PULSAR WIND NEBULAE: PROBLEMS AND PROSPECTS

1. PULSAR WIND NEBULAE:PROBLEMS AND PROSPECTS Jonathan Arons University of California, Berkeley

2. PWNe: Laboratories for Relativistic Cosmic Accelerators (Young) PWNe are Cosmic Pevatrons: Underlying Pulsar’s voltage: Synchrotron emitting PWNe have PeV particles – Chandra X-Ray emission (pulsed g-rays need TeV/particle or less) Theory: Spindown has electric current I = cF – relativistic particle loss rate in current = cF/e = Goldreich-Julian current J. Arons: COSPAR 2010

3. Large ( >>1) mass loss rate provides theoretical basis for MHD outflow model Problem #1 (a pulsar, not a PWN problem, but evidence starts with PWNe): Why is the multiplicity so large? (in young PSR/PWNe – not well measured for middle aged PWNe, except for slice of particle spectrum creating TeV by upscatter of CMB) Basic scale simple: multiplicity ~ accelerated particle energy at potential where pair creation shorts out acceleration (~ TeV)/lowest gamma-ray energy that can be absorbed emin Largest source = polar cap pair creation (? If so why don’t we see lower energy, escaping gamma rays in FERMI observations? Observed pulses gammas mostly from large r) Optical depth for gB pair production: L >> 1 Assumes angle between g momentum and B due to field line curvature J. Arons: COSPAR 2010

4. Large Multiplicity Problem (continued): Radio Core Emission (Rankin, Kramer et al): Low altitude B = dipole, R*/rB < 0.03 Always find multiplicity < few x 104; young PWN show > few x 105 – can be >> 105; can’t change rBmuch Increase particle energy? - e.g., incomplete poisoning of accelerating E by pairs? doesn’t work for young PSR in steady acceleration models; also doesn’t work for time dependent pair creation in vacuum E (Ruderman-Sutherland model) – see recent Timokinastro-ph, E poisoning voltage in full (1D) PIC+Monte Carlo similar to ancient estimates; space-charge limited flow case in progress Prospect: Polar B off set from star center (B stronger at one pole) Magnetic moment tipped with respect to radius set by gravitational bending of photon orbits ~ GM/Rc2 ~ 0.3 >> R*/rB Model explains large multiplicity while agreeing with radio beaming data? Lower emin = maximum energy of escaping gexplains lack of polar cap gamma emission in first set of FERMI PSR observations? 2 parameters, “predict” later obs? J. Arons: COSPAR 2010

5. Problem #2: Why are the injection spectra at the wind termination shock convex? Modeling injection particle spectra (most recently, to infer multiplicity) always comes up with broken power laws for the injected particle distribution at the wind termination surface (= wind termination shock wave, in standard model), hard at low energy, softer at high energy) SEDs from Bucciantini et al (2010): 3C58 Crab KES75 MSH15-52 Particle Injection Spectra at wind termination NOTE LACK OF INFRARED OBSERVATIONS! GO OUT AND USE AKARI (Im poster), WISE, Herschel, near IR from ground) – really needed to probe hard to soft 1.2 < g1 < 1.8 (Crab = 1.5) 2.15 < g2 < 2.8 (Crab = 2.4) J. Arons: COSPAR 2010

6. PWN SEDs (continued) High energy power law – diffusive shock acceleration (DSA), bouncing balls? (most think this is all there is to shock acceleration) Doesn’t work in transverse B geometry in pairs (Sironi & Spitkovsky 09 s > 10-2 – self-consistent turbulence) unless magnetic field is very, very weak – upstream s less than 10-3 – 3D PIC by AS & JA, being written) Low energy spectrum = Relativistic Maxwellian at jump condition temperature Follow the Duck Principle (“If it quacks like a duck, it must be a duck”): High energy distribution looks like relativistic DSA in the text particle limit, and in Monte Carlo with ~ isotropic up and down stream scattering as if transverse B field is very weak Nebular emission shows torus, in rotational equator where tranverse B flips – “current sheet” Transverse J. Arons: COSPAR 2010

7. High density pairs, mildly magnetized shock Low density pairs, beam, unmagnetized shock Termination Shock Location Termination Shock Structure (from del Zanna et al 04) PWN SEDs (continued) – a) MHD flow + relativistic DSA- aspherical flow idea Termination Shock = Magnetic Sandwich Oblique shock at higher latitude has post shock temperature declining with increasing latitude – is the hard low energy power law the envelope of the Maxwellians? Requires plasma flux to increase strongly with latitude – how strongly? Is that what polar cap pair creation does? Idea commented in private by many, needs calc – hard to make work in Crab, T must drop by 100, almost all mass flux at pole Violates Duck Principle – spectrum doesn’t look Maxwellian, model is a masquerade J. Arons: COSPAR 2010

8.  PWN SEDs (continued) – 2) non-MHD flow in current sheet, non-DSA acceleration for hard low energy power law “Current sheet” = asymptotic result of decay of the striped wind – upstream striped field decay does work if mass loading high ( >>104) Sheet beam, drift kink instabilities of wrinkled current sheet generate fast diffusion in the sheet, field decay at ~ Bohm rate, diffusion coefficient ~ p2, current carriers run away, form relativistic beam in and near equator Magnetic field strength increases with latitude (fairly slowly, final current sheet fairly thick DSA in weak field zone near equator? Cyclotron acceleration driven by runaway beam at somewhat higher latitude creates hard low energy power law in pairs J. Arons: COSPAR 2010

9. PWN SEDs – b) non-MHD flow in current sheet, non-DSA acceleration for hard low energy power law (continued) Flat spectrum radio emitters accelerated by cyclotron resonance in higher s flow at higher latitude? Amato & JA 1D PIC – hasn’t yet been done in 2D and 3D electrons positrons Large Larmor radius beam= “protons” How well does this work? Does low energy accelerator connect smoothly to higher energy equatorial DSA? What is the long wave turbulence needed to make DSA work all the way to PeV energies (rLarmor ~ r ~ rshock)? Does such long wavelength turbulence have direct observational consequences (perhaps seen in Crab “wisp” motions – others?) J. Arons: COSPAR 2010

10. PWN SEDs – c) Shock driven turbulence and fast 2nd order Fermi acceleration (vA~ c/3) for low energy power law? Turbulence significant for high energy DSA? Outflow from termination shock interacting with circulation in the nebula is unstable: B from Camus et al 2009 pressure If turbulence cascades to short wavelength, fast 2nd order Fermi acceleration may accelerate radio emitting spectrum from post shock pairs – spectrum flat, diffusion time to leave equatorial acceleration zone ~ acceleration time – how flat? J. Arons: COSPAR 2010

11. Turbulence can model wisps? Still 2D, lots of coherence – too much? (courtesy Komissarov) Older idea: Cyclotron instability of equatorial beam (invoked to do non-DSA shock acceleration in transverse B geometry) can drive compressional waves that look like wisps) (AS & JA 2004) In Crab, other moving features) J. Arons: COSPAR 2010

12. Prospect #2: question - Why are the injection spectra at the wind termination shock convex? All of the potential solutions a), b), c) need more theory investigation – envelope of Maxwellians needs a decent phenomenological model (not by JA); cyclotron instability of runaway beam needs a multi-D simulation in magnetic sandwich geometry; macrscopic shock turbulence needs 3D MHD simulation (coupled with 2nd order Fermi acceleration and diffusion model; improved pair creation model); dissipative relativistic wind model (“resistive” relativistic MHD) including emission from relativistic current sheets (TeV? GeV?) Observations: 1) IR SEDs (Herschel, WISE, Akari, Planck?) needed to fill in the huge holes in the SEDs, even more severe for middle aged PWNe than for young. Essential for measuring injection multiplicity as function of age, needed for pulsar pair injection into ISM (recently raised by PAMELA positron “anomaly”) 2) Is there any sign of pole to equator gradients in upstream 4 velocity and particle flux (test envelope of Maxwellians model for radio emitting particles) 3) Older PWNe? How long does a PWN last – reverse shock crushing limits life? Pulsar fade out (energy, mass loss)? 4) Emission from dissipative wind (possible pulsed TeV, GeV?) J. Arons: COSPAR 2010

13. IR prospects: 20m (PWN luminosities from Bucciantini et al 2010 model) nfn(erg/cm2-s) Crab (D = 2 kpc): 2 x 10-9 3C58 (D = 3.5 kpc): 1 x 10-11 MSH 15-52 (D = 5 kpc): 3 x 10-12 Kes 75 (D = 6 kpc): 0.9 x10-12 W.I.S.E. (Wide Field Infrared Survey Explorer – resolution = 12”, sensitivity = 2600 mJy at 23 m IRAS type survey First data release spring 2011 J. Arons: COSPAR 2010