Presentation on theme: "Acceleration and Escape of Particles in Young Supernova Remnants Vikram Dwarkadas University of Chicago Igor Telezhinsky, Martin Pohl (DESY) May 3 2012HEDLA."— Presentation transcript:
Acceleration and Escape of Particles in Young Supernova Remnants Vikram Dwarkadas University of Chicago Igor Telezhinsky, Martin Pohl (DESY) May HEDLA 2012
Interaction of Type Ia Ejecta with Constant-Density ISM May HEDLA 2012
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. May HEDLA 2012
Our Method May HEDLA Use flow profiles from the hydrodynamical simulations 2. Solve the CR transport equation for these flow profiles. Advantages: Accurate treatment of acceleration and transport Consistent account of escape Disadvantages: No CR feedback (Ok if cosmic ray pressure at shock < 10% SNR ram pressure)
1.Transformation of spatial coordinate required 2.Diffusion coefficient is coupled to MF profile 3.2 MF profiles used thus far D: density scaling P: pressure scaling 4. Multiple shocks can be accounted for 5.Self-consistent calculation of emission (Synchrotron, IC, Pion-Decay) 6.Take Alfvenic Drift into account Method (Contd) May HEDLA 2012
Magnetic Field Magnetic field can be amplified close to the shock region Assume B FS (t)=(2 0 V FS (t) 3 /c) 0.5 (Caprioli+2009) Assume B profiles scale as density B(r,t)=B FS (r,t)/ FS (r,t)
Particle spectra ElectronsProtons Here density scaling of MF with 75 G at forward shock Note the bumps in the spectra! May HEDLA 2012
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? May HEDLA 2012
Emission (Type 1a) Variation of gas and MF over shocked region is critical May HEDLA 2012
Surface Brightness Maps RADIO 1.4GHz SY 3 keV PD 1 TeV IC 1 TeV M1 M2 May HEDLA 2012
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 – E max of the escaped CR distribution
Scenarios Cloud Cloud: R MC = 4 pc M = 1000 M s n = 100 cm -3 Cloud Cloud: R MC = 4 pc M = 1000 M s n = 100 cm -3 D=22 pc, = const Ia,f D=22 pc, ~ r -2 Ic,f D=12 pc, = const Ia,n
Results: particle spectra, t=400 yrRL DBDBDBDB DBDBDBDB DGDGDGDG RL DBDBDBDB DBDBDBDB DGDGDGDG 0.01D G l
Results: particle spectra, t=2000 yrRL DBDBDBDB DBDBDBDB DGDGDGDG RL DBDBDBDB DBDBDBDB DGDGDGDG 0.01D G l Escape effective only in region close to shock!
Illumination of a Cloud By a SNR May HEDLA 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.
Results: radial distributions, E max Ia, D1Ia, D2 Ic, D1Ic, D2
Results: E max of the escaped CRs Not Sedov scaling!
Conclusions Realistic hydrodynamic evolution of SNRs is important for modeling Reverse shock can dominate in young SNRs for the first few 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 May HEDLA 2012
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 if it is nearby (within a couple of SNR radii) Refs: Telezhinsky, Dwarkadas, Pohl, 2011, Astroparticle Physics Telezhinsky, Dwarkadas, Pohl 2012, A&A, in press (arxiv)
Questions & Discussion May HEDLA 2012
Are Particles Accelerated at Reverse Shock? Sep Washington Univ 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.
Are Particles Accelerated at Reverse Shock? Sep Washington Univ 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.
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) Sep Washington Univ 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
Rho et al (2002) – RCW 86 A model of a plane shock in Fe- rich ejecta, with a synchrotron continuum, provides a natural explanation. This requires that reverse shocks in ejecta be accelerating electrons to energies of order 50 TeV. (Mosaicked three-color Chandra images of RCW 86: red represents keV photons; green represents 1-2 keV photons; and blue represents 2-8 keV photons.) Sep Washington Univ
DeLaney et al (2002) - Kepler The flat-spectrum and steep- spectrum radio emission indicate forward- and reverse-shocked material, respectively, and indicate a partial decoupling of these shocks in the southern portion of the remnant. This implies that the reverse shock is accelerating particles at least to radio energies. (Spectral index between 6 and 20 cm. Intensity is set by the 6 cm continuum image.) Sep Washington Univ
Gamma-Ray observations of SNRs Veritas – Cas AFermi – Cas A May HEDLA 2012
Diffusion ModelsRL DBDBDBDB DBDBDBDB DGDGDGDG 1 20 D1 RL DBDBDBDB DBDBDBDB DGDGDGDG 0.01D G l D2 D B = pvc/3qBD G =10 28 (E/10 GeV) 1/3 (B/3μG) -1/3 cm 2/s