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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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slide1

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
Pulsar timing for GLAST
  • Timing ~ 160 pulsars with Parkes
  • Perfect dataset to study young & energetic pulsars
slide3

Standard model for pulsar beams

Gould 1994, Rankin 1990, Rankin 1993,

Kramer et al. 1994, Gil et al. 1993

slide4

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
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
Idea: consequence for IP

If90o, we can see the interpulse

Most pulsars with interpulses should be young if there is beam evolution

observations interpulses
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
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
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
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
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
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

slide13

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
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
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
Conclusions
  • IP population suggests thatalign = 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
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.
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