Differential Frequency Dependent Delay from the Pulsar Magnetosphere

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December 05, 2012

Differential Frequency Dependent Delay from the Pulsar Magnetosphere

Tom Hassall

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transientskp

December 05, 2012
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  1. 2.

    Pulsars • Neutron star • ...with a very strong magnetic

    field • ...which rips particles from the surface of the star and accelerates them • The accelerated charges produce a beam of radio emission • As the star rotates the beam sweeps around the sky like a lighthouse Magnetic Axis Open Magnetic Field Lines Closed Magnetic Field Lines Radio Beam Rotation Axis Magneto- sphere Light Cylinder 1 1.5 2 2.5 3 Time (s) 4e+07 0 1e+07 2e+07 3e+07 4e+07 0 0.5 1 1.5 2 2.5 3 Relative Amplitude Time (s) Time
  2. 3.

    Not to scale! Magnetosphere Rotation axis Magnetic axis Pulsar Radio

    Frequency Image credit: Hessels Radius-to-Frequency Mapping
  3. 4.

    PSR B0809+74 Starts broad Gets narrower Broadens again But this

    happens in a very smooth way Looks like one component is moving and one is fixed Hassall et al 2012
  4. 5.

    • Profile evolution cannot be explained by simple RFM •

    What is the pulse profile? Why does it change with frequency? • Best theory: Birefringence • Different propagation modes through the magnetosphere • Emission all comes from a narrow height range and propagation effects cause the profile to change shape (Beskin & Philippov 2011) • One component is refracted, and one is not What is going on?
  5. 6.

    PSR B0809+74 • Bright • Slow (P ~1.29s) • Nearby

    • Lots of interesting features including “drifting subpulses”
  6. 7.

    Drifting Subpulses 1.5 2 2.5 3 ime (s) 1.5 2

    2.5 3 ime (s) 2 2.5 3 s) 2 2.5 3 s) 0.5 1 1.5 2 2.5 3 Time (s) 0.5 1 1.5 2 2.5 3 Time (s) 1 1.5 2 2.5 3 Time (s) 1 1.5 2 2.5 3 Time (s) 0 +07 +07 +07 +07 0 0.5 1 1.5 2 2.5 3 Time (s) 0 +07 +07 +07 +07 0 0.5 1 1.5 2 2.5 3 Time (s) 0.5 1 1.5 2 2.5 3 Time (s) 0.5 1 1.5 2 2.5 3 Time (s) P3 P2
  7. 11.

    Phase-step Edwards & Stappers 2003 • This can be explained

    by the surface oscillation model by the presence of a “nodal line” (Clemens & Rosen 2004, 2008; Rosen & Demorest 2011) • Area on the NS surface which does not move • But if this is the case, we expect a sudden frequency onset of the phase- step • It is either in the line-of-sight or it isn’t
  8. 12.

    Different Frequencies • Gradual onset of the phase-step • Surface

    oscillation model predicts a discrete step • NOT a nodal line • What is it?
  9. 13.

    Folded Pulse Stacks • At the lowest frequencies there are

    2 separate sets of driftbands • One stays in the same position • The other moves relative to it • At ~300MHz they overlap
  10. 14.

    Interesting Consequences... • Centroid of the moving driftband suffers a

    ~30-pulse delay • Overlapping region is where supulse phase step occurs • This could be linked narrow-band pulses (see Vlad’s talk)
  11. 15.

    Conclusions • Subpulse phase-step and asymmetric profile evolution can be

    explained by two driftbands which suffer differential frequency-dependent delay • BUT what causes this? • Is B0809+74 a special case?
  12. 16.

    Cycle 0 Proposal • Observe 14 pulsars which exhibit similar

    features • Study their single pulse properties • Polarisation data - New information • New 8-bit mode - Double the bandwidth • Coherent sum of the 40 core stations - 3x sensitivity