COSPAR 2008, Montreal, 13 July Patrick Slane (CfA) X-ray Observations of Supernova Remnant Shocks

COSPAR 2008, Montreal, 13 JulyPatrick Slane (CfA)

X-ray Observations of

Supernova Remnant Shocks


What Do SNR Shocks Tells Us?

SNR Shocks:

• Trace SNR dynamical evolution through heated CSM/ISM and ejecta, and reveal nature and environment of their progenitors• Compress and amplify ambient magnetic fields, leading to particle acceleration• Mitigate electron heating through plasma or collisional processes

Major Uncertainties Include:

• Electron-ion temperature equilibration processes and timescales• Approach to ionization equilibrium, and the effects of heating and particle acceleration• The strength of magnetic fields at the forward and reverse shock• Particle injection in the acceleration process• The roll of turbulence in ejecta mixing, field amplification, and other dynamical processesI.e., just about everything…

Here we touch on what X-ray observations of SNR shocks bring to some of these issues


SNRs: The (very) Basic Structure

ISM

Sh

ock

ed ISM

Sh

ock

ed E

ject

a

Unsh

ock

ed

Eje

cta

PW

N

Puls

ar

Win

d

Forward Shock

Reverse ShockPWN Shock

PulsarTerminationShock

• Pulsar Wind - sweeps up ejecta; shock decelerates flow, accelerates particles; PWN forms

• Supernova Remnant - sweeps up ISM; reverse shock heats ejecta; ultimately compresses PWN; particles accelerated at forward shock generate magnetic turbulence; other particles scatter off this and receive additional acceleration


• Expanding blast wave moves supersonically through CSM/ISM; creates shock - mass, momentum, and energy conservation across shock give (with =5/3)

X-ray emitting temperatures

shock

€

P0,ρ 0,v0

€

P1,ρ1,v1

v

Shocks in SNRs: Thermal Spectra

€

1 =γ +1

γ −1ρ 0 = 4ρ 0

€

v1 =γ −1

γ +1v0 =

v0

4

€

T1 =2(γ −1)

(γ +1)2

μ

km H v0

2 =1.3×107 v10002 K

€

vps =3vs

4


Tracking the EjectaType Ia:

• Complete burning of 1.4 Mo C-O white dwarf

• Produces mostly Fe-peak nuclei (Ni, Fe, Co) - very low O/Fe ratio

• Si-C/Fe sensitive to transition from deflagration to detonation; probes density structure - X-ray spectra constrain burning models

• Products stratified; preserve burning structure


Tracking the EjectaType Ia:

• Complete burning of 1.4 Mo C-O white dwarf

• Produces mostly Fe-peak nuclei (Ni, Fe, Co) - very low O/Fe ratio

• Si-C/Fe sensitive to transition from deflagration to detonation; probes density structure - X-ray spectra constrain burning models

• Products stratified; preserve burning structure

Core Collapse:

• Explosive nucleosynthesis builds up light elements - very high O/Fe ratio

• Fe mass probes mass cut

• O, Ne, Mg, Fe very sensitive to progenitor mass

• Ejecta distribution probes mixing by instabilities


DEM L71: a Type Ia

• ~5000 yr old LMC SNR

• Outer shell consistent with swept-up ISM - LMC-like abundances

• Central emission evident at E > 0.7 keV - primarily Fe-L - Fe/O > 5 times solar; typical of Type Ia

Hughes et al. 2003

• Total ejecta mass is ~1.5 solar masses - reverse shock has heated all ejecta

• Spectral fits give MFe ~ 0.8-1.5 Mo and MSi ~ 0.12-0.24 Mo - consistent w/ Type Ia progenitor


SNR 0509-67.5• SNR 0509-67.5 is a young SNR in the LMC - identified as Type Ia based on ASCA spectrum (Hughes et al. 1995)

• Observations of optical light echoes establish age as ~400 yr - spectra of light echoes indicate that SNR is result of an SN 1991T- like (bright, highly energetic, Type Ia) SN (Rest et al. 2008)

• Comparison of hydrodynamic and non-equilibrium ionization models for emission from Type Ia events with X-ray image and spectrum indicates E = 1.4 x 1051 erg s-1 and M(56Ni)=0.97 Mo (i.e. energetic, high-luminosity, 1991T-like event) - X-ray analysis can be used to infer type and details of SNe!

Rest et al. 2005

Badenes et al. 2008



• Shock velocity gives temperature of gas - can get from X-rays (modulo NEI effects)

• If cosmic-ray pressure is present the temperature will be lower than this - radius of forward shock affected as well

X-ray emitting temperatures

shock

€

P0,ρ 0,v0

€

P1,ρ1,v1

v

Shocks in SNRs: Dynamics

Ellison et al. 2007

=0=.63

€

1 =γ +1

γ −1ρ 0 = 4ρ 0

€

v1 =γ −1

γ +1v0 =

v0

4

€

T1 =2(γ −1)

(γ +1)2

μ

km H v0

2 =1.3×107 v10002 K

€

vps =3vs

4


Aside: Evidence for CR Ion Acceleration

Tycho Ellison et al. 2007

Warren et al. 2005

• Efficient particle acceleration in SNRs affects dynamics of shock - for given age, FS is closer to CD and RS with efficient CR production

• This is observed in Tycho’s SNR - “direct” evidence of CR ion acceleration

Forward Shock(nonthermal electrons)


Aside: Evidence for CR Ion Acceleration Ellison et al. 2007Tycho

Warren et al. 2005



Reverse Shock(ejecta - here Fe-K)


Aside: Evidence for CR Ion Acceleration Ellison et al. 2007Tycho

Warren et al. 2005

Warren et al. 2005



ContactDiscontinuity



• Shock velocity gives temperature of gas - can get from X-rays (modulo NEI effects)

• If cosmic-ray pressure is present the temperature will be lower than this - radius of forward shock affected as well

shock

€

P0,ρ 0,v0

€

P1,ρ1,v1

v

Shocks in SNRs: Nonthermal Spectra

Ellison et al. 2007

€

1 =γ +1

γ −1ρ 0 = 4ρ 0

€

v1 =γ −1

γ +1v0 =

v0

4

€

vps =3vs

4


Broadband Emission from SNRs

Ellison, Slane, &Gaensler (2001)

• synchrotron emission dominates spectrum from radio to x-rays - shock acceleration of electrons (and protons) to > 1013 eV - Emax set by age or energy losses - observed as spectral turnover

• inverse-Compton scattering probes same electron population; need self- consistent model w/ synchrotron

• pion production depends on density - thermal X-ray emission provides constraints


-rays from G347.3-0.5 (RX J1713.7-3946)

• X-ray observations reveal a nonthermal spectrum everywhere in G347.3-0.5 - evidence for cosmic-ray acceleration - based on X-ray synchrotron emission, infer electron energies of ~50 TeV

• This SNR is detected directly in TeV gamma-rays, by HESS - -ray morphology very similar to x-rays; suggests I-C emission - spectrum seems to suggest -decay WHAT IS EMISSION MECHANISM?

ROSAT PSPC HESS

Slane et al. 1999 Aharonian et al. 2006


Modeling the EmissionMoraitis & Mastichiadis 2007

• Joint analysis of radio, X-ray, and -ray data allow us to investigate the broad band spectrum - data can be accommodated by synch. emission in radio/X-ray and pion decay (with some IC) in -ray - however, two-zone model for electrons fits -rays as well, without pion-decay component

• Pion model requires dense ambient material - but, implied densities appear in conflict with thermal X-ray upper limits

• Origin of emission NOT YET CLEAR


Modeling the Emission

1d

1m

1y

• Joint analysis of radio, X-ray, and -ray data allow us to investigate the broad band spectrum - data can be accommodated by synch. emission in radio/X-ray and pion decay (with some IC) in -ray - however, two-zone model for electrons fits -rays as well, without pion-decay component

• Pion model requires dense ambient material - but, implied densities appear in conflict with thermal X-ray upper limits

• Origin of emission NOT YET CLEAR - GLAST MAY SETTLE ISSUE

Moraitis & Mastichiadis 2007

For kT > 0.25 keV,

while hadronic modelsrequire n0 > 0.8 cm-3€

n0 ≤ 0.2d1−1/2 cm−3

Slane et al. 1999


• Particles scatter from MHD waves in background plasma - pre-existing, or generated by streaming ions themselves - scattering mean-free-path

(i.e., most energetic particles have very large and escape)

• Maximum energies determined by either: age – finite age of SNR (and thus of acceleration)

radiative losses (synchrotron)

escape – scattering efficiency decreases w/ energy

€

=ηrg =ηE /eB

€

η =δBB

⎛

⎝ ⎜

⎞

⎠ ⎟−2

≥1

Diffusive Shock Acceleration

€

Emax (age) ~ 0.5v82t3BμG (ηRJ )

−1TeV

€

Emax (loss) ~ 100v8(BμGηRJ )−1/ 2 TeV

€

Emax (escape) ~ 20BμGλ17TeV

see Reynolds 2008

High B => High Emax

High B => Low Emax for e+-

High B => High Emax

magnetic field

amplification

important!


• Particles scatter from MHD waves in background plasma - pre-existing, or generated by streaming ions themselves - scattering mean-free-path

(i.e., most energetic particles have very large and escape)

• Maximum energies determined by either: age – finite age of SNR (and thus of acceleration)

radiative losses (synchrotron)

escape – scattering efficiency decreases w/ energy

€

=ηrg =ηE /eB

Diffusive Shock Acceleration

€

Emax (age) ~ 0.5v82t3BμG (ηRJ )

−1TeV

€

Emax (loss) ~ 100v8(BμGηRJ )−1/ 2 TeV

€

Emax (escape) ~ 20BμGλ17TeV

see Reynolds 2008 Electrons: - large B lowers max energy due to synch. losses

Ions: - small B lowers max energy due to inability to confine energetic particles

Current observations suggest high B fields


Thin Filaments: B Amplification?• Thin nonthermal X-ray filaments are now observed in many SNRs, including SN 1006, Cas A, Kepler, Tycho, RX J1713, and others - observed drop in synchrotron emissivity is too rapid to be the result of adiabatic expansion

• Vink & Laming (2003) and others argue that this suggests large radiative losses in a strong magnetic field:

• Diffusion length upstream appears to be very small as well (Bamba et al. 2003) - we don’t see a “halo” of synchrotron emission in the upstream region

• Alternatively, Pohl et al (2005) argue that field itself is confined to small filaments due to small damping scale

€

B ~ 200v82 / 3 l

0.01pc

⎛

⎝ ⎜

⎞

⎠ ⎟

−2 / 3

μG

€

lD ~κ

v, but κ ∝ B−1


Rapid Time Variability: B Amplification?

• Along NW rim of G347.3-0.5, brightness variations observed on timescales of ~1 yr - if interpreted as synchrotron-loss or acceleration timescales, B is huge: B ~ 1 mG

• This, along with earlier measurements of the nonthermal spectrum in Cas A, may support the notion of magnetic field amplification => potential high energies for ions

• Notion still in question; there are other ways of getting such variations (e.g. motion across compact magnetic filaments); more investigation needed

Lazendic et al. 2004

Uchiyama et al. 2008

€

tsyn ~ 1.5BmG−3 / 2εkeV

−1/ 2yr

€

tacc ~ 9BmG−3 / 2εkeV

1/ 2 v1000-2 yr


Time Variations in Cas A• Cas A is expanding rapidly

• Significant brightness variations are seen on timescales of years - ejecta knots seen lighting up as reverse shock crosses

• Variability seen in high energy continuum as well - similar to results from RX J1713.7-3946

• Uchiyma & Aharonian (2008) identify variations along region of inner shell, suggesting particle accelerations at reverse shock - many more observations needed to understand this!


Summary• X-ray observations of SNRs provide spectra that probe the composition and thermodynamic state of ejecta - provides constraints on progenitor types and explosion physics

• X-ray imaging and spectroscopy trace position of FS/CD/RS - provides dynamical information about evolutionary state as well as evidence for particle acceleration X-ray dynamics indicated strong hadronic component - X-ray spectra (thermal and nonthermal) place constraints on nature of VHE -ray emission

• Several lines of argument lead to conclusion that the magnetic field is amplified to large values in shock - thin filaments; rapid variability this could allow acceleration of hadrons to ~knee - other explanations for above exist, without large B; further study needed

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COSPAR 2008, Montreal, 13 July Patrick Slane (CfA) X-ray Observations of Supernova Remnant Shocks