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Scuola nazionale de Astrofisica Radio Pulsars 1: Pulsar Basics

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  1. Scuola nazionale de Astrofisica Radio Pulsars 1: Pulsar Basics Dick Manchester Australia Telescope National Facility, CSIRO Outline • Rotating neutron stars, SN associations, Binaries, MSPs • Pulse profiles, polarisation, beaming, RVM model • Pulse fluctuations: drifting, nulling, mode changing

  2. Basic References Books • Manchester & Taylor 1977 “Pulsars” • Lyne & Smith 2005 “Pulsar Astronomy” • Lorimer & Kramer 2005 “Handbook of Pulsar Astronomy” Review Articles • Rickett 1990, ARAA - Scintillation • Science, 23 April 2004 - Three articles: NS, Isolated Pulsars, Binary Pulsars • Living Reviews articles: (http://relativity.livingreviews.org/Articles) • Stairs 2003: GR and pulsar timing • Lorimer 2005: Binary and MS pulsars • Will, 2006: GR theory and experiment • SKA science: New Astron.Rev. 48 (2004) • Cordes et al.: Pulsars as tools • Kramer et al.: Strong-field tests of GR

  3. The Discovery of Pulsars Jocelyn Bell and Tony Hewish Bonn, August 1980 The sound of a pulsar:

  4. Spin-Powered Pulsars: A Census • Number of known pulsars: 1765 • Number of millisecond pulsars: 170 • Number of binary pulsars: 131 • Number of AXPs: 12 • Number of pulsars in globular clusters: 99* • Number of extragalactic pulsars: 20 * Total known: 129 in 24 clusters (Paulo Freire’s web page) Data from ATNF Pulsar Catalogue, V1.25 (www.atnf.csiro.au/research/pulsar/psrcat; Manchester et al. 2005)

  5. Pulsar Model • Rotating neutron star • Light cylinder RLC = c/ = 5 x 104 P(s) km • Charge flow along open field lines • Radio beam from magnetic pole (in most cases) • High-energy emission from outer magnetosphere • Rotation braked by reaction to magnetic-dipole radiation and/or charge acceleration:  = -K -3 • Characteristic age: c = P/(2P) • Surface dipole magnetic field: Bs ~ (PP)1/2 . . . (Bennet Link)

  6. Pulsar Formation • ~30 young pulsars associated with SNR • Core of red giant collapses when its mass exceeds “Chandrasekhar Mass” • Energy release ~ 3GM/5R2 ~ 3 x 1053 erg ~ 0.1 Mc2 • Kinetic energy of SNR ~ 1051 erg; 99% of grav. energy radiated as neutrinos and anti-neutrinos • Asymmetry in neutrino ejection gives kick to NS • Measured proper motions: <V2D> = 211 km s-1 • <V3D> = 4<V2D>/ = 2<V1D> for isotropic velocities ESO-VLT Guitar Nebula PSR B2224+65 (Cordes et al. 2003) (Hobbs et al. 2005)

  7. Neutron Stars • Formed in Type II supernova explosion - core collapse of massive star • Diameter 20 - 30 km • Mass ~ 1.4 Msun (MT77) (Stairs 2004) (Lattimer & Prakash 2004)

  8. . P vs P Galactic disk pulsars . • Most pulsars have P ~ 10-15 • MSPs have P smaller by about 5 orders of magnitude • Most MSPs are binary • Only a few percent of normal pulsars are binary • AXPs are slow X-ray pulsars with very strong fields - “magnetars” • Some young pulsars are only detected at X-ray or -ray wavelengths . ATNF Pulsar Catalogue (www.atnf.csiro.au/research/pulsar/psrcat)

  9. Pulsar Recycling • Young pulsars live for 106 or 107 years • MSPs have c 109 or 1010 years and most are binary • Accretion from an evolving binary companion leads to: • Increased spin rate for NS - angular momentum transferred from orbit to NS • Decreased Bs - mechanism not understood. Could be simple “burial” of field by accreted matter . . • Minimum spin period: Pmin ~ (B9)6/7 (M/MEdd)-3/7 • Short-period MSPs from low-mass binary companions - long evolution time • Recycling is very effective in globular clusters - more than half of all MSPs in globular clusters: 22 in 47 Tucanae, 33 in Terzan 5 (Ransom et al. 2005, Friere 2007) • Old NS in core of cluster captured by low-mass stars and then recycled • About 30% of MSPs are single - what has happened to companion? • Blown away by relativistic wind from pulsar - ? • Lost in 3-body encounter - only in core of globular cluster 47 Tucanae

  10. Pulsar Energetics Spin-down Luminosity: Radio Luminosity:

  11. Pulsar Electrodynamics • For a typical pulsar, P = 1s and P = 10-15, Bs ~ 108 T or 1012 G. • Typical electric field at the stellar surface E ~ WRBs/c ~ 109 V/cm • Electrons reach ultra-relativistic energies in < 1 mm. • Emit g-ray photons by curvature radiation. These have energy >> 1 MeV and hence decay into electron-positron pairs in strong B field. • These in turn are accelerated to ultra-relativistic energies and in turn pair-produce, leading to a cascade of e+/e- pairs. • Relativistic pair-plasma flows out along ‘open’ field lines. • Instabilities lead to generation of radiation beams at radio to g-ray energies.

  12. Rotating neutron-star model: magnetospheric gaps W.B = 0 Regions of particle acceleration! Inner (polar cap) gap Outer gaps Cheng et al. (1986); Romani (2000)

  13. Coherent Radio Emission • Source power is very large, but source area is very small • Specific intensity is very large • Pulse timescale gives limit on source size ~ ct • Brightness temperature: equivalent black-body temperature in Rayleigh-Jeans limit Radio emission must be from coherent process!

  14. Frequency Dependence of Mean Pulse Profile • Pulse width generally increases with decreasing frequency. • Consistent with ‘magnetic-pole’ model for pulse emission. • Lower frequencies are emitted at higher altitudes. Phillips & Wolsczcan (1992)

  15. Magnetic-Pole Model for Emission Beam • Emission beamed tangential to open field lines • Radiation polarised with position angle determined by projected direction of magnetic field in (or near) emission region(Rotating Vector Model)

  16. Mean pulse shapes and polarisation P.A. Stokes I Linear Stokes V Lyne & Manchester (1988)

  17. Orthogonal-mode emission – PSR B2020+28 V P.A. %L I Stinebring et al. (1984)

  18. Mean pulse profile of PSR J0437-4715 P.A. Stokes I • Binary millisecond pulsar • P = 5.75 ms • Pb = 5.74 d Linear Stokes V • Complex profile, at least seven components • Complex PA variation, including orthogonal transition I L V Navarro et al. (1997)

  19. Wide Beams from Young and MS Pulsars • Pulsed (non-thermal) X-ray and -ray profiles from young pulsars have wide “double” shape • Emitted from field lines high in magnetosphere associated with a single magnetic pole • Some young radio pulsars have a similar pulse profile, e.g. PSR B1259-63 • Class of young pulsars with very high (~100%) linear polarisation, e.g. Vela, PSR B0740-28 • Radio emission from high in pulsar magnetosphere? • MSPs also have very wide profiles - also single-pole emission from high in magnetosphere? Crab (Ulmer et al. 1994) PSR B1259-63 PSR B0740-28

  20. Other Examples: Vela PSR B0950+08 PSR J0737-3039A

  21. Drifting subpulses and periodic fluctuations Drifting subpulses Periodic fluctuations PULSE LONGITUDE Taylor et al. (1975) Backer (1973)

  22. Pulse Modulation • Extensive survey of pulse modulation properties at Westerbork - 187 pulsars • Observations at 1.4 GHz, 80 MHz bw • Modulation indices, longitude-resolved and 2D fluctuation spectra computed • 42 new cases of drifting subpulses • At least 60% of all pulsars show evidence for drifting behaviour • “Coherent” drifters have large characteristic age, but drifting seen over most of P - P diagram . (Weltevrede et al. 2006)

  23. Pulsar Nulling • Parkes observations of 23 pulsars, mostly from PM survey • Large null fractions (up to 96%) - mostly long-period pulsars • Nulls often associated with mode changing (Wang et al. 2006)

  24. PSR B0826-34 • P = 1.848 s, pulsed emission across whole of pulse period • In “null” state ~80% of time • 5-6 drift bands across profile, variable drift rate with reversals • Weak emission in “null” phase, ~2% of “on” flux density • Different pulse profile in “null” phase: Null is really a mode change. On “Null” (Esamdin et al. 2005)

  25. PSR B1931+24 - An extreme nuller • Quasi-periodic nulls: on for 5-10 d, off for 25-35 d • Period derivative is ~35% smaller when in null state! • Implies cessation of braking by current with G-J density • Direct observation of current responsible for observed pulses (Kramer et al. 2006)

  26. Giant Pulses Intense narrow pulses with a pulse energy many times that of an average pulse - characterised by a power-law distribution of pulse energies. First observed in the Crab pulsar - discovered through its giant pulses! Crab Giant Pulses • Arecibo observations at 5.5 GHz • Bandwidth 0.5 GHz gives 2 ns resolution • Flux density > 1000 Jy • implies Tb > 1037 K! • Highly variable polarisation • Suggests emission from plasma turbulence on scales ~ 1 m (Hankins et al. 2003)

  27. Giant Pulses from Millisecond Pulsars PSR B1937+21 • Giant pulses seen from several MSPs with high BLC • Most also have pulsed non-thermal emission at X-ray energies • Giant pulses occur at phase of X-ray emission RXTE PSR J0218+4232 BeppoSAX GBT 850 MHz Radio Chandra 0.1-10kev (Cusumano et al. 2003) (Knight et al. 2006, Kuiper et al. 2004, Rutledge et al. 2004)

  28. Transient Pulsed Radio Emission from a Magnetar • AXP XTE J1810-197 - 2003 outburst in which X-ray luminosity increased by ~100 • X-ray pulsations with P = 5.54 s observed • Detected as a radio source at VLA, increasing and variable flux density: 5 - 10 mJy at 1.4 GHz (Halpern et al. 2005) • Within PM survey area, not detected in two obs. in 1997, 1998, S1.4 < 0.4 mJy • Observed in March 2006 at Parkes (Camilo et al. 2006) • Pulsar detected! • S1.4 ~ 6 mJy • Very unusual flat spectrum - individual pulses detected in GBT observations at 42 GHz! Earlier unconfirmed detections (e.g. Malofeev et al 2005) accounted for by transient and highly variable nature of pulsed emission?

  29. End of Part 1