Pulsar Wind Nebulae - Stellenbosch...
Transcript of Pulsar Wind Nebulae - Stellenbosch...
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
Pulsar Wind Nebulae
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
http://www.astroscu.unam.mx/neutrones/home.html
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
• 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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
• 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
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Harvard-Smithsonian Center for AstrophysicsPatrick Slane
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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
The Crab Nebula
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
• 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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
The Crab Nebula in X-rays
• Fine structure observed in Chandraimage- poloidal loop structures surround torus
as well (seen w/ HST too)
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
• 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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
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?
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
HESS Detection of PSR B1509-58
4’ = 6 pc
10 arcmin
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
Reverse-Shock/PWN Interaction
• As PWN sweeps up material, reverse shock forms in ejecta
van der Swaluw, Downes, & Keegan 2003
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
Reverse-Shock/PWN Interaction
• As PWN sweeps up material, reverse shock forms in ejecta- this propagates back to the SNR center and eventually interacts w/ PWN
van der Swaluw, Downes, & Keegan 2003
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
Reverse-Shock/PWN Interaction
• 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
van der Swaluw, Downes, & Keegan 2003
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
Reverse-Shock/PWN Interaction
• 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- eventually bow-shock can form from supersonic pulsar motion
van der Swaluw, Downes, & Keegan 2003
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
G292.0+1.8: Sort of Shocking…
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
• “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
Harvard-Smithsonian Center for AstrophysicsPatrick Slane
• “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)