Differential Frequency Dependent Delay from the Pulsar Magnetosphere

Tom Hassall

Transcript

1. Tom Hassall and the LOFAR Pulsar Working Group Differential Frequency-dependent Delay from the Pulsar Magnetosphere
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
3. Not to scale! Magnetosphere Rotation axis Magnetic axis Pulsar Radio Frequency Image credit: Hessels Radius-to-Frequency Mapping
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
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?
6. PSR B0809+74 • Bright • Slow (P ~1.29s) • Nearby • Lots of interesting features including “drifting subpulses”
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
8. Carousel Pulsar Spark Carousel Line-of-sight
9. Pulsar Surface Oscillations Line-of-sight
10. Subpulse Phase Tracks
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
12. Different Frequencies • Gradual onset of the phase-step • Surface oscillation model predicts a discrete step • NOT a nodal line • What is it?
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
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)
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?
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
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       Is this slide still necessary Rene?