Pulsar Acceleration: The Chicken or the Egg?

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

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

4. Force-free magnetosphere Goldreich & 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

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

6. Observer angle z Inclination angle a Accelerators and global models Accelerator gaps Global currents Charges (e+e-) Global B-field structure

7. Polar cap accelerators e+

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

9. 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

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

11. Accelerating electric field Lense-Thirring effect Near polar cap, inertial frame-dragging! Muslimov & Tsygan 1992

12. Daugherty & Harding 1982 Zhang & Harding 2000 Sturner & Dermer 1994 Hibschmann & Arons 2001 e± SYN e± e± SYN e± e± e± ICS g+ B  e X(surface) ICS CR < 50 GeV X(surface) e (1-10 TeV) e (0.05-500 GeV) 3 6 -6 -3 0 Log Energy (MeV) Polar cap pair cascades Magnetic pair production Threshold eth = mc2/sinq Spectral attenuation is “super-exponential” SR CR ICS kT Mp < 10 Mp ~ 102 - 105

13. Pair production spectral cutoff

14. Measuring spectral cutoffs Super-exponential (PC) or exponential cutoff (OG) ? Is there a real EC vs. B0 trend?

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

16. Morini 1983 Caustic emission • 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

17. 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

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

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

20. (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

21. Slot gap particle acceleration and radiation W Resonant absorption of radio photons when e+e-pairs primary e-

22. a = 450, z = 1000 Harding et al. 2008 Crab pulsar Model profiles X-rays from pairs g-rays from primaries Observer Angle z Radio cone emission Harding et al. 2008 Phase

23. 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

24. Contopoulos, Kazanas & Fendt 1999 Global models Force-free electrodynamics: everywhere No accelerator gaps! a = 00 Spitkovsky 2008 a = 600

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

26. Toward a self-consistent magnetosphere • Allow component ofin 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

27. Pulsars detected by CGRO Princeton Pulsar Catalogc. 1995 • Only the youngest and/or nearest pulsars were detectable

28. 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

29. 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

30. Predicted GLAST pulsar populations (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

31. 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!