The structure of the pulsar magnetosphere via particle simulation

Shinpei Shibata (1), Shinya Yuki (1), Tohohide Wada (2),Mituhiro Umizaki (1)

(1)Department of Phys. Yamagata University(2)National Astronomical Obvsevatory of Japan

Introduction

Pulsars

Neutron Starabout 1M_sun10km in size

Pulsars:B_d ～ 10^9 – 10^13GP ～ 1.5msec – several seconds

Emf ～ 10^14 Volt rotation powered pulsars

Magnetars: Small subclass of magnetic neutron starsmagnetic active regions with B ～ (maybe)10^15G

Emf ～ 10^14 Volt magnetic powered pulsars

Rotation axis

PulsarWind(relativisticoutflow ofmagnetizedplasmaγ ～ 10^6)

Size of the magnetosphere: c/Ω ～ 4.8×10^4 R_ns

1 ly

Beamed radiationObserved as pulsed radiation

SED(spectral energy density plot)

magnetospheric

Nebula

2. Pulsar Wind Lwind=ηw Lrot

Aharonian, F.A. & Atoyan, A.M., 1998

Unpulsed emission

Pulsed emission

E// + e/p

BB 加熱

Ｅ // 加速

ＩＣ

sync

RL=c/Ω

Rs=(Lwind/4πPext)^1/2

Vacc=RL*BL=μΩ^2/c^2

Vacc=Rs*Bn with Pext=Bn^2/8π

keVGeV

TeV

垂直衝撃波加速の困難

1. High Energy Pulses1. High Energy Pulses3. Radio Pulses

E// (field-aligned acceleration)

Roation × magnetizationmakes emf charge separation

Unipolar Inductor

E ⊥

If pair creation is suppressed, charged particles are emitted from the neutron star and forms steady clouds.

• Particle Simulation for the screening. (ref. Wada and Shibata 2003)

gap

The gap is unstable against pair creation.

1. EMF and charge separation

Unipolar Induction

Basic properties of the pulsar magnetosphere

Motional field

As compared with required charge separation, plasma source is limited gap E//

Goldreich-Julian model (1969)

In reality, plasma is extracted from the stellar surface by E//: maybe, complete charge separation

Positive space charge

Negative space charge

Corotation speed becomes the light speed

Relativistic

centrifugal wind

Goldreich-Julian model (1969)

Strong charge separation in a rotating magnetosphere makes the gap, non-zero E//

Positive space charge

Negative space charge Null c

harge surfa

ce

Gap formation

Ω B

Dead zone

Null面

Light cylinder

Polar cap

Slot gap

Outer gap

Models based on observatons: PC, SG, OG

Closed field(dead zone)

Open field region

Ω B

Dead zone

Null面

Light cylinder

Polar cap

Slot gap

Outer gapClosed field

(dead zone)

Open field region

γ-ray pulse shape and relation to radio pulses are well explained if γ from OG and radio from PC

Two-pole caustic (TPC) geometry (Dyks & Rudak, 2003)

Radio pulse

Models based on observatons: PC, SG, OG

Particle simulation

part

icle

code

―

acceleration

Gamma-ray―

―

radiation from the starStrong B

Particle codeParticle motion and the electromagnetic fields are solved iteratively for the axis-symmetric steady solution.

Emf is included in the BC

For the EM field

For the particle motion

• Gravitational interaction

• For the electric field • For the magnetic field

We use Grape-6, the special purpose computer for astornonomical N-body problem at NAOJ.

- Particles are emitted from the star if there is E// on the surface.

- On the spot approximation: e+/e- are created if E//>Ec

- Particles are removed through the outer boundary: loss by the puslar wind.

The system settles in a steady state when the system charge becomes constant:seteadily pairs are created in the magnetoschpere and lossed as the wind.

Particle creation and loss

Result

Modificaton of the magnetic field

Light cylinder

Outer gap

The outer gaps steadily create pairs.Gap electric fileld is kept above Ec as to create pairs.

Particle distribution and motion Strength of E//

Pair creation

Not so strong: limitation of particle number

Pola

r cap

current circulation seen

Slot

gap

Outer gap

Outward current

Retun current

Current-neutral dead zone

Dead zone

Light cylinder E/B map

E>BBread down of the ideal-MHD cond.

磁気リコネクション

Summary and Discussion1. The outer gap, which is the candidate place of

the particle acceleration and gamma-ray emission, is proven from the first principles by particle simulation.

2. Due to radiation reaction force, some particles escape through the closed field lines.

3. At the top of the dead zone, we find strong E field larger than B, i.e., break down of the ideal-MHD condition, and in addition PIC simulation indicates possibility of reconnection driven by the centrifugal force.

There are two places in which magnetic reconnection may play an important role.-Close-open boundary near the light cylinder-Termination shock of the pulsar wind

Ω

Magnetic axis

Ｔｈｉｃｋ　ｗｉｎｄNeutral sheet

Magnetic reconnection

パルサーオーロラ

Rotation axis

Lig

ht

cylin

der

Outer gap

Polar cap

Slot gap

SED(spectral energy density plot)

magnetospheric

Nebula

2. Pulsar Wind Lwind=ηw Lrot

Aharonian, F.A. & Atoyan, A.M., 1998

Unpulsed emission

Pulsed emission

E// + e/p

BB 加熱

Ｅ // 加速

ＩＣ

sync

RL=c/Ω

Rs=(Lwind/4πPext)^1/2

Vacc=RL*BL=μΩ^2/c^2

Vacc=Rs*Bn with Pext=Bn^2/8π

keVGeV

TeV

垂直衝撃波加速の困難

1. High Energy Pulses1. High Energy Pulses3. Radio Pulses