Pulsar Wind Nebulae

COSPAR 2008, Montreal, 18 July Patrick Slane (CfA)

Pulsar Wind Nebulae

Observations of


Jet/Torus Structure in PWNe

Lyubarsky 2002

• Anisotropic flux with maximum energy flux in equatorial zone - radial particle outflow - striped wind from Poynting flux decreases away from equator

• Wind termination shock is farther from pulsar at equator than along axis

• Magnetization is low in equatorial region due to dissipation in striped wind (reconnection?) - no collimation along equator; an equatorial disk (i.e. torus) forms

• At higher latitudes, average B field is a maximum - this can turn the flow inward at high latitudes, collimating flow and forming a jet beyond TS, where flow is mildly (or non-) relativistic

€

F ≈Ω2ψ 0

2

4πc 2R2sin2 θ +

1

σ 0

⎛

⎝ ⎜

⎞

⎠ ⎟


Pulsar Wind Nebulae• Expansion boundary condition at forces wind termination shock at - wind goes from v = c/31/2 inside Rw to v ~ RN/t at outer boundary

wR

NR

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

BlastWave

H or ejecta shell

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

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

Slane et al. 2004

bPulsar

WindMHD Shock

Particle Flow

logarithmicradial scale wR

} ++ + + +

R

F

€

˙ E = ˙ E 0 1+t

τ 0

⎡

⎣ ⎢

⎤

⎦ ⎥

−n+1

n−1

€

Rw =˙ E

4πcPN

⎡

⎣ ⎢

⎤

⎦ ⎥

1/2

€

br ≈ 1021BμG−3 t3

-2 Hz


Broadband Emission from PWNe• Spin-down power is injected into the PWN at a time-dependent rate

• Based on studies of Crab Nebula, there appear to be two populations – relic radio-emitting electrons and electrons injected in wind (Atoyan & Aharonian 1996)

• Get associated synchrotron and IC emission from electron population, and some assumed B field (e.g. Venter & dE Jager 2006)

Zhang et al. 2008


Broadband Emission from PWNeDel Zanna et al. 2006

Volpi et al. 2008

• More realistically, assume wind injected at termination shock, with radial particle distribution and latitude-dependent magnetic component:

• Evolve nebula considering radiative and adiabatic losses to obtain time- and spatially-dependent electron spectrum and B field (e.g. Volpi et al. 2008) - integrate over synchrotron and IC emissivity to get spectrum

€

= B2

4πργ 2c2 =σ (θ )


Connecting the Synchrotron and IC Emission

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εkeVs ≈ 2 ×10−4 ETeV

2 B−5

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εTeVic ≈ 3×10−3 ETeV

2

€

εkeVs ≈ 0.06ε TeV

ic B−5

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fic(ε ic )fs (ε s )

=UB

UCMB

≈ 0.1B−5−2

• Energetic electrons in PWNe produce both synchrotron and inverse-Compton emission - for electrons with energy ETeV,

synchrotron

inverse-Compton

- comparing photon energies from given electron population gives B (e.g. Atoyan & Aharonian 1999)

• Similarly, relative fluxes fE = E2 f(E) = S give B:

• For low B, synchrotron lifetime is long, and fic/fs is large - can expect bright TeV emission from collection of long-lived electrons


A Point About Injection: 3C 58

Slane et al. 2004

• 3C 58 is a bright, young PWN - morphology similar to radio/x-ray; suggests low magnetic field - low-frequency spectral break suggests possible injection break

• PWN and torus region observed in Spitzer/IRAC and CFHT observations - jet structure not seen above diffuse emission


A Point About Injection: 3C 58• 3C 58 is a bright, young PWN - morphology similar to radio/x-ray; suggests low magnetic field - low-frequency spectral break suggests possible injection break


E

Flu

x D

ensi

ty

Injection

Nebula

SynchrotronBreak


Spitzer Observations of 3C 58VLA

IRAC 3.6mChandra

Bietenholz 2006

IRAC 4.5m

Slane et al. 2004 Slane et al. 2008




Spitzer Observations of 3C 58• 3C 58 is a bright, young PWN - morphology similar to radio/x-ray; suggests low magnetic field - low-frequency spectral break suggests possible injection break


• IR flux for entire nebula falls within extrapolation of x-ray spectrum - indicates single break just below IR - sub-mm observations would be of interest • Torus spectrum requires change in slope between IR and x-ray bands - challenges assumptions of single power law for injection into nebula; TeV observations should provide constraints

Slane et al. 2008

Slane et al. 2008


Spitzer Observations of 3C 58

PRELIMINARY



• IR flux for entire nebula falls within extrapolation of x-ray spectrum - indicates single break just below IR - sub-mm observations would be of interest • Torus spectrum requires change in slope between IR and x-ray bands - challenges assumptions of single power law for injection into nebula; TeV observations should provide constraints


Kes 75

• Bright wind nebula powered by PSR J1846-0258 (Edot = 1036.9) - jet-like structure defines rotation axis (Helfand et al. 2003)

• Deep Chandra observation reveals moving clumps, arc-like structure, Crab-like bays, inner/outer jet features, and abrupt jet termination in south (Ng et al. 2008) - best-fit structure to ordered structure yields jet/torus with clump in north - jet spectrum is harder than surrounding regions, suggesting high-velocity flow

Ng et al. 2008


Kes 75

See also poster E11.62: (S. Safi-Harb et al.)

• Spectral index shows general steepening with radius in diffuse nebula • HESS observations reveal VHE -ray emission - Lx/L B ~ 15 G , consistent w/ large X-ray size

• RXTE observations reveal magnetar-like bursts from PSR J1846-0258 (Gavril et al. 2008) - Chandra observation reveal brightening of pulsar as well - also see brightening of northern clump and inner jet (though unrelated to bursts given flow timescales)

Ng et al. 2008

Djannati-Atai et al. 2008


HESS J1640-465

5 arcmin

Lemiere et al. 2008

• Extended source identified in HESS GPS - no known pulsar associated with source - may be associated with SNR G338.3-0.0

• XMM observations (Funk et al. 2007) identify extended X-ray emission, securing an associated X-ray PWN

• Chandra observations (Lemiere et al. 2008) reveal point source within extended nebula, apparently identifying associated neutron star - HI absorption indicates a distance d ~ 8 – 13 kpc - Lx ~ 1033.1 erg s-1 Edot ~ 1036.7 erg s-1

- X-ray and TeV spectrum well-described by leptonic model with B ~ 6 G and t ~ 15 kyr


PWNe and Their SNRs

ISM

Sh

ock

ed ISM

Sh

ock

ed E

ject

a

Unsh

ock

ed

Eje

cta

PW

N

Puls

ar

Win

dForward Shock

Reverse ShockPWN Shock

PulsarTerminationShock

• Pulsar Wind - sweeps up ejecta; shock decelerates flow, accelerates particles; PWN forms

• Supernova Remnant - sweeps up ISM; reverse shock heats ejecta; ultimately compresses PWN; particles accelerated at forward shock generate magnetic turbulence; other particles scatter off this and receive additional acceleration

Gaensler & Slane 2006


• Vela X is the PWN produced by the Vela pulsar - located primarily south of pulsar - apparently the result of relic PWN being disturbed by asymmetric passage of the SNR reverse shock (e.g. Blondin et al. 2001)

• Elongated “cocoon-like” hard X-ray structure extends southward of pulsar - clearly identified by HESS as an extended VHE structure - this is not the pulsar jet (which is known to be directed to NW); presumably the result of reverse shock interaction

Vela X

van der Swaluw, Downes, & Keegan 2003

t = 10,000 yr t = 20,000 yr t = 30,000 yr t = 56,000 yr

Blondin et al. 2001

RS interaction displaces PWN, produces turbulent structures, and mixes in ejecta


Vela X

LaMassa et al. 2008

• XMM spectrum shows nonthermal and ejecta-rich thermal emission from end of cocoon - reverse-shock crushed PWN and mixed-in ejecta?

• Radio, X-ray, and -ray measurements appear consistent with synchrotron and I-C emission from power law particle spectrum w/ two spectral breaks - density derived from thermal emission 10x lower than needed for pion-production to provide observed -ray flux - much larger X-ray coverage of Vela X is required to fully understand structure


Vela X

de Jager et al. 2008

• Radio and VHE spectrum for entire PWN suggests presence of two distinct electron populations - radio-emitting particles may be relic population; higher energy electrons injected by pulsar

• Maximum energy of radio-emitting electrons not well-constrained - this population will generate IC emission in GLAST band; spectral features will identify indentify emission from distinct up-scattered photon populations - upcoming observations will provide strong constraints on this electron population


G327.1-1.1: Another Reverse-Shock InteractionTemim et al. 2008 • G327.1-1.1 is a composite SNR

with a bright central nebula - nebula is offset from SNR center - “finger” of emission extends toward northwest

• X-ray observations reveal compact source at tip of radio finger - trail of emission extends into nebula - Lx suggests Edot ~ 1037.3 erg s-1

See poster E11.57: Chandra and XMM Observations of the Composite SNR G327.1-1.1 (Tea Temim et al.)


Conclusions• PWNe are reservoirs of energetic particles injected from pulsar - morphology of nebulae reveals underlying geometry - synchrotron and inverse-Compton emission places strong constraints on the underlying particle spectrum and magnetic field

• Modeling of broadband emission constrains evolution of particles and B field - modeling form of injection spectrum and full evolution of particles still in its infancy

• Reverse-shock interactions between SNR and PWNe distort nebula and may explain TeV sources offset from pulsars - multiwavelength observations needed to secure this scenario (e.g. Vela X. HESS J1825-137, and others)

• Low-field, old PWNe may fade from X-ray view, but still be detectable sources of TeV emission - VHE -ray surveys are likely to continue uncovering new members of this class

Pulsar Wind Nebulae

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