The population of pulsars with interpulses and the implications for beam evolution
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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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Patrick weltevrede simon johnston

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 implications for beam evolution

  • Timing ~ 160 pulsars with Parkes

  • Perfect dataset to study young & energetic pulsars


Patrick weltevrede simon johnston

Standard model for pulsar beams implications for beam evolution

Gould 1994, Rankin 1990, Rankin 1993,

Kramer et al. 1994, Gil et al. 1993


Patrick weltevrede simon johnston

Pulse width distribution implications for beam evolution

  • 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 implications for 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 implications for beam evolution

If90o, we can see the interpulse

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


Observations interpulses
Observations: interpulses implications for beam evolution

  • 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 implications for beam evolution

  • 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 implications for beam evolution

  • 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 implications for beam evolution

  • 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 implications for beam evolution

  • 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 implications for beam evolution

  • IP fraction: 2.3% (observed: < 1.8%)

  • There are too many

    fast IP pulsars

  • W  P -1/2

    Model fails


Patrick weltevrede simon johnston

Alignment of the magnetic axis implications for beam evolution

  • 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 implications for beam evolution

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 implications for beam evolution

  • 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 implications for beam evolution

  • 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? implications for beam evolution

  • 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.