Pulsar Wind Nebulae

Harvard-Smithsonian Center for AstrophysicsPatrick Slane

Pulsar Wind Nebulae


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


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

E&410−≈

• Magnetic polarity in wind alternates spatially - magnetically “striped” wind - does reconnection result in conversion to kinetic energy? (e.g. Coroniti 1990, Michel 1994, Lyubarsky 2003)

The Pulsar Wind Zone


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

E&410−≈

• Magnetic polarity in wind alternates spatially - magnetically “striped” wind - does reconnection result in conversion to kinetic energy? (e.g. Coroniti 1990, Michel 1994, Lyubarsky 2003)

• Wind expands until ram pressure is balanced by surrounding nebula - flow in outer nebula restricts inner wind flow, forming pulsar wind termination shock

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 boundary

€

v ≈ c / 3

wRNR

wR

€

v ≈ RN / t

• Pulsar accelerates particle wind

- wind inflates bubble of particles and magnetic flux- particle flow in B-field creates synchrotron nebula

bPulsar

WindMHD Shock

Particle Flow

BlastWave

H or ejecta shell

logarithmicradial scale wR

1

1

00 ô

1−+

−

⎥⎦

⎤⎢⎣

⎡+=

nn

tEE &&

}

- spectral break at

where synchrotron lifetime of particles equals SNR age - radial spectral variation from burn-off of high energy particles

Hz ô10í 3324 −−≈ Bbr

• Pulsar wind is confined by pressure in nebula - wind termination shock

obtain by integratingradio spectrum

€

Rs =˙ E

4πcPN

⎡

⎣ ⎢

⎤

⎦ ⎥

1/ 2

• Wind is described by magnetization parameter = ratio of Poynting flux to particle flux in wind

€

≡FE×B

Fparticle

++ + + +

R


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

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

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

• As SNR/PWN ages, reverse shock approaches/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 field result in elongation of nebula along pulsar 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 in detail, but similar results overall - B field shows variations in interior - turbulent flow and cooling could result in additional structure in emission

pu

lsa

r 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; still expanding into cold ejecta?

• Rayleigh-Taylor fingers produced as relativistic 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 – H cont

Filamentary Structure in PWNe: Crab Nebula

• Radio emission shows structure similar to optical line emission - interaction with relativistic fluid in PWN forms 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; still expanding into cold ejecta?

• Rayleigh-Taylor fingers produced as relativistic 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 Chandra image - 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 appears to extend all the way to the neutron star - jet axis aligned with pulsar proper motion; same is true of Vela pulsar

• inner ring of x-ray emission associated with shock wave produced by matter rushing away 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 radio pulsar with the 2nd highest spin-down power has recently been discovered (Camilo et al. 2005) - - ~ 4.8 kyr; true age more likely < 1 kyr

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

€

P = 61.8 ms; ˙ 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 toroidal field 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 break from synchrotron cooling

• Suggests E-W axis for pulsar - consistent with E-W elongation of 3C 58 itself 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 = ˙ E /4πcPneb ≈ 0.2 pc ⇔ 13 arcsec

12 arcsec

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

• For minimum energy field, the jet length 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 equatorial region due to wound-up B-field - termination shock radius decreases with increasing angle from equator

• For sufficiently high magnetization parameter ( ~ 0.01), magnetic stresses can divert particle flow back inward - collimation into jets may occur - asymmetric brightness profile from Doppler beaming

• Collimation is subject to kink instabilities - magnetic loops can be torn off near TS and expand 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 and size (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 ejecta are enhanced above levels expected; very little iron observed (Park et al. 2003) - reverse shock appears to still be progressing toward center; not all material synthesized in center of star has been shocked - pressure in PWN is higher than in ejecta as well 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)

Documents

Pulsar Wind Nebulae