Pulsar Acceleration:Pulsar Acceleration:The Chicken or the Egg?The Chicken or the Egg?

Alice Harding NASA Goddard Space Flight Center

Compton Gamma-Ray Observatory Compton Gamma-Ray Observatory (CGRO)(CGRO)

Compton Gamma-Ray Observatory Compton Gamma-Ray Observatory (CGRO)(CGRO)

• 7 (+3) gamma-ray pulsars detected

Force-free magnetosphereForce-free magnetosphereForce-free magnetosphereForce-free magnetosphereGoldreich & Julian 1969

• In vacuum E|| >> Fgrav at NS surface

• Vacuum conditions (Deutsch 1955) cannot exist!

• If charge supply creates force-free conditions,

• Goldreich-Julian charge density

• Corotating dipole field

• NO particle acceleration

v BE

c

4 2GJ

E B

c

Possible sites of particle Possible sites of particle accelerationacceleration

Possible sites of particle Possible sites of particle accelerationacceleration

slot gap

Ideal MHD in most of

magnetosphere

Ideal MHD in most of

magnetosphere0E B

Deficient charge supply

acceleration

Deficient charge supply

acceleration

0E B

Solve Poisson’s Eqn

Solve Poisson’s Eqn

|| 4 ( )GJE

Accelerators and global modelsAccelerators and global modelsAccelerators and global modelsAccelerators and global models

Inclination

angle

Observer angle

AcceleratAccelerator gapsor gaps

ChargeCharges (es (e++ee--))

Global B-Global B-field field structurestructure

Global Global currentcurrentss

Polar cap acceleratorsPolar cap acceleratorsPolar cap acceleratorsPolar cap accelerators

B 0 B 0

SPACE CHARGE "GAP"

T Ts e i ,

VACUUM GAP

T Ts e i ,

e-

NS SURFACE

e-

e-

NS SURFACE

i

e-

e+

e-

e+

e+

e-

0 GJ

4 ( )GJE 4 GJE

(0) 0

e+

NS SURFACE

E|| 0

E|| 0

e+

e-

E|| 0

Polar Cap Pair Formation Front Polar Cap Pair Formation Front (SCLF)(SCLF)

Polar Cap Pair Formation Front Polar Cap Pair Formation Front (SCLF)(SCLF)

Closed field region

• Curvature radiation pair front

complete screening• Inverse Compton scattering pair front

incomplete screening

Slot gap modelSlot gap model• Pair-free zone

near last open field-line

(Arons 1983, Muslimov &

Harding 2003, 2004) Slower accelerationPair formation front at

higher altitudeSlot gap forms

between conducting walls

• E|| acceleration is not screened || 0E

Harding & Muslimov 2002Polar Cap Pair Death lines

SLOT GAPS NO SLOT

GAPS

Lense-Thirring effect Lense-Thirring effect 2 3

2 LG

c r

Accelerating electric field Accelerating electric field Accelerating electric field Accelerating electric field

Near polar cap, inertial frame-dragging!Near polar cap, inertial frame-dragging!Muslimov & Tsygan 1992

Daugherty & Harding 1982

Zhang & Harding 2000

Sturner & Dermer 1994

Hibschmann & Arons 2001

e (1-10 TeV)

CRCR< 50 GeV< 50 GeV

SYN

ICS

e±

X(surface)

X(surface)

ICS

SYNe±

e±

e±

e±

e±

e (0.05-500 GeV)

+ B e

Polar cap pair cascadesPolar cap pair cascades

-6 -3 30 6

Log Energy (MeV)

SR

kT

CRICS

Magnetic pair productionThreshold th = mc2/sinSpectral attenuation is “super-exponential”

Magnetic pair productionThreshold th = mc2/sinSpectral attenuation is “super-exponential”1

1

( ) exp( )

8( )exp

3 'sin

af A

C BB

Mp ~ 102 - 105

Mp < 10

Pair production spectral cutoffPair production spectral cutoff

Measuring spectral cutoffsMeasuring spectral cutoffsMeasuring spectral cutoffsMeasuring spectral cutoffs

Is there a real EIs there a real ECC vs. B vs. B00 trend?trend?

Is there a real EIs there a real ECC vs. B vs. B00 trend?trend?

Super-exponential Super-exponential (PC) or exponential (PC) or exponential

cutoff (OG) ?cutoff (OG) ?

Super-exponential Super-exponential (PC) or exponential (PC) or exponential

cutoff (OG) ?cutoff (OG) ?

B

closed fieldregion

Polar cap model - low-altitude slot gapPolar cap model - low-altitude slot gapPolar cap model - low-altitude slot gapPolar cap model - low-altitude slot gapDaugherty & Harding 1996

Measure off-pulse Measure off-pulse emissionemission

Caustic emissionCaustic emission Morini 1983

• Particles radiate along last open field line from Particles radiate along last open field line from polar cap to light cylinderpolar cap to light cylinder

• Time-of-flight, aberration and phase delay cancel Time-of-flight, aberration and phase delay cancel on trailing edge emission from many altitudes on trailing edge emission from many altitudes arrive in phase arrive in phase causticcaustic peaks in light curve peaks in light curve

Caustic emissionCaustic emission• Dipole magnetic fieldDipole magnetic field• Outer edge of open Outer edge of open

volumevolume

Emission on trailing field Emission on trailing field lineslines

• Bunches in phaseBunches in phase• Arrives at inertial Arrives at inertial

observer observer simultaneouslysimultaneously

Emission on leading field Emission on leading field lineslines

• Spreads out in phaseSpreads out in phase• Arrives at inertial Arrives at inertial

observer at different observer at different timestimes

Formation of causticsFormation of caustics

Slot gap and outer Slot gap and outer gap geometrygap geometry

Slot gap and outer Slot gap and outer gap geometrygap geometry

Vela

Dyks & Rudak 2003Dyks, Harding & Rudak 2004

B

closed fieldregion

Slot Slot gapgapSlot Slot gapgap

Vela

B

closed fieldregion

Slot gap and outer Slot gap and outer gap geometrygap geometry

Slot gap and outer Slot gap and outer gap geometrygap geometry

Cheng, Ruderman & Zhang 2000Dyks, Harding & Rudak 2004

No off pulse emission in traditional OG model

outer gapouter gapouter gapouter gap

(New) Outer gap model(New) Outer gap model

Hirotani 2006, Takata et al. 2006

Outer gap exists below the null surface

visible emission from both poles

More like extended slot gap!

Improved profile for Crab

Slot gap particle acceleration and radiationSlot gap particle acceleration and radiationSlot gap particle acceleration and radiationSlot gap particle acceleration and radiation

Resonant absorption of radio photons when

(1 cos )B R

R

primary e-

e+e- pairs

Crab pulsar Model profilesCrab pulsar Model profilesCrab pulsar Model profilesCrab pulsar Model profiles

X-rays from pairs

-rays from primaries

Radio cone emission

Ob

serv

er

An

gle

Phase

= 450, = 1000

Harding et al. 2008

Harding et al. 2008

Phase-averaged spectrumPhase-averaged spectrumPhase-averaged spectrumPhase-averaged spectrum

Primary CR

Primary SR

Primary ICS

Pair SR

Simple exponential cutoff of CR spectrum

Correlations with radio variability only below 200 MeV

Kuiper et al. 2000

GLAST

Harding et al. 2008

Global modelsGlobal modelsGlobal modelsGlobal models

Spitkovsky 2008

Contopoulos, Kazanas & Fendt 1999

Force-free electrodynamics:

everywhere

No accelerator gaps!

0E B

= 600

= 00

Global currents Global currents Global currents Global currents

Timokhin 2006

Timokhin 2007

Global current solutions

Pair cascade (assumed) current

They don’t match!

Toward a self-consistent magnetosphereToward a self-consistent magnetosphereToward a self-consistent magnetosphereToward a self-consistent magnetosphere

• Allow component of in global model

• Input global model currents as BC to acceleration model (i.e. Poisson’s Eqn)

• Do pair cascades generate enough multiplicity?

• If not, unscreened E|| generates new global field structure

• Check output profiles, spectra with 3D radiation model

0E B

Pulsars detected by CGROPulsars detected by CGROPulsars detected by CGROPulsars detected by CGROPrinceton Pulsar Catalog

c. 1995 Only the youngest and/or nearest pulsars were detectable

More pulsars detectable with AGILE and More pulsars detectable with AGILE and GLASTGLAST

More pulsars detectable with AGILE and More pulsars detectable with AGILE and GLASTGLAST

ATNF catalogc. 2007

~53 radio pulsars in error circles of EGRET unidentified sources (18-20 plausible counterparts)

AGILE will discover new -ray pulsars associated with EGRET sources

GLAST will detect sources 25 times fainter or 5 times further away – possibly 50 – 200 new -ray pulsars

Will be able to detect -ray pulsars further than the distance to the Galactic Center

Middle-aged and older pulsars, including millisecond pulsars should be detected in -rays

AGILEAGILE

GLASTGLAST

Better profiles measured with GLASTBetter profiles measured with GLASTBetter profiles measured with GLASTBetter profiles measured with GLAST

PSR B1055-52

Courtesy D. Thompson

• With larger numbers of photons detected for each pulsar, much sharper and well-defined pulse profiles will be measured by LAT.

• How are the pulse shapes, peak separation, and relationship to pulses seen at other wavelengths explained in different models?

• Is the emission away from the pulse associated with the pulsar (as predicted by the polar cap and slot gap) or not (predicted by outer gap)?

2 year

Predicted GLAST pulsar Predicted GLAST pulsar populationspopulations

Predicted GLAST pulsar Predicted GLAST pulsar populationspopulations

Normal pulsars Millisecond pulsars

Radio-loud

Radio-quiet

Radio-loud

Radio-quiet

Low Altitude Slot gap

84 41 12 37 (6)

High Altitude Slot gap 4 28

Outer gap178

258740

Few radio-loud pulsars for high-altitude Few radio-loud pulsars for high-altitude acceleratorsaccelerators

Gonthier et al. 2007Jiang & Zhang 2006Story et al. 2007

Gonthier et al. 2007Jiang & Zhang 2006Story et al. 2007

(20)

( ) – bright enough for GLAST blind pulsation search

SummarySummarySummarySummary

• Exciting future for -ray pulsar astrophysics• AGILE will detect pulsars coin. with unID EGRET

sources• GLAST will possibly detect 50 – 100 radio loud,

including ms pulsars – many radio-quiet

• Population trends: L vs. LSD, Spectral index vs. age

• Ratio of radio-loud/radio-quiet pulsars discriminates between high and low altitude accelerators

• Better definition of pulse profiles • Spectral components and cutoffs• Phase-resolved spectroscopy of more sources• Improved sensitivity above 10 GeV

May finally understand pulsar physics!