Radio Pulsar Phenomenology: What can we learn from pulsar profiles? Simon Johnston (ATNF, CSIRO) Aris Karastergiou (Oxford, UK)

Transcription

1 Radio Pulsar Phenomenology: What can we learn from pulsar profiles? Simon Johnston (ATNF, CSIRO) Aris Karastergiou (Oxford, UK)

2 Outline Brief tour through the basics P-Pdot diagram The Rotating Vector Model Obtaining pulsar geometry What we ve learned from integrated profiles Beam models Thoughts

3 Basic Observables and Derivables. I. Measure spin period, P, and its derivatives, dp/dt etc Derive Spin down energy: Edot = 4π 2 I Pdot/P 3 Magnetic Field: B = (P Pdot) Braking index: n = νν/ν 2 Characteristic Age: τ c = Pdot / (n-1)p Light cylinder radius: R lc = P

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16 Basic Observables and Derivables. II. Measure DM, RM, Tscatt, HI absorption Derive Interstellar medium density Interstellar medium magnetic field Interstellar medium turbulence Pulsar distances Global picture of Galaxy structure

17 Basic Observables and Derivables. II. Measure DM, RM, Tscatt Johnston et al Loehmer et al Image: Reiner Beck

18 Basic Observables and Derivables. III. Measure Pulse arrival times, glitch parameters, timing noise, binary parameters Derive Tests of general relativity Detection of gravitational waves Pulsar distances NS interior density profile and structure Equation of state

19 Kramer et al Image: NASA Verbiest et al. 2010

20 Basic Observables and Derivables. IV. Single pulses an area rich in phenomenology Karastergiou et al Weltevrede et al van Leeuwen et al Derive Emission physics, emission models Hankins & Eilek 2007

21 Basic Observables and Derivables. V. Measure Integrated Pulsar Profiles Derive Geometry, emission heights, beam structure, emission physics, beam models

22 Assumptions about radio emission Emission from open field lines above polar cap Dipolar magnetic field Emission from near surface Must be coherent emission (unlike at high energies) Could argue about all these assumptions!

23 Pulsar geometry and the rotating vector model Important angles are: α angle between the rotation and magnetic axis β closest approach of line of sight to magnetic axis ρ cone opening angle The RVM uses the PA swing to derive the geometry

24 The Rotating Vector Model (RVM) for polarization 150 o Longitude

25 The Rotating Vector Model (RVM) for polarization Longitude

26 The Rotating Vector Model (RVM) for polarization Longitude

27 The Rotating Vector Model (RVM) for polarization Longitude

28 The Rotating Vector Model (RVM) for polarization Longitude

29 The Rotating Vector Model (RVM) for polarization 90 o Longitude

30 The Rotating Vector Model (RVM) for polarization Longitude

31 The Rotating Vector Model (RVM) for polarization Longitude

32 The Rotating Vector Model (RVM) for polarization Longitude

33 The Rotating Vector Model (RVM) for polarization Longitude

34 The Rotating Vector Model (RVM) for polarization Longitude

35 The Rotating Vector Model (RVM) for polarization 30 o Longitude

36 α = 30, β = -1.7 α = 86, β = -3.2 RVM fit in alpha-beta space

37 α = 30, β = -1.7 α = 86, β = -3.2 Ambiguity due to restricted longitude range!!

38 RVM is a useful tool and geometry is crucial for testing models of γ-ray emission (and radio emission). In practice, hard to constrain. This PA swing is forbidden by the RVM, and is likely the result of magnetospheric or propagation effects.

39 Mapping the magnetosphere We can use the radio observations to make a 3D map of the emission zones Need geometry Need a method of computing altitudes Need a large sample of pulsars BUT Geometry often uncertain Conflicting results from altitude derivations Models of the beam

40 Beam models of Rankin developed in a series of papers since In a nutshell: Two different types of emission, from the core and from cones. Emission heights are typically 300 km at 1 GHz. Circular polarization sign change in the core. Elliptical beams.

41 Beam model of Lyne & Manchester In a nutshell: No distinction between core and cone emission. Patchy beams with random emission locations. Circular beams.

42 Emission height versus longitude from Gangadhara & Gupta In a nutshell: Emission far from the centre occurs at higher heights than emission from the centre at a given observing frequency.

43 Young pulsar study of Johnston & Weisberg In a nutshell: Highly polarized, double component (simple) pulsars with high emission heights. Some have large widths.

44 Linear polarization fraction versus Edot Weltevrede & Johnston show that Edot is a crucial parameter! Abrupt transition between from low to high polarization states around Edot of No orthogonal mode jumps. One mode dominates but this is split 50:50 between parallel and orthogonal modes!!

45 Polarization and the P-Pdot diagram Weltevrede & Johnston 2008 Harding et al Links with high energy slot-gap models??

46 Pulse width versus Edot Orthogonal rotators High Edot, wide beam single pole Low Edot aligned rotators Weltevrede & Johnston study of interpulse pulsars. In a nutshell: Random orientation of B-Ω axis at birth, becoming aligned over time.

47 Orthogonal Rotators Weltevrede & Wright 2009 Kramer & Johnston 2008 Beam mapping of B (left) and B (above) following RVM fits. Emission heights similar above both poles. Evidence for emission from the closed field lines (see also Keith et al. 2010).

48 Beam model of Karastergiou & Johnston High Edot Low Edot In a nutshell: High single emission height for high Edot pulsars, multiple heights for low Edot pulsars. Explains high beaming fraction of high Edot pulsars.

49 Summary The integrated profiles give information on Geometry, emission heights, the static magnetosphere Beam models and observations tell us Clear evidence of conal emission, beam is patchy, emission height varies across polar cap High Edot (non MSP) pulsars Highly polarized, from high emission heights, implying a high beaming fraction Geometry poorly constrained in most cases. Distances are poorly known (until the SKA?) Often (always?) γ-ray emitters MSPs Polarization low Better handle on (low DM) distances

50 Questions What is the radio/gamma beaming fraction? For young, high Edot pulsars? For MSPs? Is there a link between the radio and the γ-ray apart from geometric considerations? Locations of the emission regions? Why so many γ-ray MSPs? What do the orthogonal rotators tell us? Very well determined geometry known radio emission regions