Pulsar Acceleration: The Chicken or the Egg? Alice Harding NASA Goddard Space Flight Center

Compton Gamma-Ray Observatory (CGRO) 7 (+3) gamma-ray pulsars detected

Force-free magnetosphere Goldreich & Julian 1969 In vacuum E || >> F grav 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

Possible sites of particle acceleration slot gap Ideal MHD in most of magnetosphere Deficient charge supply acceleration Deficient charge supply acceleration Solve Poisson’s Eqn

Accelerators and global models   Inclination angle Observer angle Accelerator gaps Charges (e + e - ) Global B-field structure Global currents

Polar cap accelerators e+e+

e+e+ e-e- Polar Cap Pair Formation Front (SCLF) Closed field region Curvature radiation pair front complete screening Inverse Compton scattering pair front incomplete screening

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

Harding & Muslimov 2002 Polar Cap Pair Death lines SLOT GAPS NO SLOT GAPS

Lense-Thirring effect Accelerating electric field 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) CR < 50 GeV SYN ICS e±e± X (surface) ICS SYN e±e± e±e± e±e± e±e± e±e± e ( GeV)  + B  e  Polar cap pair cascades Log Energy (MeV) SR kT CR ICS Magnetic pair production Threshold  th = mc 2 /sin  Spectral attenuation is “super-exponential” Magnetic pair production Threshold  th = mc 2 /sin  Spectral attenuation is “super-exponential” M p ~ M p < 10

Pair production spectral cutoff

Measuring spectral cutoffs Is there a real E C vs. B 0 trend? Super-exponential (PC) or exponential cutoff (OG) ?

B closed field region Polar cap model - low-altitude slot gap Daugherty & Harding 1996 Measure off-pulse emission

Caustic emission Morini 1983 Particles radiate along last open field line from polar cap to light cylinderParticles 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 curveTime-of-flight, aberration and phase delay cancel on trailing edge emission from many altitudes arrive in phase caustic peaks in light curve

Caustic emission Dipole magnetic fieldDipole magnetic field Outer edge of open volumeOuter edge of open volume Emission on trailing field lines Bunches in phaseBunches in phase Arrives at inertial observer simultaneouslyArrives at inertial observer simultaneously Emission on leading field lines Spreads out in phaseSpreads out in phase Arrives at inertial observer at different timesArrives at inertial observer at different times Formation of caustics

Slot gap and outer gap geometry Vela Dyks & Rudak 2003 Dyks, Harding & Rudak 2004 B closed field region Slot gap

Vela B closed field region Slot gap and outer gap geometry Cheng, Ruderman & Zhang 2000 Dyks, Harding & Rudak 2004 No off pulse emission in traditional OG model outer gap

(New) Outer gap model Hirotani 2006, Takata et al 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 radiation Resonant absorption of radio photons when  primary e - e + e - pairs

Crab pulsar Model profiles X-rays from pairs  -rays from primaries Radio cone emission Observer Angle  Phase  = 45 0,  = Harding et al. 2008

Phase-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 GLAST Harding et al. 2008

Global models Spitkovsky 2008 Contopoulos, Kazanas & Fendt 1999 Force-free electrodynamics: everywhere No accelerator gaps!  = 60 0  = 0 0

Global currents Timokhin 2006 Timokhin 2007 Global current solutions Pair cascade (assumed) current They don’t match!

Toward 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

Pulsars detected by CGRO Princeton Pulsar Catalog c  Only the youngest and/or nearest pulsars were detectable

More pulsars detectable with AGILE and GLAST ATNF catalog c  ~53 radio pulsars in error circles of EGRET unidentified sources ( 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 AGILE GLAST

Better profiles measured with GLAST PSR B 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 populations Normal pulsarsMillisecond pulsars Radio-loudRadio-quietRadio-loudRadio-quiet Low Altitude Slot gap (6) High Altitude Slot gap 428 Outer gap Few radio-loud pulsars for high-altitude accelerators Gonthier et al Jiang & Zhang 2006 Story et al Gonthier et al Jiang & Zhang 2006 Story et al (20) ( ) – bright enough for GLAST blind pulsation search

SummarySummary 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. L SD, 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!