PULSAR WIND NEBULAE:PROBLEMS AND PROSPECTS

Jonathan AronsUniversity of California,

Berkeley

J. Arons: COSPAR 2010 27/23/10

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 37/23/10

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

Large ( >>1) mass loss rate provides theoretical basis for MHD outflow model

Assumes angle between g momentum and B due to field line curvature

J. Arons: COSPAR 2010 47/23/10

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 rB much

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 Timokin astro-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 g explains lack of polar cap gamma emission in first set of FERMI PSR observations? 2 parameters, “predict” later obs?

J. Arons: COSPAR 2010 57/23/10

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):

Crab 3C58 MSH15-52 KES75

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 67/23/10

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 77/23/10

PWN SEDs (continued) – a) MHD flow + relativistic DSA- aspherical flow idea

Termination Shock = Magnetic Sandwich

Termination Shock Location

Termination Shock Structure(from del Zanna et al 04)

Low density pairs, beam,unmagnetizedshock

High density pairs, mildlymagnetizedshock

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 thatwhat polar cap pair creation does? Idea commented in private by many, needscalc – 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 87/23/10

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 thickDSA 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 97/23/10

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”

PWN SEDs – b) non-MHD flow in current sheet, non-DSA acceleration for hard low energy power law (continued)

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 107/23/10

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 pressureIf 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 117/23/10

Turbulence can model wisps? Still 2D, lots of coherence – too much?

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)

(courtesy Komissarov)

(AS & JA 2004)

In Crab, other moving features)

J. Arons: COSPAR 2010 127/23/10

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 137/23/10

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 surveyFirst data release spring 2011