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The population of pulsars with interpulses and the implications for beam evolution ( astro-ph/0804.4318). Patrick Weltevrede & Simon Johnston. ATNF. Low-Frequency Pulsar Science Leiden 2008. Pulsar timing for GLAST. Timing ~ 160 pulsars with Parkes
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The population of pulsars with interpulses and the implications for beam evolution(astro-ph/0804.4318) Patrick Weltevrede & Simon Johnston ATNF Low-Frequency Pulsar Science Leiden 2008
Pulsar timing for GLAST • Timing ~ 160 pulsars with Parkes • Perfect dataset to study young & energetic pulsars
Standard model for pulsar beams Gould 1994, Rankin 1990, Rankin 1993, Kramer et al. 1994, Gil et al. 1993
Pulse width distribution • Expect W P -1/2 • Large scatter because of unknown geometry • Correlation is flatter (slope is ~ - 0.3) • Same as in the Gould & Lyne (1998) data
Idea: beam evolution The magnetic axis evolves towards alignment with the rotation axis (Tauris & Manchester 1998) Long period pulsar older more aligned beams W P -1/2 (P large, W small) W increasing with P W - P correlation flatter
Idea: consequence for IP If90o, we can see the interpulse Most pulsars with interpulses should be young if there is beam evolution
Observations: interpulses • Literature: 27/1487 slow pulsars have an interpulse (1.8%) IP pulsars • Includes 3 new weak interpulses • Some “interpulses” will be aligned rotators observed fraction is an upper-limit J0905-5127 J1126-6054 J1637-4553 slow pulsars
The model: beam geometry • Pick a random pairs from the pulsar catalogue (slow pulsars) • Calculate beam size: • Pick random birth and a random line of sight (both and + distributions are sinusoidal) • Allow alignment:
The model: elliptical beams • If polar cap is bounded by the last open field lines, the beam could be elliptical • Axial ratio: • Axial ratio between 1 ( = 00) and 0.62 ( = 900) • Model most likely oversimplified, but interesting to investigate consequences • We can force circular beams by setting for all (McKinnon 1993)
Model: detection condition • We can check with the following conditions if the beams intersect the line of sight: • We keep picking new ’s and ’s until at least one beam is detected
No alignment and circular beams • IP fraction: 4.4% (observed: < 1.8%) • There are too many fast IP pulsars • W P -1/2 Model fails
No alignment and elliptical beams • IP fraction: 2.3% (observed: < 1.8%) • There are too many fast IP pulsars • W P -1/2 Model fails
Alignment of the magnetic axis • IP fraction 1.8% (for align = 70 Myr) • P distribution fits • W P -0.4 • Elliptical beams: - align = 2 Gyr - P distribution no longer fits data
Implications of alignment Orthogonal (young) • Beaming fraction = fraction of the celestial sphere illuminated by the pulsar = probability to see the pulsar • Older pulsars are less likely to be found in a pulsar survey • Average beaming fraction is 8% instead of 17% inferred total population of pulsars is 2x larger Aligned (old)
Implications for spin-down • Braking torque can change • Braking torque depends on • Characteristic age, B, Edot etc. is a function of • Vacuum dipole: Edot sin2 • Why timescale so slow?
Conclusions • IP population suggests thatalign = 7x107 yr • Consistent with align found by Tauris & Manchester • The model is simple and intuitive. No ad-hoc assumptions are required. • Different - P relations without alignment is not able to fit the data • Elliptical beams are inconsistent with the data • Older pulsars are more difficult to find and total inferred population is 2x larger • Standard spin-down formula is questionable
What can LOFAR/SKA do? • Find many more pulsars. • Constrain beam shapes • Constrain functional forms evolution • Better understanding braking torques • Comparison of the high and low frequency IP populations provides information about frequency dependence of pulsar beams.
