Pulsar Acceleration: The Chicken or the Egg?

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Pulsar AccelerationThe Chicken or the Egg?
Alice Harding
NASA Goddard Space Flight Center
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(No Transcript)
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Compton Gamma-Ray Observatory (CGRO)

• 7 (3) gamma-ray pulsars detected

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Force-free magnetosphere
Goldreich Julian 1969

• In vacuum E gtgt 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

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Possible sites of particle acceleration
Ideal MHD in most of magnetosphere
Deficient charge supply acceleration
Solve Poissons Eqn
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Accelerators and global models
Accelerator gaps
Global currents
Charges (ee-)
Global B-field structure
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Polar cap accelerators
e
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Polar Cap Pair Formation Front (SCLF)
e-

• Curvature radiation pair front
• complete screening

e

• Inverse Compton scattering pair front
• incomplete screening

Closed field region
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Slot gap model

• Pair-free zone near last open field-line
• (Arons 1983, Muslimov Harding 2003,
  2004)
• Slower acceleration
• Pair formation front at higher altitude
• Slot gap forms between conducting walls
• E acceleration is not screened

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Polar Cap Pair Death lines
Harding Muslimov 2002
SLOT GAPS
NO SLOT GAPS
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Lense-Thirring effect
Accelerating electric field
Near polar cap, inertial frame-dragging!
Muslimov Tsygan 1992
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Polar cap pair cascades
Magnetic pair production Threshold eth
mc2/sinq Spectral attenuation is
super-exponential
SR
CR
ICS
kT
Mp 102 - 105
Mp lt 10

• Daugherty Harding 1982
• Zhang Harding 2000
• Sturner Dermer 1994
• Hibschmann Arons 2001

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Pair production spectral cutoff
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Measuring spectral cutoffs
Super-exponential (PC) or exponential cutoff (OG)
?
Is there a real EC vs. B0 trend?
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Polar cap model - low-altitude slot gap
Daugherty Harding 1996
Measure off-pulse emission
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Caustic emission
Morini 1983

• Particles radiate along last open field line from
  polar cap to light cylinder
• Time-of-flight, aberration and phase delay cancel
  on trailing edge emission from many
  altitudes arrive in phase caustic peaks
  in light curve

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Formation of caustics

• Emission on trailing field lines
• Bunches in phase
• Arrives at inertial observer simultaneously

• Emission on leading field lines
• Spreads out in phase
• Arrives at inertial observer at different times

• Caustic emission
• Dipole magnetic field
• Outer edge of open volume

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Slot gap and outer gap geometry
Slot gap
Dyks Rudak 2003 Dyks, Harding Rudak 2004
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Slot gap and outer gap geometry
outer gap
Cheng, Ruderman Zhang 2000 Dyks, Harding
Rudak 2004
No off pulse emission in traditional OG model
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(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
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Slot gap particle acceleration and radiation
W
Resonant absorption of radio photons when
ee- pairs
primary e-
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Crab pulsar Model profiles
a 450, z 1000
Harding et al. 2008
X-rays from pairs
g-rays from primaries
Observer Angle z
Radio cone emission
Harding et al. 2008
Phase
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Phase-averaged spectrum
Harding et al. 2008
GLAST
Correlations with radio variability only below
200 MeV
Primary CR
Pair SR
Primary SR
Simple exponential cutoff of CR spectrum
Kuiper et al. 2000
Primary ICS
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Global models
Contopoulos, Kazanas Fendt 1999
Force-free electrodynamics
everywhere No accelerator gaps!
a 00
Spitkovsky 2008
a 600
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Global currents
Timokhin 2007
Pair cascade (assumed) current
Global current solutions
They dont match!
Timokhin 2006
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Toward a self-consistent magnetosphere

• Allow component of in global
  model
• Input global model currents as BC to acceleration
  model (i.e. Poissons Eqn)
• Do pair cascades generate enough multiplicity?
• If not, unscreened E generates new global field
  structure
• Check output profiles, spectra with 3D radiation
  model

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Pulsars detected by CGRO
Princeton Pulsar Catalogc. 1995

• Only the youngest and/or nearest pulsars were
  detectable

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More pulsars detectable with AGILE and GLAST
ATNF catalogc. 2007

• 53 radio pulsars in error circles of EGRET
  unidentified sources (18-20 plausible
  counterparts)
• AGILE will discover new g-ray pulsars associated
  with EGRET sources
• GLAST will detect sources 25 times fainter or 5
  times further away possibly 50 200 new g-ray
  pulsars
• Will be able to detect g-ray pulsars further
  than the distance to the Galactic Center
• Middle-aged and older pulsars, including
  millisecond pulsars should be detected in g-rays

GLAST
AGILE
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Better profiles measured with GLAST
PSR B1055-52

• 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
Courtesy D. Thompson
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Predicted GLAST pulsar populations
Normal pulsars Normal pulsars Millisecond 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 gap 1 78 258 740
(20)
Gonthier et al. 2007 Jiang Zhang 2006 Story et
al. 2007
Few radio-loud pulsars for high-altitude
accelerators
( ) bright enough for GLAST blind pulsation
search
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Summary

• Exciting future for g-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 Lg 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!