03/19/2007N. Bucciantini: Hubble Symposium 2007 1 Relativistic Outflows from Compact Rotators:...

03/19/2007 N. Bucciantini: Hubble Symposium 2007 1

Relativistic Outflows from Compact Rotators: Pulsar Winds, Pulsar-Wind

Nebulae, and GRBs environment

Niccolo’ BucciantiniAstronomy Department UC Berkeley

http://www.astro.berkeley.edu

In collaboration with: Jonathan Arons, Eliot Quataert, Todd A. Thompson, Luca Del Zanna, Elena Amato, Delia Volpi, Brian Metzger


Outline

• Introduction to pulsar winds• Acceleration of relativistic winds• Winds from proto-magnetar. Proto-magnetars as engine for

GRBs.• Interaction of the wind with the environment• PWNe - MHD models and what we can learn from

observations.• GRBs as scaled version of PWNe. Reviving the magnetar

scenario.

03/19/2007 N. Bucciantini: Hubble Symposium 2007

The NS magnetosphere

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RNS =10kmP = 0.001− 7s

Bsurf =108−12G

Acceleration of particle pair plasma- ions from the surface:

Initial Lorentz factor ~ 100Cold wind (Sync. losses)

Pair creations - change density exceeding the GJ valueMHD regime (ideal?)

Unipolar inductor (AGN, GRB, Magnetar)EM extraction of rotational energy.


At best we require a 2D axisymmeric RMHD “Steady At best we require a 2D axisymmeric RMHD “Steady state”state”

No exact solution even for the simple monopole-like case

Conversion of magnetic energy is logarithmic after the fast p.No collimation for a relativistic wind

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γ∞ ~ σ 1/ 3

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σ∞ ~ σ 2 / 3

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γ>>1⇒ ρ q E + j × B ≈ 0

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σ ≈Φ2Ω / ˙ M

Terminal γ = σ1/3


MHD simulations (split monopole)

Pressureless RMHD trans alfvenic solution (Bogovalov 2001)Full GRMHD simulations, subslow injection (Bucciantini et al.)

γ=2

γ=7

Magnetic surface close to monopole shape for Magnetic surface close to monopole shape for γγ > 10 > 10


The force-free dipole

Gruzinov 05ff - steady state

Spitkovsky 05

Closed zone extends to the LCThe wind in the far region is monopolarNo evidence for collimation


Proto-magnetars and GRBs

Proto-Magnetar model - initial neutrino driven wind (Thompson et al. 2003)

Rotating magnetar ( ~0.2c)Non cold atmosphere (Cs ~ 0.1c, H ~ 0.1 RNS)Surface Density ~ 1013 g/cm3

Surface Magnetic field B ~ 1014-16 G

Pulsars Proto-MagnetarsPulsar winds are almost force free Conditions changes in few seconds from mass loaded

to magnetic dominated

Particles are injected with high Lorentz factor Injection is subsonic and the flow velocity at the base of the star is much smaller than c

Low density in the wind might lead to charge starvation and non ideal MHD

High particle density can guarantee MHD currents


Validity of the Pulsar analogy

Can a magnetar trigger a GRB (efficient acceleration)Does mass flux depend on the value of B (i.e. acceleration)How energy and angular momentum losses scale with B2/r

Can the magnetar wind inflate a bubble of magnetic field and relativistic particles inside the progenitor?

Can we use what we know about pulsar winds for proto-magnetars?

How good is the force-free regimeHow collimation and acceleration depend on

What about the interaction with the environment?


2D Monopole

Uniform radial magnetic field, density and pressureAt the star boundary Vr, Bq, Bf, are extrapolated

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Vθ = Vr ⋅Bθ /Br

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Vφ = Ωr /α + Vr ⋅Bφ /Br

(B=2 1015G, r=1010g/cm3, cs=0.2c) - 10-5 M¤/s


Energy and angular momentumAngular momentum flux ~ sin2(θ)Energy flux ~ sin2(θ)

Poynting flux dominated outflowPoynting flux dominated outflow

EEmattermatter/E/Eem em ~ 0.1~ 0.1

The ratio of particle The ratio of particle to magnetic energy to magnetic energy ~ A + B sin ~ A + B sin ((θθ))


Dipole

Closed zone (Rc ~ 2 RNS)Equatorial channel (most of the mass, angular mom., and energy flux in the equator)

Collimation is efficient

The far zone resemble a The far zone resemble a monopole - lower monopole - lower equatorial velocityequatorial velocity γ


Far wind region and evolution

Collimation in the far region (109cm) -efficient collimation and energy conversion

isotropic wind and close to equipartition

energy flux close to FF, high magnetization


Magnetars as engine for GRBs

Can a magnetar be the engine producing the GBR jet?

NO

It is not possible in Ideal MHD to achieve both, relativistic speeds and energy collimation (no evidence for sheet-like explosions).

Relativistic speeds require high sigma (later evolution)Collimation requires low sigma What happen when this wind interacts with the progenitor?Can somehow collimation be achieved not because of the source but because of the interaction?


Pulsar Wind Nebulae• PWNe are hot bubbles (plerions)

emitting non-thermal radiation (synchrotron) at all wavelengths: require injection of relativistic particles and magnetic fields

• Originated by the interaction of the ultra-relativistic magnetized pulsar wind with the expanding SNR dense ejecta

• Crab Nebula in optical: central amorphous mass (continuum) + external filaments (lines)

PWN

SNR


Jet-torus structure: Chandra X-ray images

• Crab nebula (Weisskopf et al., 2000; Hester et al., 2002)• Vela pulsar (Helfand et al., 2001; Pavlov et al., 2003)• Other objects: PSR 1509-58, G0.9+01, G54.1+0.3

Crab

Vela


TS structure and flow patternσ=0.003• The wind anisotropy shapes the

TS structure. A complex flow pattern arises:

• A: ultrarelativistic pulsar wind• B: subsonic equatorial outflow• C: supersonic equatorial funnel• D: super-fastmagnetosonic flow• a: termination shock front• b: rim shock• c: fastmagnetosonic surface

03/30/2007 N. Bucciantini: Modeling PWNe

PWN elongation

• Shape of the nebula - magnetic pinching (Begelman & Li, 1992, van der Swaluw 2003, Del Zanna et al 2004) - average magnetization in the wind:

03/30/2007 N. Bucciantini: Modeling PWNe 18

Internal pressure structure

Due to magnetic pinch the pressureis much higher on the axis than at the equator

Hot plasma + toroidal magnetic field

If the high-scale is small then the pressure acts along the axis as for a jet

Begelman & Li 92

03/19/2007 N. Bucciantini: Modeling PWNe 19

Magnetic cavity inside progenitorsWhat happens if a magnetar wind inflates a relativistic bubble inside a progenitor?

Elongation is a strong function of magnetization


Dependence on the field shape• Initial magnetic field with a narrow equatorial neutral sheet

b=10


Features originMain torusInner ring (wisps structure)KnotBack side of the inner ring

No jet - Axisymmetric assumption

Knot

Ring

TorusHester et al. 1995


Striped wind and jet propertiesWhat effect has the striped wind region size on the appearance of the jet?Where the wind magnetization is maximum? Equatorial region or polar?Wind with higher polar magnetization proposed by Arons 1998.

σ=0.025, b=10


Photon Index

Brighter inner ringBrighter inner ring

Mori 02

Higher index in the torusHigher index in the torus(recompression and boosting)

Jet not correctly reproducedJet not correctly reproduced(required reacceleration)


Formation of polar jets by hoop stresses

σ=0.003 σ=0.03σ=0.01

• The global nebular flow changes with σ• For high magnetization (σ > 0.01) a supersonic jet is formed• PSR winds are below equipartition (dissipation non-ideal MHD)• Proto-magnetar wind are more magnetized -> faster jet?


Summary and conclusions

• PSR magnetosphere are prototype of compact accelerators• Difficulty in achieving high Lorentz factor • Collimation of relativistic wind is prevented by El.field• Magnetar might develop late time relativistic winds• Magnetar are unlikely candidate for GRB jet• Many PWNe show a jets (Crab, Vela, …) • Jet collimation forbidden in the wind. Inside PWN.• PWNe are elongated - axial pressure is higher despite equatorial energy

flow.• Magnetars inside progenitor might produce elongated cavities• Elongation might be at the origin of jet-like flows - no need for collimation

at the source Thank you

03/19/2007N. Bucciantini: Hubble Symposium 2007 1 Relativistic Outflows from Compact Rotators:...

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