Pulsar Wind Nebulae - Stellenbosch Universityacademic.sun.ac.za/summerschool/2006/documents/lecture...Patrick Slane Harvard-Smithsonian Center for Astrophysics Pulsar Wind Nebulae

Harvard-Smithsonian Center for AstrophysicsPatrick Slane

Pulsar Wind Nebulae


http://www.astroscu.unam.mx/neutrones/home.html


• Rotating magnetospheregenerates E X B wind- direct particle accelerationas well, yielding(e.g. Michel 1969; Cheng,Ho, & Ruderman 1986)

E&410−≈

• Magnetic polarity inwind alternates spatially- magnetically “striped” wind- does reconnection result in

conversion to kinetic energy?(e.g. Coroniti 1990, Michel1994, Lyubarsky 2003)

The Pulsar Wind Zone


• Rotating magnetospheregenerates E X B wind- direct particle accelerationas well, yielding(e.g. Michel 1969; Cheng,Ho, & Ruderman 1986)

E&410−≈

• Magnetic polarity inwind alternates spatially- magnetically “striped” wind- does reconnection result in

conversion to kinetic energy?(e.g. Coroniti 1990, Michel1994, Lyubarsky 2003)

• Wind expands until rampressure is balanced bysurrounding nebula- flow in outer nebula restrictsinner wind flow, formingpulsar wind terminationshock

The Pulsar Wind Zone

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.


Pulsar Wind Nebulae• Expansion boundary condition at forces wind termination shock at - wind goes from inside to

at outer boundaryv ≈ c / 3

wRNR

wRv ≈ RN / t

• Pulsar acceleratesparticle wind- wind inflates bubbleof particles and magnetic flux

- particle flow in B-fieldcreates synchrotronnebula

bPulsar

WindMHD Shock

Particle Flow

BlastWave

Hα or ejecta shell

logarithmicradial scale wR

11

00 τ

1−+

−

⎥⎦

⎤⎢⎣

⎡+=

nn

tEE &&

}

- spectral break at

where synchrotron lifetime of particlesequals SNR age

- radial spectral variationfrom burn-off of highenergy particles

Hz τ10ν 3324 −−≈ Bbr

• Pulsar wind is confined by pressurein nebula- wind termination shock

obtain by integratingradio spectrumRs =

Ý E 4πcPN

⎡

⎣ ⎢

⎤

⎦ ⎥ 1/ 2

• Wind is described by magnetizationparameterσ = ratio of Poynting flux to particleflux in wind

σ ≡FE×B

Fparticle

++ + + +

R

Γ


Pulsar Wind Nebulae• Young NS powers a particle/magneticwind that expands into SNR ejecta- toroidal magnetic field results in axisymmetric equatorial wind

• Termination shock forms where pulsarwind meets slowly expanding nebula- radius determined by balance of ram pressure and pressure in nebula

• As PWN accelerates higher densityejecta, R-T instabilities form- optical/radio filaments result

• As SNR/PWN ages, reverse shockapproaches/disrupts PWN- as pulsar reaches outer portions of SNR,a bow-shock nebula can form

Gaensler & Slane 2006


Begelman & Li 1992

• Dynamical effects of toroidal fieldresult in elongation of nebula alongpulsar spin axis- profile similar for expansion into ISM,

progenitor wind, or ejecta profiles- details of structure and radio vs. X-ray

depend on injection geometry and B

• MHD simulations give differences indetail, but similar results overall- B field shows variations in interior- turbulent flow and cooling could result in

additional structure in emission

puls

ar a

xis

van der Swaluw 2003

pulsar axis

Elongated Structure of PWNe


G5.4-0.1

Lu et al. 2002

Elongated Structure of PWNe

Crab Nebula

pulsar spin

G11.2-0.3

Roberts et al. 2003

PSR B1509-58

Gaensler et al. 2002

3C 58

Slane et al. 2004


The Crab Nebula


• Optical filaments show dense ejecta- total mass in filaments is small; stillexpanding into cold ejecta?

• Rayleigh-Taylor fingers produced asrelativistic fluid flows past filaments- continuum emission appears to reside

interior to filaments; filamentary shell

Filamentary Structure in PWNe: Crab Nebula

Jun 1996


Crab Nebula - radio

Crab Nebula – Hα + cont

Filamentary Structure in PWNe: Crab Nebula

• Radio emission shows structuresimilar to optical line emission- interaction with relativistic fluid in PWNforms nonthermal filaments?

• Overall morphology is elliptical- suggestive of pulsar rotation axis with

southeast-northwest orientation

• What do we see in in X-rays?

• Optical filaments show dense ejecta- total mass in filaments is small; stillexpanding into cold ejecta?

• Rayleigh-Taylor fingers produced asrelativistic fluid flows past filaments- continuum emission appears to reside

interior to filaments; filamentary shell

N

W


The Crab Nebula in X-rays

• Fine structure observed in Chandraimage- poloidal loop structures surround torus

as well (seen w/ HST too)


The Crab Nebula in X-raysHow does pulsar energizesynchrotron nebula?Pulsar: P = 33 ms

dE/dt = 4.5 x 10 erg/s38

Nebula: L = 2.5 x 10 erg/s37x

• X-ray jet-like structure appearsto extend all the way to theneutron star- jet axis aligned with pulsar propermotion; same is true of Vela pulsar

• inner ring of x-ray emissionassociated with shock waveproduced by matter rushingaway from neutron star- corresponds well with optical wisps

delineating termination shock boundary

jet

ring

v


G21.5-0.9: Home of a Young Pulsar

Slane et al. 2000

Matheson & Safi-Harb et al. 2005

Camilo et al. 2005

• G21.5-0.9 is a composite SNR for which a radiopulsar with the 2nd highest spin-down power hasrecently been discovered (Camilo et al. 2005)-- τ ~ 4.8 kyr; true age more likely < 1 kyr

• Merged 351 ks HRC observation reveals pointsource embedded in compact nebula (torus?)- no X-ray pulsations observed- column density is > 2 x 10 cm , distance ~5 kpc

P = 61.8 ms; Ý E = 3.3×1037ergs s−1

22 -2


Filamentary Structure in 3C 58

• X-ray emission shows considerable filamentary structure- particularly evident in higher energy X-rays

• Radio structure is remarkably similar, both for filaments and overall size

Slane et al. 2004


• Radial steepening of spectral index shows aging of synchrotron-emitting electrons- consistent with injection from central pulsar

• Modeling of spectral index in expected toroidalfield is unable to reproduce the observed profiles- model profile has much more rapid softening of spectrum(Reynolds 2003)

- diffusive particle transport and mixing may be occurring

Spectral Structure of 3C 58


van der Swaluw 2003

pulsar axis3C 58: Structure of the Inner Nebula

• Central core is extended in N/S direction- suggestive of inner Crab region with structure

from wind termination shock zone

• 65 ms pulsar at center:

• Pressure in radio nebula:

• X-ray spectrum of jet shows no breakfrom synchrotron cooling

• Suggests E-W axis for pulsar- consistent with E-W elongation of 3C 58itself due to pinch effect in toroidal field

Slane et al. 2002

pulsar

jet

torus

145

37 s ergs 106.2 −×= IE&

-22.3

10 cmdyn 102.3 dPneb−×=

Rs = Ý E /4πcPneb ≈ 0.2 pc ⇔13 arcsec

12 arcsec

s 103.5v/ 2/311 −×≈<=∴ Gsynchflowflow Btlt μ

• For minimum energy field, the jetlength gives a flow speed of

c 10.0v ≥flow


PWN Jet/Torus Structure

Komissarov & Lyubarsky 2003

pulsar

jet

torus

spin axis

• Poynting flux from outside pulsar light cylinder is concentrated in equatorialregion due to wound-up B-field- termination shock radius decreases withincreasing angle from equator

• For sufficiently high magnetization parameter (σ ~ 0.01), magnetic stressescan divert particle flow back inward- collimation into jets may occur- asymmetric brightness profile from Dopplerbeaming

• Collimation is subject to kink instabilities- magnetic loops can be torn off near TS andexpand into PWN (Begelman 1998)

- many pulsar jets are kinked or unstable,supporting this picture


Inner Structure in PWNe: Jets

Crab Nebula (Weisskopf et al 2000)

40” = 0.4 pc

PSR B1509-58 (Gaensler et al 2002)

4’ = 6 pc

Vela PWN (Pavlov et al 2003)

Kommisarov & Lyubarsky (2003)

• Collimated features- some curved at ends – why?

• Wide range in brightness andsize (0.01–6 pc)- how much energy input?

• Perpendicular to inner ring- directed along spin axis?

• Relativistic flows:- motion, spectral analysis

give v/c ~ 0.3–0.6

• Primarily one-sided - Doppler boosting?

• Magnetic collimation / hoop stress?


HESS Detection of PSR B1509-58

4’ = 6 pc

10 arcmin


Reverse-Shock/PWN Interaction

• As PWN sweeps up material, reverse shock forms in ejecta

van der Swaluw, Downes, & Keegan 2003



• As PWN sweeps up material, reverse shock forms in ejecta- this propagates back to the SNR center and eventually interacts w/ PWN





• PWN can be disrupted, then re-form





• PWN can be disrupted, then re-form- eventually bow-shock can form from supersonic pulsar motion



G292.0+1.8: Sort of Shocking…


G292.0+1.8: Sort of Shocking…

• Oxygen and Neon abundances seen in ejectaare enhanced above levels expected; verylittle iron observed (Park et al. 2003)- reverse shock appears to still be progressing

toward center; not all material synthesized incenter of star has been shocked

- pressure in PWN is higher than in ejecta aswell reverse shock hasn’t reached PWN

• Pulsar is offset from geometric center of SNR- age and offset can give velocity estimate

• Emission from around pulsar is also elongated- if interpreted as jet, this could imply kick velocity

is (somewhat) aligned with rotation axis


A Disrupted PWN in G327.1-1.1?

• Radio image shows shell with bright PWN in center- distinct “radio finger” observed in PWN

• Extended X-ray emission observed in central region- ROSAT image shows X-ray emission trails away from central plerion- faint compact X-ray source located at tip of radio finger (Slane et al. 1999)

Slane et al. 1999Whiteoak & Green 1996


A Disrupted PWN in G327.1-1.1?

• Radio image shows shell with bright PWN in center- distinct “radio finger” observed in PWN

• Extended X-ray emission observed in central region- ROSAT image shows X-ray emission trails away from central plerion- faint compact X-ray source located at tip of radio finger (Slane et al. 1999)

• Chandra observation shows compact source w/ trail of emission- has PWN been disrupted by reverse shock?

Slane et al. 2004


• “Stand-off distance” of shock set by ram pressure balance:

• Analytic solution for shape of bow shock: (Wilkin 1996)

• Forward (“bow”) and reverse (“termination”) shocks, separated by contact discontinuity

• Termination shock elongated by ratio of ~ 5:1

Bow Shocks: Theory

224

VPcr

Eram

w

ρπ

==

•

)tan/1(3sin/)( θθθθ −= wrr

ambientISM

unshocked pulsar wind

shocked pulsar wind

shocked ISM

bow shock

contactdiscontinuity

terminationshock

Bucciantini (2002)

}

From B. Gaensler


• “Stand-off distance” of shock set by ram pressure balance:

• Analytic solution for shape of bow shock: (Wilkin 1996)

• Forward (“bow”) and reverse (“termination”) shocks, separated by contact discontinuity

• Termination shock elongated by ratio of ~ 5:1

Bow Shocks: Observation

224

VPcr

Eram

w

ρπ

==

•

)tan/1(3sin/)( θθθθ −= wrr

From B. Gaensler

PSR J1747-2958 / “the Mouse” (Gaensler et al 2003)

30”

X-rays Radio

}

2D hydro simulation (van der Swaluw & Gaensler 2004)

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Pulsar Wind Nebulae - Stellenbosch Universityacademic.sun.ac.za/summerschool/2006/documents/lecture...Patrick Slane Harvard-Smithsonian Center for Astrophysics Pulsar Wind Nebulae