Particle acceleration in Supernova Remnants from X-ray observations

Anne Decourchelle

Service d’Astrophysique, CEA Saclay

I- Ejecta dominated SNRs: Cas A, Tycho and KeplerII- Synchrotron-dominated SNRs: SN 1006, G347.3-0.5

Tycho (XMM-Newton) PWN in G0.9+0.1 (XMM-Newton)

Shell SNRs and plerions

=>Restriction to shell SNRs

-> talk of Sidoli

Particle acceleration in SNRs

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

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

RADIUS

Blondin and Ellison 2001, ApJ 560, 244Decourchelle, Ellison, Ballet 2000, ApJ 543, L57

Efficient particle acceleration in young supernova remnants

<---Ejecta --->

Interstellar medium

2 shocks

=>Modification of the X-ray morphology and temperature

of the gas

Cas A

RadioBleeker et al. 2001, A&A 365

XMM-Newton 8-15 keV

Allen et al. 1997, ApJ 487, L97

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

• 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 A

XMM-Newton 8-15 keVBleeker et al. 2001, A&A 365

Radio

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 ~ 2.2 INSIDE: 11% of the 4-6 keV continuum, power law > 4.5

=>width of the rim inconsistent with thermal emission

Vink and Laming 2003, ApJ 584, 758

VLA radioX-ray continuumChandra

(x 10)

Spectrum of the forward shock in Cas A

Cassam-Chenai et al., 2003, A&A in press

DeLaney et al., 2002, ApJ 580, 914

Kepler (1604)

XMM-Newton Si K

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:

morphology expected when particle acceleration is nonlinear !

Shocked ejecta

Shocked ambient medium

Shocked ejecta => thermal non ionization equilibrium emissionShocked ambient medium => synchrotron emission using radio constraints: Maximum energy of accelerated electrons ~ 60 TeV

XMM-Newton 4-6 keV Radio VLA

XMM spectrum of the forward shock

Tycho (1572)•Continuum associated with the forward shock:

synchrotron emission ?

• Forward shock very close to the interface ejecta/ambient medium: RFS/RCD = 11/10:

morphology expected when particle acceleration is nonlinear !

4-6 keV

Decourchelle et al. 2001, A&A 365, L218

No emission line features !

Thermal interpretation:

kT = 2.1 keV and net = 108 cm-3s => strong

ionization delay but problem with the morphology

Non-thermal interpretation: synchrotron

X-ray power law: ~ 2.8

Rolloff frequency: ~7 1016 Hz

=> maximum electron energies ~ 1-12 TeV

Broad bandRadio 22cmcontinuum

Chandra Chandra

Chandra spectrum of the forward shock

Hwang et al, 2002, ApJ

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 < 1.56109 s => B 60-100 G Vink and Laming 2003, ApJ 584, 758

Tycho: D = 3.2 kpc, Vsh~ 4600 km/s, 4”, t = 1.65109 s => B 60 GKepler: D = 4.8 kpc, Vsh~ 5400 km/s, 3”, t = 1.59109 s => B 60 G

Sharp rims at the forward shock. Radiative ?

Requires magnetic field amplification (Lucek and Bell 2000, MNRAS 314,65) or nonlinear particle acceleration and large compression ratio at the shock (~8)

X-ray continuumChandra

Sharp filaments observed all along the periphery of these young SNRs => synchrotron emission, width determined by synchrotron losses of ultrarelativistic electrons

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) ?

Summary on particle acceleration in young SNRs

SN 1006: first evidence of electrons acceleration up to TeV energies (Koyama et al. 1995, Nature 378, 255)

• 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)

(Tanimori et al. 1998, ApJ 497, L33)

SN 1006, CANGAROO

γ-ray emission

• Inverse Compton on 3 K CMB (or local photons) by the same TeV electrons emitting the X-rays by synchrotron

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)

or

SynchrotronIC

Radio

νFν

small B large B

SN 1006 with XMM-Newton : Geometry of the acceleration

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

R. Rothenflug et al.

Transverse profile: principleHow 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

φ = π/3

In

Out

Fin/Fout > π/2φ – 1 = 0.5

If equatorial belt: Fin/Fout > 0.5Observed: 0.8-2 keV: 0.300 ± 0.014 2-4.5 keV: 0.127 ± 0.074

=> Polar caps

Radio/X-ray comparison

Combined Molonglo + Parkes radio image at 843 MHz with 44" x 66"  (FWHM) spatial resolution

(Roger et al., ApJ 332, 940).

Extract spectra in boxes just behind the shock all around the remnant, and as a function of radius toward the North-East

XMM-Newton: 2 – 4.5 keV bandRADIO

Spectral modelling

North East

South East

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.

Cut-off frequency

- 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)

Center

Shock

- Spatial variations of the cut-off frequency of the synchrotron emission exist in SN 1006.

- The very strong radial variations confirm that the bright limbs look like polar caps.

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

Uchiyama et al. 2002, PASJ 54, L73

ASCA

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

XMM-Newton results => talk of G. Cassam-Chenai