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First It’s Hot & Then It’s Not Extremely Fast Acceleration of Cosmic Rays In A Supernova Remnant

First It’s Hot & Then It’s Not Extremely Fast Acceleration of Cosmic Rays In A Supernova Remnant. Peter Mendygral Journal Club November 1, 2007. Outline. Poor man’s outline of diffusive shock acceleration (DSA) Issue in DSA Background of SNR RX J1713.7-3946

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First It’s Hot & Then It’s Not Extremely Fast Acceleration of Cosmic Rays In A Supernova Remnant

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  1. First It’s Hot & Then It’s NotExtremely Fast Acceleration of Cosmic Rays In A Supernova Remnant Peter Mendygral Journal Club November 1, 2007

  2. Outline • Poor man’s outline of diffusive shock acceleration (DSA) • Issue in DSA • Background of SNR RX J1713.7-3946 • Chandra observations of SNR RX J1713.7-3946 • Conclusions Journal Club

  3. Diffusive Shock Acceleration Shock moving out Journal Club

  4. Diffusive Shock Acceleration Shock moving out Journal Club

  5. Diffusive Shock Acceleration Shock moving out Journal Club

  6. Diffusive Shock Acceleration Shock moving out Journal Club

  7. Diffusive Shock Acceleration • Original mechanism proposed by Fermi in 1949 as an attempt to explain the power-law nature of the cosmic ray spectrum • Particles accelerated in some region by successive scattering events where the recoil of the scatterer is negligible (i.e. particle hits a wall) Journal Club

  8. Diffusive Shock Acceleration • In the presence of a shock • Particle scatters off of B┴ on either side of shock • In particle’s frame, B┴ on either side of shock appears to be approaching (walls moving at it) • A resonance forms and particle gains lots of energy • Particle has energy-independent escape probability Journal Club

  9. Diffusive Shock Acceleration • B┴ is first generated by plasma instabilities due to the high energy thermal particles passing through the shock • For these systems a spectrum of Alfvén waves are produced yielding B┴ • Shock will amplify B┴ produced upstream • Particles will scatter approximately over the gyroradius of the interaction Journal Club

  10. I helped him. DSA Outline Alfvén waves generate turbulent B Higher energy particle escapes as CR B0,ISM = B|| B = turbulent Gyroradius increases with increased energy High energy thermal proton/electron encounters shock Bounces off previously made Alfvén wave and gains some energy Shock moving out Journal Club

  11. Shock Amplification • Collisionless shocks can produce a compression ratio (post-shocked to pre-shocked) given by • For γ = 5/3, as M→∞ r→4 • B┴ can be amplified by a factor of 4 • Amplifications beyond this are not well understood Journal Club

  12. Field Amplification • Observations of some SNRs suggest amplifications beyond 4 • Tycho • Cassiopeia A • > 4 amplification is predicted by non-linear DSA • Bell & Lucek can get ~100 • An independent measurement of the field strength in an SNR would verify if amplifications of this order are real Journal Club

  13. SNR RX J1713.7-3946 • Discovered in the ROSAT All-Sky Survey • Brightest source of non-thermal X-rays among shell-type SNRs • Core collapse of type II/Ib of massive progenitor • Age is ~1600 yr • Distance is ~1 kpc • Vshock ~3000 km s-1 XMM-Newton (Hiraga et. al., 2005) Journal Club

  14. Power-law X-ray Spectrum • XMM-Newton spectra of the rim are consistent for power-law with Γ ranging from 2.1−2.6 Hiraga et. al., 2005 Journal Club

  15. Broadband X-ray Spectrum • Suzaku data agrees well with theoretical expectation for spatially integrated synchrotron spectrum Uchiyama et. al., 2007 Journal Club

  16. Broken Power-law γ–ray Spectrum • Gamma-ray spectra are consistent with a model of π0 decay following inelastic proton-proton interactions • Imply proton acceleration in the shell up to 200 TeV • Could be consistent with IC scattering by 100 TeV electrons if B ~ 10μG ~ ISM value • Difficult to reconcile weak field with prediction that DSA will greatly amplify B 2004, 2005 gamma-ray excess HESS images (counts / smoothed region) (Aharonian et. al., 2007) Journal Club

  17. Evidence For SNR RX J1713.7-3946 • We have significant evidence that the system is a CR accelerator • X-ray data is a non-thermal power-law spectrum consistent with synchrotron spectrum • γ-ray data suggests presence of 200 TeV protons • Those regions are coincident • Fits description of candidate accelerator through DSA process Journal Club

  18. Chandra Observations • 1-2.5 keV Chandra ACIS image • Color scale is (0-1.2)x10-7 photons cm-2 s-1 pixel-1 • TeV γ-ray HESS contours overlaid • γ-ray contours coincident with x-ray Uchiyama et. al., 2007 Journal Club

  19. Chandra Observations • Top is 1-2.5 keV observations made in July 2000, July 2005, July 2006 (region b) • Bottom is hard-band (3.5-6 keV) observations (region c) • Color scale same as last image Uchiyama et. al., 2007 Journal Club

  20. Chandra Observations • Top arrow is a 10σ “hot spot” • Bottom arrow is a 6σ “hot spot” Journal Club

  21. Chandra Observations • Any arbitrary x-ray variation over the course of one year must take place in a compact region of angular size cΔt (θ< 1 arcmin) • Doesn’t alone rule out thermal processes • Also occur from a process where losses happen sufficiently fast over one year • Rules out any thermal processes • Thermal Bremsstrahlung and Free-Free emission ruled out Journal Club

  22. Timescales • Synchrotron loss timescale for electrons given by • DSA acceleration timescale of electrons given by • Average energy of synchrotron photon Journal Club

  23. Field Magnitude • To have seen the “hot spots”, tacc can’t significantly exceed the x-ray variability • Spots appeared within a few years • Assuming particle acceleration proceeds at maximum effective (Bohm-diffusion) regime with η 1 • B ~ 1mG • Independent of the acceleration mechanism, tsynch must also be on the order of one year • B ~ 1mG Journal Club

  24. Field Magnitude • Lower limits on the magnitude of B were estimated indirectly by measuring the width of x-ray filaments • Interpretation of these structures in terms of diffusion and synchrotron cooling gives B ~ 0.07-0.25 mG • The variability seen by Uchiyama represents the strongest amplification Journal Club

  25. Implications • Interpretation of γ-ray data as hadronic proton-proton interactions is most likely • IC is ruled out by B field measurement • Protons and nuclei are accelerated to PeV energies (electrons are short-lived at that energy) • Confirms that field amplifications over several orders of magnitude are possible • Non-linear DSA produces observed amplification but many microscopic process remain unexplored Journal Club

  26. References • Aharonian, F. A., many others, 2005, arXiv:astro-ph/0511678v2 • Aharonian, F. A., many others, 2006, arXiv:astro-ph/0511678v2 • Berezhko, E. G., Völk, H. J., 2006, A&A 451, 981–990 • Drury, L., 1983, Rep. Prog. Phys., Vol. 46, pp. 973-1027 • Hiraga, J. S., Uchiyama, Y., Aharonian, F. A., 2005, A&A 431, 953–961 • Uchiyama, Y., Aharonian, F. A., Tanaka, T., Takahashi, T., Maeda, Y., 2007, Nature, Volume 449, Issue 7162, pp. 576-578 Journal Club

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