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Fundamental Problems: Pulsars to their Winds

O. de Jager, C. Venter, M. Vorster (NWU, S.A.) North-West University, Potchefstroom South Africa. Fundamental Problems: Pulsars to their Winds. NS & GB – 30 March to 5 April: Cairo & Alexandria. Primary acceleration fields (unscreened by pair production)

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Fundamental Problems: Pulsars to their Winds

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  1. O. de Jager, C. Venter, M. Vorster (NWU, S.A.) North-West University, Potchefstroom South Africa Fundamental Problems: Pulsars to their Winds NS & GB – 30 March to 5 April: Cairo & Alexandria

  2. Primary acceleration fields (unscreened by pair production) • ensemble average of pulsed gamma-ray emission from a globular cluster. • Caviats: pair production in ms pulsars • Pulsar Wind Nebulae probing pulsar properties: • Birth periods from dynamics and MWL observations. • Pair production multiplicities. • Caviats: Stabbed PWN – Vela X Contents

  3. Select unscreened pulsars to test basic semi self consistent models for particle acceleration. Screening complicates matters. • Average over a large sample of similar pulsars to obtain a statistical average over uncertain magnetic inclination and viewer line of sights. • N=50 millisecond pulsars in 47 Tucanae Globular cluster provide an ideal sample. How can we test first principles of pulsar electrodynamics experimentally?

  4. Calculate 3D rotating model magnetospheres for sample of N=100 cluster ms pulsars in 47 Tuc with random P,Pdot. • Take random observer & magnetic angles and calculate flux slices cut by observer. • Follow escape of radiation reaction limited electrons through the light cylinder (about 1% of spindown power). GR-Frame dragging model of Muslimov & Harding. Magnetic inclination and EoS enters as main uncertainty.

  5. Venter & de Jager (2008) and • Venter, de Jager & Clapson (2009), ApJL • EoS and number N of pulsars main uncertainties. Predicted pulsed flux within a factor 2 of Fermi if we reduce N to 50.

  6. Venter, de Jager & Clapson (2009), ApJL • Shocked “Pulsar Winds” from ms pulsars? • Assume Bohm diffusion in cluster. No re-acceleration assuming sigma parameter does not decrease beyond light cylinder. Also, no reacceleration in cluster space. Even HESS becomes constraining for 5 – 10 G cluster fields given N=50. However, if field lines in cluster are not tangled then diffusion faster than Bohm is possible and predicted unpulsed cluster flux should drop. Predicted unpulsed nebular flux to be tested with CTA.

  7. However, disaster !!! Look at the radio, X-ray and Fermi pulse profiles of ms pulsars !

  8. Fermi Pulse profiles implies: • Copious pair production: • Field strength too low for magnetic pair production. • However, photon-photon pair production • Cascading – include synchrotron for X-rays • Screening • Slot gaps etc. to explain Fermi fan beams. • Let us be self-critical and face our demons: Why did we predict Fermi a-priori correct within a factor 2 ? Implications:

  9. Galactic Radio pulsar statistics (F-G & Kaspi 06): • P0=0.3±0.15 s • Pmin=0.01 s (SN157B in LMC) • Millisecond Birth Period would destroy the SNR shell • (1/2)I2 = 2E52 (1 ms/P)2 erg > ESN =1051 E51 erg • Leptons ejected during spindown history give a relic wind. X-ray synchrotron emitting leptons would most likely burn away, but surviving lower energy leptons should give IC radiation gamma-ray nebula. Lack of such a nebula gives a lower limit on birth period. • Estimating the Birth Period of Pulsars through GLAST LAT Observations of Their Wind Nebulae(O. C. de Jager 2008 ApJ) At least three constraints on pulsar birth periods.

  10. Kes 75: MWL studies of SNRTotal number of leptons: Ne>8x1048 Integrated number of Goldreich Julian pairs: NGJ=1.6x1044Pair production multiplicity: 2.5x105 > M > 2.4x104

  11. We will see that Vela X has been stabbed ! How reliable are these calorimetric measurements?

  12. X-rays in annular rings measure flux & photon index vs. radius • Connect profiles for radial flow velocity V(r) and radial field strength B(r) profile via Faraday’s induction equation. • Assume spherical and planar (cylindrical) geometry, obtain trial (B,V) solutions coupled via Faraday. • Free parameters at pulsar wind termination shock and downstream: • Conversion efficiency of spindown power to leptons • Field compression ratio • Maximum lepton energy (constrained by gyroradius size) • Sigma parameter • Radial dependence of flow profile (depends on geometry) Measurements of the sigma parameter and pulsar conversion efficiency in from Vela compact nebula from X-rays and MHD principles. How does that square up with HESS observations?

  13. Zoom in on compact PWN: Pulsar/Torus/Jet

  14. Solving 1D particle transport equation downstream including synchrotron & adiabatic losses. Synchronize grid with X-ray resolution.Calculate synchrotron spectra from lepton spectra

  15. Model & observed spectral evolution vs. radius. Constrain to freshly injected compact nebula (30 years) – avoid outer regions of reverse shock.

  16. Parameters for Vela X:

  17. What does it predict for HESS? Too little ! Because we see relic electrons accumulated over 11 kyr

  18. Vela Torus/Jet/Pulsar REVERSE SHOCK HESS detection of Vela X (Aharonian et al. 2006)

  19. Who pricked the bag of Vela X ? • The total lepton energy from TeV (+ MWL) observations correspond to a conversion efficiency of 0.001 of spindown power to leptons. • Estimate from X-rays is ~0.1 => factor 100 higher. • Reverse shock could have punched the field line structure of the PWN => protective azimuthal configuration lost. • Leptons leak out. • How serious is this leakage problem for other post-reverse shock (Vela-like) HESS PWN ? Are they reliable calorimeters ? The pre-reverse shock HESS PWN (Crab-like) maintain their leptons.

  20. Conclusions • Galactic plane seems to be dotted with PWN along |b|0, where Type II SNR are typically formed following massive star formation in molecular clouds. • Ground-based VHE gamma-ray observations probe the electron component of extended PWN, which corresponds to the EUV domain (in synchrotron), which cannot be done with EUV instruments (absorption). • Target photon field for PWN is CMBR/galactic photons fields => spatial map of VHE emission truly reflects the electron distribution. Same cannot always be said about X-ray observations, due to • Contamination from thermal emission and • Possible gradients in the PWN magnetic field strength complicates conclusions about resident electron distributions. • Energy dependent morphology is a proof of electron origin in G18.0-0.7. First detection of such morphology in HESS J1825-137. • Particle dominated PWN are ideal VHE sources (de Jager & Venter 2005). This allow them to (a) have radiation maxima in IC rather than synchrotron, and (b) maximal expansion sizes from electron survival considerations.

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