Transcript Cours Goutelas juin 2003

Particle acceleration in Supernova
Remnants from X-ray observations
Anne Decourchelle
Service d’Astrophysique, CEA Saclay
I- Ejecta dominated SNRs: Cas A, Tycho and Kepler
II- Synchrotron-dominated SNRs: SN 1006, G347.3-0.5
Shell SNRs and plerions
=>Restriction to shell SNRs
Tycho (XMM-Newton)
PWN in G0.9+0.1 (XMM-Newton)
-> talk of Sidoli
Particle acceleration in SNRs
SNRs : main source of cosmic-rays with E ≤ 1015 eV ?
► SNRs have enough total power
► Strong shocks in SNRs: First-order Fermi shock
acceleration (radio emission → relativistic GeVelectrons)
► Composition of bulk of CRs, typical of mixed ISM
→ acceleration of ISM gas and dust by shocks in SNRs
► X-ray observations of synchrotron emission
→ TeV electrons
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Pending questions:
How efficient is cosmic-ray acceleration in SNRs ?
What is the maximum energy of accelerated particles ?
How large is the magnetic field ? Is it very turbulent ?
Is it amplified ?
No unambiguous evidence for ion acceleration in SNRs
Efficient particle acceleration in young supernova remnants
2 shocks
Interstellar medium
<---Ejecta --->
=>Modification of the X-ray
morphology and temperature
of the gas
RADIUS
Decourchelle, Ellison, Ballet 2000, ApJ 543, L57
Blondin and Ellison 2001, ApJ 560, 244
Morphology of the X-ray continuum in young SNRs
• Strong radio emission “associated” with the ejecta => amplified magnetic
field due to R-T instabilities at the interface ejecta/ambient medium
Allen et al. 1997, ApJ 487, L97
• Strong X-ray emission “associated” with the ejecta and weaker plateau in
X-rays associated with the blast wave => inconsistent with synchrotron:
non-thermal bremstrahlung ? Particle acceleration at secondary shocks ?
(Vink & Laming 2003, pJ 584, 758) ?
Cas
CasA
A
XMM-Newton 8-15 keV
XMM-Newton
8-15 keV
Bleeker et al. 2001, A&A 365
Bleeker et al. 2001, A&A 365
Radio
Radio
Spectrum of the forward shock in Cas A
Vink and Laming 2003, ApJ 584, 758
VLA radio
X-ray continuum
Chandra
(x 10)
Thermal component: kT ~ 3.5 keV and net ~ 4 1010 cm-3s
Non-thermal component:
RIM: 84 % of the 4-6 keV continuum, power law a ~ 2.2
INSIDE: 11% of the 4-6 keV continuum, power law a > 4.5
=>width of the rim inconsistent with thermal emission
Radio VLA
XMM-Newton 4-6 keV
XMM-Newton Si K
DeLaney et al., 2002, ApJ 580, 914
Cassam-Chenai et al., 2003, A&A in press
Kepler (1604)
Cassam-Chenai et al., 2003
•Continuum associated with the forward shock:
synchrotron emission ?
• Forward shock very close to the interface
ejecta/ambient medium: RFS/RCD = 11/10:
Shocked ambient
medium
Shocked ejecta
morphology expected when particle
acceleration is nonlinear
!
XMM spectrum
of the forward shock
Shocked ejecta => thermal non ionization equilibrium emission
Shocked ambient medium => synchrotron emission using radio
constraints: Maximum energy of accelerated electrons ~ 60 TeV
4-6 keV
Decourchelle et al. 2001, A&A 365, L218
No emission line features !
Tycho (1572)
Chandra spectrum of the forward shock
interpretation:
•ContinuumThermal
associated
with the forward shock:
8 cm-3s => strong
kT = 2.1 keV
and
n
t
=
10
e
synchrotron emission ?
ionization delay but problem with the morphology
• Forward shock very close to the interface
Non-thermalmedium:
interpretation:
ejecta/ambient
RFS/RCD synchrotron
= 11/10:
Hwang et al, 2002, ApJ
continuum
morphology
X-ray power
law: aexpected
~ 2.8 when particle
acceleration is nonlinear !
Radio 22cm
Broad band
Rolloff frequency: n ~7 1016 Hz
=> maximum electron energies ~ 1-12 TeV
Chandra
Chandra
Sharp rims at the forward shock. Radiative ?
X-ray continuum
Chandra
Sharp filaments observed all along the periphery of these young SNRs => synchrotron
emission, width determined by synchrotron losses of ultrarelativistic electrons
Time to move out t = r / ugas with ugas = 1/R*Vsh , R: compression ratio
Equating tloss and t gives B.
Cas A: D = 3.4 kpc, Vsh~ 5200 km/s, <4", t < 1.56109 s => B  60-100 G
Vink and Laming 2003, ApJ 584, 758
Tycho: D = 3.2 kpc, Vsh~ 4600 km/s, 4”, t = 1.65109 s => B  60 G
Kepler: D = 4.8 kpc, Vsh~ 5400 km/s, 3”, t = 1.59109 s => B  60 G
Requires magnetic field amplification (Lucek and Bell 2000, MNRAS 314,65) or
nonlinear particle acceleration and large compression ratio at the shock (~8)
Summary on particle acceleration in young SNRs
Morphology of the high energy X-ray continuum
- Bright emission associated with the ejecta in Cas A: nonthermal bremsstrahlung.
Particle acceleration at secondary shocks ?
- Ejecta interface close to the forward shock in Kepler and Tycho => nonlinear particle
acceleration with shock modification
- Sharp filaments all along the rim => magnetic field randomly oriented around the 3
remnants (unlike in SN 1006)
Forward shock: nature of the spectrum and width of the rim
- Synchrotron emission at the forward shock
=> First order Fermi acceleration with electrons up to energies of 1-60 TeV
- Sharp rims due to the limited lifetime of the ultrarelativistic electrons in the SNR
=>large magnetic field values ~ 60-100 G derived for Cas A, Tycho, Kepler
Amplification of the magnetic field or nonlinear particle acceleration with large
compression ratio (>>4) ?
SN 1006: first evidence of electrons acceleration
up to TeV energies (Koyama et al. 1995, Nature 378, 255)
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X-ray thermal emission in the faint areas
X-ray synchrotron emission in the bright rims
Power-law distribution of electrons, cut-off at high energy
νcut = 240 eV (Dyer et al. 2001, ApJ 551, 439)
νFν
γ-ray emission
Synchrotron
IC
Radio
SN 1006, CANGAROO
(Tanimori et al. 1998, ApJ 497, L33)
small B
• Inverse Compton on 3 K CMB (or local
photons) by the same TeV electrons
emitting the X-rays by synchrotron
or
large B
• 0 decay following nuclear reactions by
the accelerated ions on the thermal gas
• Only spectral range where the protons
may be directly detected (electrons make
up only a few % of the energy density of
CRs)
SN 1006 with XMM-Newton :
Geometry of the acceleration
R. Rothenflug et al.
MOS images (square root scaling), 4 pointings GT
Oxygen band (0.5 – 0.8 keV) :
thermal emission
2 – 4.5 keV band :
Non-thermal emission
Transverse profile: principle
How is the magnetic field oriented ?
A symmetry axis seems to be running from south-east to north-west, BUT if the bright
limbs were an equatorial belt, non-thermal emission should also be seen in the interior
=> Polar caps
φ = π/3
Out
In
Fin/Fout > π/2φ – 1 = 0.5
If equatorial belt: Fin/Fout > 0.5
Observed: 0.8-2 keV: 0.300 ± 0.014
2-4.5 keV: 0.127 ± 0.074
Radio/X-ray comparison
RADIO
Combined Molonglo + Parkes radio image
at 843 MHz with 44" x 66" (FWHM)
spatial resolution
(Roger et al., ApJ 332, 940).
XMM-Newton: 2 – 4.5 keV band
Extract spectra in boxes just behind the
shock all around the remnant, and as a
function of radius toward the North-East
Spectral
modelling
Fit the spectra with a two-component model:
• synchrotron emission from a cut-off electron
power law (SRCUT).
• thermal emission from a plasma out of
ionisation equilibrium behind a shock
(VPSHOCK) with variable abundances.
Interstellar absorption fixed: NH = 7 1020 cm-2.
Slope of the e- power-law fixed: 2.2.
(α = 0.6 in the radio)
Normalisation of the synchrotron component
fixed using the radio data
=> only the cut-off frequency was left free.
North East
South East
Cut-off frequency
Center
- Spatial variations of the cut-off
frequency of the synchrotron emission
exist in SN 1006.
Shock
- The very strong radial variations
confirm that the bright limbs look like
polar caps.
- Very strong azimuthal variations, cannot
be explained by variations of the
magnetic compression alone.
=> Maximum energy of accelerated
particles higher at the bright limbs than
elsewhere.
- If B ~ 50 μG, the maximum energy
reached by the electrons at the bright
limb is around 100 TeV.
νcut (eV) ~ 0.02 B(μG) E2cut (TeV)
The X-ray geometry of SN 1006 favors cosmic-ray acceleration where the
magnetic field was originally parallel to the shock speed (polar caps)
An extreme case of synchrotron-dominated SNR:
G347.3-0.5
ASCA
XMM-Newton results => talk of G. Cassam-Chenai
Uchiyama et al. 2002, PASJ 54, L73
GeV emission (EGRET)
associated with cloud A ?
Butt et al. 2001, ApJ, 562, L167
TeV emission (CANGAROO) in the NW:
Inverse compton or 0 decay process ?
Muraishi et al. 2000, A&A, 365, L57, Enomoto et al. 2002,
Nature, 416, 25, Reimer & Pohl 2002, A&A, 390, L43