Acceleration and Escape of Particles in Young Supernova Remnants

1. Acceleration and Escape of Particles in Young Supernova Remnants Vikram Dwarkadas University of Chicago Igor Telezhinsky, Martin Pohl (DESY) HEDLA 2012

2. Interaction of Type IaEjecta with Constant-Density ISM HEDLA 2012

3. Young SN Expansion • There are two shocks, a forward shock expanding into the ambient medium, and a reverse shock going back into the ejecta. • The shocks are collisionless. • Particles are accelerated, presumably by Diffusive Shock Acceleration, at the shock. In principle both shocks can accelerate particles. • We wish to study the acceleration of particles at the shocks, escape and transport of high-energy particles, and the resulting γ-ray emission. HEDLA 2012

4. Our Method 1. Useflowprofilesfromthehydrodynamicalsimulations 2. SolvetheCR transportequationfortheseflowprofiles. Advantages: Accuratetreatmentof acceleration and transport Consistentaccountofescape Disadvantages: NoCR feedback(Okifcosmicraypressure at shock < 10% SNR rampressure) HEDLA 2012

5. Method (Contd) Transformation of spatial coordinate required Diffusion coefficientis coupled to MF profile 2 MF profiles used thus far D: density scaling P: pressure scaling 4. Multiple shocks can be accounted for Self-consistent calculation of emission (Synchrotron, IC, Pion-Decay) Take Alfvenic Drift intoaccount HEDLA 2012

6. Magnetic Field • Magnetic field can be amplified close to the shock region • Assume BFS(t)=(2pr0xVFS(t)3/c)0.5 (Caprioli+2009) • Assume B profiles scale as densityB(r,t)=BFSr(r,t)/rFS(r,t)

7. Particle spectra Electrons Protons Here density scaling of MF with 75 mG at forward shock Note the bumps in the spectra! HEDLA 2012

8. Observed Spectra of SNRs in GeV and TeV range (from Caprioli, JCAP 2011) Note that spectral index is smaller in GeV range as compared to TeV range for almost all SNRs. This implies that spectra are steeper at higher energies, and flatter at lower energies (with some exceptions – Tycho). This is exactly opposite to what is predicted by basic non-linear Diffusive Shock Acceleration (NLDSA) theory. So, what gives? HEDLA 2012

9. Emission (Type 1a) Variation of gas and MF overshockedregioniscritical HEDLA 2012

10. Surface Brightness Maps M1 M2 RADIO 1.4GHz PD 1 TeV SY 3 keV IC 1 TeV HEDLA 2012

11. Transport Equation • Test-particle approximation • Numerical evolution SNR • Account for two shocks • Spherically-symmetric geometry • CR dilution is taken into account • Can trace escaped particles up to a few tens of SNR radii • CR escape is intrinsic part of solution • allows obtain CR spectral shape at the given time • Emax of the escaped CR distribution

12. Scenarios Ia,n Ia,f D=12 pc, r = const D=22 pc, r = const Ic,f Cloud: RMC= 4 pc M = 1000 Ms n = 100 cm-3 D=22 pc, r~ r-2

13. l 0.01DG Results: particle spectra, t=400 yr L R R L DB DB DG DB DG DB

14. l 0.01DG Results: particle spectra, t=2000 yr L R R L DB DB DG DB DG DB Escape effective only in region close to shock!

15. Illumination of a Cloud By a SNR 1 TeV Intensity distribution of Type Ia/Molecular Cloud (top), and Core Collapse SNR/Molecular Cloud (bottom), at the age of 1000 years (left) and 2000 years (right). Log-scaled colormap spans roughly 2.5 orders of magnitude per image. HEDLA 2012

16. Results: radial distributions, Emax Ia, D1 Ia, D2 Ic, D1 Ic, D2

17. Results: Emax of the escaped CRs Not Sedov scaling!

18. Conclusions • Realistichydrodynamicevolutionof SNRs is important for modeling • Reverse shock can dominate in young SNRs forthefirstfew 100 years (depends on MF) • Adiabatic cooling/heating throughout the system • Complicated particle / emission spectra • Hard gamma-ray spectra don‘t always imply CR modification HEDLA 2012

19. Conclusions • We have reconstructed the shapes as well as the maximum energies of the escaped CR distribution directly from simulations. • We account for dilution of CR energy density ahead of the spherical shock • In case Bohm diffusion is assumed in the upstream region, CRs are trapped around the SNR for a long time • Illumination of a Molecular cloud is effective only ifit is nearby (within a couple of SNR radii) • Refs: Telezhinsky, Dwarkadas, Pohl, 2011, Astroparticle Physics Telezhinsky, Dwarkadas, Pohl 2012, A&A, in press (arxiv)

20. Questions & Discussion HEDLA 2012

21. Are Particles Accelerated at Reverse Shock? Depends on the magnetic field in the ejecta just ahead of the reverse shock. Ellison, Decourchelle and Ballet (2005): “The expanded ejecta bubble may be one of the lowest magnetic field regions in existence” If so, then the reverse shocks may not be accelerating particles to high energies, at least not in comparison with the forward shock. Therefore most authors have neglected the reverse shock. This may also be technically because they don’t know how to deal with it. Washington Univ

22. Are Particles Accelerated at Reverse Shock? Depends on the magnetic field in the ejecta just ahead of the reverse shock. But there are several indications from X-ray and radio observations that particles can be accelerated at the reverse shock. Washington Univ

23. Helder & Vink 2008 (Cas A) [Possibly the best evidence of acceleration at reverse shock] • The power-law index of the spectrum between 4.2 and 6.0 keV is an indicator of X-ray synchrotron emission; there is a correlation between filaments, dominated by continuum emission and hard spectra. • Hard X-ray spectra are not exclusively associated with filaments, dominated by continuum emission, suggesting that nonthermal emission also comes from other regions. • The nonthermal X-ray emission is likely to be synchrotron radiation. • The nonthermal emission accounts for about 54% of the overall continuum emission in the 4-6 keV band. • In the western part of Cas A, most X-ray synchrotron emission comes from the reverse shock. • (See also Gotthelf et al 2001) Spectrum of Cas A as observed by Chandra. Below is the spectrum of the featureless filament (D) described by Hughes et al. (2000), extracted from the megasecond observation; above is a spectrum of the whole remnant of one single observation (obsID 4638), multiplied by 0.1. Washington Univ

24. Rho et al (2002) – RCW 86 • A model of aplane shock in Fe-richejecta, with a synchrotroncontinuum, provides a naturalexplanation. This requires thatreverse shocks in ejectabe accelerating electrons toenergies of order 50TeV. • (Mosaicked three-color Chandra imagesof RCW 86: redrepresents 0.5-1 keV photons;green represents 1-2 keVphotons; and blue represents2-8 keV photons.) Washington Univ

25. DeLaney et al (2002) - Kepler • The flat-spectrum andsteep-spectrum radio emission indicateforward- and reverse-shocked material,respectively, and indicate apartial decoupling of theseshocks in the southernportion of the remnant. • This implies that the reverse shock is accelerating particles at least to radio energies. • (Spectral index between 6and 20 cm. Intensityis set by the6 cm continuum image.) Washington Univ

26. Gamma-Ray observations of SNRs Veritas – Cas A Fermi – Cas A HEDLA 2012

27. Diffusion Models D1 R L DB DG DB D2 0 2 1 R L l 0.01DG DB DG DB DB = pvc/3qB DG=1028 (E/10 GeV)1/3 (B/3μG)-1/3 cm2/s 2 1 1.05 0