Non-thermal acceleration mechanismsin supernova remnant shells

Anne DecourchelleService d’Astrophysique, DAPNIA

CEA Saclay, France

I- X-ray observationsII- Shock physics and particle accelerationIII- Contributions of SIMBOL-X

Collaborators:J. Ballet, G. Cassam-Chenaï (CEA Saclay)D. Ellison (NCSU)J. Hughes (Rutgers)U. Hwang (Goddard)G. Dubner, E. Giacani (IAFE)

SNRs : main source of cosmic-rays with energies up to 3 1015 eV ?

Emission processes:- synchrotron from radio (GeV e-) to X-rays (TeV e-)- π0 following p-p collisions in GeV-TeV range- inverse Compton on CMB in TeV range

Thermalinterior

Nonthermalrims

Ellison, Berezhko and Baring 2000, ApJ 540, 292

GeV e-

TeV e-

synchpion

IC

brems

First evidence of electrons upto TeV energies in SN 1006(Koyama et al. 1995, Nature 378, 255)

Tycho(SN 1572)

Chandra

Si K line

4-6 keV continuumSi K lineFe L line

4-6 keV continuum

SN material ejected at high velocity=> Heating of the ejecta and ISM

• Powerful X-ray production usually dominated bythermal emission at ~ 1 keV of optically thin plasma.

• Spatially resolved spectroscopy => non-thermalemission as well.

SN Ia

X-ray observations of young SNRs below 10 keV

Acceleration of electrons at the forward shock.• powerful X-ray emitters of soft thermal emission.• Above 2 keV, synchrotron emission dominates.

O K line band

2 - 4.5 keV band

SN 1006

SN Ia

X-ray observations of synchrotron-dominated SNRs below 10 keV

XMM-Newton

X-ray observations of shell SNRs above 10 keV

Above 10 keV: observations with RXTE,Beppo-Sax and INTEGRAL satellites.

No spectro-imagerie available=> nature and origin of the emissionobserved above 10 keV still unknown.

Cas ARXTE/ PCA

Allen et al. 1999, ICRC

INTEGRAL/ISGRI

Cas A

Renaud et al. 2006, ApJL

Allen et al. 1997, ApJ 487, L97

Nature of the hard X-ray emission observed in young SNRs ?

Ejecta

ColdISM

interface ejecta/ ISM

2 shocks

ThermalX-raysfrom

shockedISM

ThermalX-raysfrom

shockedejecta

cold

X-raysynchrotron:

electronsacceleratedby diffusionon magneticturbulence

on bothsides of the

shock

Non-thermalbremsstrahlung:acceleration ofelectrons at the

interface(in some cases)

Nature of the hard X-ray emission observed in young SNRs ?

- Thermal emission from the shocked ambient medium ?

- Synchrotron emission from relativistic electrons accelerated at the forward shock ?

- Nonthermal bremsstrahlung from suprathermal electrons accelerated at the

contact discontinuity ?

Spatially resolved spectroscopy above 10 keV requiredto distinguish and quantify the contribution of each potential processus

⇒ SIMBOL-X awaitedto resolve a number of pending questions on shock physics and particle

acceleration

1. Shock physics

Thermal emission from theshocked ambient medium ?

- Constraints on the degree ofequilibration of the electrons atthe shock through collision-lessprocesses.

- Constraints on the supernovakinetic energy

However, faint level of thermalemission below 10 keV from theshocked ambient medium (inTycho, SN 1006 )

=> not in favour of a thermal origin ofthe hard X-ray emission.

Hughes et al. 2000, ApJ 528, L109

10-3

10-2

10-1

100

101

1 10C

ounts

/sec

/keV

RXTE/PCA

ASCA/SIS

Ejecta

MIS

Energy (keV)

Spectre en rayons X du reste de Kepler

SiS

ArCa

FeISM

X-ray spectrum of the Kepler SNR

Full equipartition between Te and Ti

2. Electron acceleration

X-ray synchotron emission

o Maximum energy of accelerated particles

- obtained through the measurement of the cut-offfrequency of the synchrotron emission, observable inX-rays (if the magnetic field is known)

=> energy on the order of 10 TeV.- knowledge of the full extent of the synchrotron emission

above 10 keV is required to better determine it.

o Azimuthal variation of Emax along the SNR shock

- spatially resolved spectroscopy required

What is the maximum energy of accelerated particles ?Electrons are a few % of cosmic rays

but can reveal a lot on the mechanism of diffusive shock acceleration=> accelerated like protons, so their spectrum is expected to be the same.

SN 1006

XMM-Newton

Azimuthal variations of the cut-off frequency

Very strong azimuthal variations, cannotbe explained by variations of the

magnetic compression alone.

=> Maximum energy of acceleratedparticles higher at the bright limbs thanelsewhere.

- - If B ~ 10 µG, the maximum energyreached by the electrons outside thelimbs is around 25 TeV.

- - If B is amplified at the limbs,Emax(protons) is certainly much larger (>1000 TeV) there.

νcut (eV) ~ 0.02 B(μG) E2cut

(TeV)The X-ray geometry of SN 1006 favors cosmic-ray acceleration where themagnetic field was originally parallel to the shock speed (polar caps)

XMM-Newton

SN 1006:a SN Ia

νcut (eV) ~ 0.02 B(μG) E2cut (TeV)

Rothenflug et al. 2004, A&A 425, 121

2. Electron acceleration in young SNRs

o Constraints on the magnetic fieldconfiguration and intensity

- either the magnetic field is large enough (~100 µG) to induce strong radiative losses inthe high energy electrons (Vink and Laming2003).

- or the magnetic field is damped at theshock (Pohl et al. 2005).

These models predict distinct morphology andspectral shape in X-rays (Cassam-Chenaïet al. 2007).

The X-ray spectrum must be observed over abroad band (extending to hard X-rays) totell one from the other.

How large is the magnetic field ? Is it very turbulent ? Is it amplified ?Morphology of the X-ray synchrotron emission:

filamentary emission in thin sheets just behind the blast wave.

Cassam-Chenai et al. 2004, A&A 414, 545

Hwang et al, 2002, ApJ 581, 1101

Blondin and Ellison 2001, ApJ 560, 244

3. Proton acceleration: morphological signature

o Back-reaction of the accelerated particle on the shock and hydrodynamics

Evidence for ion acceleration in SNRs ? How efficient is cosmic-ray acceleration ?What fraction of the shock energy can be tapped by the cosmic rays ?

RADIUS10.95 1.05 1.1

interfaceejecta/ISM

reverseshock

forward shock

For efficient ion injection, large fraction of energy goes into accelerated particles• larger compression ratio and lower post-shock temperature (but observed Te < Ti )• modified hydrodynamics: narrower interaction region, observable in X-rays

Decourchelle, Ellison, Ballet 2000, ApJ 543, L57Ellison, Decourchelle, Ballet 2004, A&A 413, 189

1-D

2-D

Modified hydrodynamicsin 1E 0102.2-7219 SNR:

lower post-shock temperature

Mean post-shock temperature withoutefficient ion acceleration:

kTs = 3/16 µmp Vs2

Shock velocity Vs~ 6200 km/s => expected Ts ~ 45 keV

Electronic temperature measured withChandra: ≤ 1 keV

(Coulomb collisions predict > 2.5 keV)

=> Efficient proton accelerationat the forward shock

Hughes et al. 2000, ApJ 543, L61

2-D

Chandra

1E 0102.2-7219 in the SMC

Modified hydrodynamics in Tycho’s SNR:narrower interaction region

Forward shock very close to the contactdiscontinuity :

Observed value: RFS/Rcd < 1.1

Test-particle value: RFS/Rcd ~1.18

=> efficient particle acceleration ofprotons

Decourchelle 2004, Warren et al. 2005, ApJ 634, 376

2-D

Forward shock

Contact discontinuity

3-color imagered = Fe Lgreen = Si + S Kblue = 4 to 6 keV continuum

Azimuthal variation of RFS/Rcd

Test-particle

Contours: X-ray (ASCA)

Aharonian et al. 2006, A&A 449, 223

3. Proton acceleration: spectral signature at TeV energies

Evidence for ion acceleration in SNRs ?o Pion decay (TeV observations with HESS, GLAST,…)

=> knowledge of the contribution of the IC emission required

First resolved TeV sourceFirst shell SNR detected in γ-raysGood correlation with X-ray emission

HESS0.3 – 40 TeV

Large nearby remnant (1° diameter) with onlysynchrotron emissionBrighter emission, softer spectrum (cloud interaction)

Cassam-Chenaï et al. 2004, A&A 427, 199

RXJ1713-: a core collapse supernova

XMM-NEWTON0.5-10 keV

4. Acceleration by secondary shocks

Are electrons accelerated to supra-thermal energies (sub-MeV) close to the contactdiscontinuity due to secondary shocks and enhanced turbulence ?

- High energy continuum associated with the ejecta => inconsistent with X-ray synchrotron- Non-thermal bremsstrahlung at the interface due to particle acceleration at secondary

shocks ? (Vink & Laming 2003, ApJ 584, 758)

XMM-Newton 8-15 keV

Bleeker et al. 2001, A&A 365

Cas A: a core collapse supernova

Ellison et al.

Contribution of Simbol-X

Major contribution of Simbol-X expected on particle acceleration

thanks to its spectro-imaging capabilityfrom thermal (below 10 keV) to non-thermal regime (above 10 keV).

⇒ Contribution of each potential emission processus (thermal, suprathermal,nonthermal).

⇒ Particle acceleration: coupling between thermal and non-thermal populations throughthe back-reaction of accelerated protons on the shock structure and hydrodynamics.

⇒ Physics at the interface between the ejecta and the ambient medium through theacceleration to suprathermal energies.

⇒ Shock physics at the forward shock through the heating of the electrons.

SourcesYoung ejecta-dominated galactic SNRs: relatively bright sources, adapted to the FOV

(less than 10 arcmin).Synchrotron-dominated SNRs: specific sites of particle acceleration

Cas A (1670): the youngest and brightest known galactic SNR

Suprathermal electrons accelerated at the interface and relativistic electronsaccelerated at the forward shock ?

Simulation with SIMBOL–X:Region west of Cas A with a bright andrelatively hard spectrum.

SIMBOL–X > 20 keVField of 10 x 10 arcmin2

Total exposure time = 30 ks

CZT

MPD

EPIC PN1 arcmin2, 30 ks

SN 1006

Simulation with SIMBOL–XNE region of SN 1006 with bright

and relatively hard emission

Simulation with SIMBOL–X > 10 keVField of 10 x 10 arcmin2

Total exposure time = 30 ks

Spectrum and maximum energy of the electrons accelerated at parallel shocks ?

1 arcmin2, 30 ks

CZT

MPD

Kepler (1604)

Spectrum and maximum energy of theelectrons accelerated at the forward

shock ? Azimuthal variations ?

Simulation with SIMBOL–XBright and relatively hard regionTobs = 100 ks

1 arcmin2, 100 ks

CZT

MPDMPD: