B2 ~ 10 G
Thermal X-ray constraint  IC
dominates GeV-TeV emission in
J1713
Ellison, Patnaude, Slane & Raymond ApJ (2007, 2010)
Note: Fukui et al (2012) reach conclusion that pion-decay from protons
dominates emission in J1713 based on spatial correlation of gas and -rays
Contours: HESS TeV -rays
Color: Np(H2 + H1) column density
Fukui et al. 2012
Also, high resolution Suzaku images may show that spatial correlation
between X-ray and TeV gamma-ray emission may not be as good as thought
from low-resolution images
15
Mult-component model (Inoue et al 2012; Fukui et al 2012):
Average density of ISM
protons: ~130 cm-3
Total mass ~2 104 Msun
over SNR radius
~0.1% of supernova
explosion energy in CR
protons !!
This may be a problem for
CR origin
Inoue et al (2012)
High densities needed for pion-decay may be in cold clumps that don’t
radiate thermal X-ray emission
Warning: many parameters and uncertainties in model, but :
For homogeneous model of SNR RX J1713 :
Inverse-Compton is best explanation for GeV-TeV (Note: other
remnants can certainly be Hadronic, e.g. Tycho’s SNR see G.
Morlino’s talk)
Note: For DSA most CR energy (~17% of ESN) is still in ions even
with IC dominating the radiation  SNRs produce CR ions !
Beyond CR question: Careful modeling of SNRs provides
constraints on critical parameters for shock acceleration:
a) Shape and normalization of CR ions from particular SNRs
b) Kep, electron/proton injection ratio
c) Acceleration efficiency
d) Magnetic Field Amplification
e) Properties of escaping CRs
f) Geometry effects in SNRs such as SN1006
18
Extra slides
19
Forward shock of SNR produces 3 particle distributions that will
contribute to the photon emission
1) Ions accelerated and trapped within SNR and precursor
2) Electrons accelerated and trapped within SNR and precursor
3) CRs escaping upstream (mainly ions)
Ellison & Bykov 2011
Escaping CRs
Some fraction of the highest
energy CRs will always
escape upstream in DSA
trapped
Vsk
Qesc
Shock
wave
CRs need self-generated
turbulence to diffuse.
This B/B will be lacking
far upstream
T. Bell & M. Malkov’s talks
Word on observations of rapid time variability in SNR synchrotron
emission
RX J1713 Nature 2007
1-2.5 keV X-rays
Interpreted by
Uchiyama et al. as
time-scale for
synchrotron losses
in B > 1 mG fields !
We predict much
lower (integrated)
B-fields
Alternative explanation that doesn’t set time scale of variations by
radiation losses (Bykov, Uvarov & Ellison 2008)
 Combine turbulent magnetic field with steep electron distribution
 For given synchrotron emission energy, local regions with high B have
many more electrons to radiate than regions of low B
5 keV
High B
Log
Ne
Low B
20 keV
50 keV
 Local high-B regions dominate line-of-sight
projection
 Varying magnetic turbulence produces
intermittent, clumpy emission
 Time scales consistent with SNR
observations
 No need for ~ 1000 µG magnetic fields
Log Electron Energy
SNR J1713: Tanaka et al 2008
Hadronic model
XIS
spectrum
Leptonic model
Simulated Suzaku XIS spectra
(nH = 7.9 1021 cm-2)
Lines produced by Hadronic model
would have been seen !
To be consistent with Suzaku observations. That
is, to have lines weaker than synchrotron
continuum, must have low ISM density and
accelerated e/p ratio, Kep~ 10-2
This determines GeV-TeV emission mechanism