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
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Scuola nazionale de Astrofisica Radio Pulsars 1: Pulsar Basics
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 - PowerPoint PPT Presentation
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Scuola nazionale de AstrofisicaRadio Pulsars 1: Pulsar Basics
Dick ManchesterAustralia Telescope National Facility, CSIRO
• Manchester & Taylor 1977 “Pulsars”• Lyne & Smith 2005 “Pulsar Astronomy”• Lorimer & Kramer 2005 “Handbook of Pulsar Astronomy”
Books
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
The sound of a pulsar:
Jocelyn Bell and Tony Hewish Bonn, August 1980
The Discovery of Pulsars
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
Data from ATNF Pulsar Catalogue, V1.25 (www.atnf.csiro.au/research/pulsar/psrcat; Manchester et al. 2005)
* Total known: 129 in 24 clusters (Paulo Freire’s web page)
Pulsar Model
(Bennet Link)
• 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
..
.
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
(Hobbs et al. 2005)
Guitar Nebula
PSR B2224+65
(Cordes et al. 2003)
ESO-VLT
Neutron Stars• Formed in Type II supernova explosion - core collapse of massive star
• 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
..
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
. .
Pulsar Energetics
Spin-down Luminosity:
Radio Luminosity:
• For a typical pulsar, P = 1s and P = 10-15, Bs ~ 108 T or 1012 G.
• Typical electric field at the stellar surface E ~ RBs/c ~ 109 V/cm
• Electrons reach ultra-relativistic energies in < 1 mm.
• Emit -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 -ray energies.
Pulsar Electrodynamics
Cheng et al. (1986); Romani (2000)
Rotating neutron-star model: magnetospheric gaps
Inner (polar cap) gap
Outer gaps
Regions of particle acceleration!
.B = 0
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 ~ ct• Brightness temperature: equivalent black-body temperature in Rayleigh-Jeans limit
Radio emission must be from coherent process!
Frequency Dependence of Mean Pulse Profile
Phillips & Wolsczcan (1992)
• Pulse width generally increases with decreasing frequency.
• Consistent with ‘magnetic-pole’ model for pulse emission.
• Lower frequencies are emitted at higher altitudes.
• 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)
Magnetic-Pole Model for Emission Beam
Mean pulse shapes and polarisation
Lyne & Manchester (1988)
P.A.
Stokes I
Linear
Stokes V
Orthogonal-mode emission – PSR B2020+28
P.A.
Stinebring et al. (1984)
V
I
%L
Mean pulse profile of PSR J0437-4715
Binary millisecond pulsar• P = 5.75 ms• Pb = 5.74 d
Navarro et al. (1997)
Stokes I
Stokes V
Linear
I
L
V
P.A.
• Complex profile, at least seven components
• Complex PA variation, including orthogonal transition
Wide Beams from Young and MS PulsarsCrab
(Ulmer et al. 1994)
PSR B1259-63
• 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?
PSR B0740-28
Other Examples:
Vela
PSR J0737-3039A
PSR B0950+08
Drifting subpulses and periodic fluctuations
PULSE LONGITUDE
Drifting subpulses
Taylor et al. (1975)
Periodic fluctuations
Backer (1973)
(Weltevrede et al. 2006)
• 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
Pulse Modulation
• 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
.
Pulsar Nulling
(Wang et al. 2006)
• 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
(Esamdin et al. 2005)
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”
PSR B1931+24 - An extreme nuller
(Kramer et al. 2006)
• 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
(Hankins et al. 2003)
Giant Pulses
First observed in the Crab pulsar - discovered through its 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.
• 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
Crab Giant Pulses
(Knight et al. 2006, Kuiper et al. 2004, Rutledge et al. 2004)
PSR J0218+4232
(Cusumano et al. 2003)
PSR B1937+21
Giant Pulses from Millisecond Pulsars• 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
BeppoSAX
Radio
Chandra 0.1-10kev
GBT 850 MHz
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?