Radio Pulsars

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Radio Pulsars
R. N. Manchester
Australia Telescope National Facility, CSIRO
Sydney, Australia
Summary

• Introduction to pulsar basics
• Multibeam searches at Parkes
• Supernova remnants and globular clusters
• Applications of pulsar timing

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The sound of a pulsar
The Vela pulsar
Located in the Vela supernova remnant Pulse
period 89 ms Age 11,000 years
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What are pulsars?

• Pulsars are rotating neutron stars.
• Neutron stars are tiny stars with the mass of
  the sun but a diameter of about 20 km.
• They rotate tens or even hundreds of times every
  second and send out a beam of emission.
• If we lie in the path of the beam, we see a
  pulse every revolution - the light-house model.

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How are pulsars formed?

• Pulsars are formed at the end of the life of a
  massive star.
• The inner core of the star collapses to form a
  rapidly rotating, highly magnetised neutron star.
• The outer layers of the star are blown off in a
  supernova explosion.
• We are left with a pulsar in the centre of an
  expanding supernova remnant.

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The Crab Nebula and its Pulsar

• Exploded in 1054 AD - observed by the Chinese.
• Pulsar at centre spins 30 times a second.
• Pulses from radio band to gamma-rays

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Distribution of Pulsar Periods
Total number known 1500

• Normal pulsars 0.1 - 8.5 seconds
• Millisecond pulsars 1.5 - 25 ms. About 80
  known.

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Formation of millisecond pulsars

• MSPs are very old (109 years).
• They have been recycled by accretion from an
  evolving binary companion.

• This accretion spins up the neutron star to
  millisecond periods.
• During the accretion phase the system may be
  detectable as an X-ray pulsar.

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Where are pulsars found?

• Most known pulsars are in the disk of our Galaxy
  - The Milky Way.
• Twenty are in our nearest neighbour galaxies,
  the Magellanic Clouds.
• About 30 young pulsars are associated with
  supernova remnants.
• More than one third of the known millisecond
  pulsars are in globular clusters.

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Interstellar Dispersion
Ionised gas in the interstellar medium causes
lower radio frequencies to arrive at the Earth
with a small delay compared to higher
frequencies. Given a model for the distribution
of ionised gas in the Galaxy, the amount of delay
can be used to estimate the distance to the
pulsar.
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Pulsars as Clocks

• Pulsar periods are generally very stable.
• However, they are not constant - all pulsars are
  slowing down.
• The ratio of period P to slowdown rate P gives
  an estimate of the pulsar age - typically 106
  years.
• Young pulsars have unpredictable changes in
  period - glitches and period noise.
• Millisecond pulsars have extremely stable
  periods.

.
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Binary pulsars

• Some pulsars are in orbit around another star.
  Orbital periods range from 1.6 hours to several
  years.
• Only a few percent of normal pulsars, but more
  than half of all millisecond pulsars, are
  binary.
• Pulsar companion stars range from very low-mass
  white dwarfs (0.01 solar masses) to heavy normal
  stars (10 - 15 solar masses).
• Seven pulsars have neutron-star companions.
• One pulsar has three planets in orbit around it.

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The Parkes radio telescope has found more than
twice as many pulsars as the rest of the worlds
telescopes put together.
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Parkes Multibeam Pulsar Surveys

• Multibeam receiver installed in mid-1997.
• Discovered more than 800 pulsars, more than 50
  of all known pulsars.
• The Parkes Multibeam Pulsar Survey has found
  more than 700 of these.
• High-latitude surveys have found about 100
  pulsars including 12 millisecond pulsars.

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Parkes multibeam pulsar surveys
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Distribution of Pulsar Periods
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Distribution of Dispersion Measures
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Distribution of pulsars on the Galactic Plane
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P- P Diagram
PSR J1119-6127

• Young pulsars have rapid slow-down rates
  t P/(2P)
• High-B pulsars also slow down rapidly Bs
  (PP)1/2
• Most millisecond pulsars are binary

.
.
Multibeam surveys

• New sample of young high-B pulsars
• Several mildly recycled binary pulsars, filling
  gap between MSPs and normal pulsars

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PSR J1119-6127 - G292.2-0.5
ATCA 1.4 GHz

• P 407 ms
• Age 1.7 kyr
• No catalogued SNR
• Faint ring on MOST GPS
• Deep ATCA observation revealed shell SNR exactly
  centred on pulsar!

New SNR! New Association!
PSR Camilo et al. (2000) SNR Crawford et al.
(2001)
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Pulsar SNR Associations
Cumulative Distribution by Year of Discovery
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Pulsars in 47 Tucanae

• 11 millisecond pulsars discovered 1991-1995. All
  but two single (non-binary)
• 12 more discovered since 1998 using multibeam
  receiver. All but two binary.

(Camilo et al. 2000)
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Positions of 47 Tuc Pulsars

• Positions from pulse timing observations -
  typical uncertainty lt 1 milliarcsecond
• All pulsars lie within central region of cluster

Camilo et al. (2000)
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Proper motion of 47 Tuc pulsars

• Timing measurements over 10 years
• Proper motion due to motion of 47 Tuc through
  halo at 150 km s-1
• Motion of individual pulsars within cluster too
  small to detect
• Mean proper motion of pulsars more accurate than
  and marginally inconsistent with Hipparchos
  value.

Hipparchos
Freire et al. (2003)
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Ionized gas in 47 Tucanae
.

• Correlation of DM and P
• P due to acceleration in cluster potential
• Pulsars on far side of cluster have higher DM
• Gas density 0.07 cm-3, about 100 times local
  density
• Total mass of gas in cluster 0.1 Msun

.
(Freire et al. 2001)
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Millisecond pulsars in other clusters
NGC 6266 NGC 6397
NGC6544 NGC 6752 PSR J1701-30
PSR J1740-53 PSR J1807-24 PSR J1910-59
P 5.24 ms 3.65 ms
3.06 ms 3.27 ms Pb 3.81 d
1.35 d (eclipse) 0.071 d (1.7 h) 0.86 d
Mc gt0.19 Msun gt0.18 Msun gt0.009
Msun (10 MJup) gt0.19 Msun d 6.7 kpc
2.2 kpc 2.5 kpc
3.9 kpc
DAmico et al. (2001)
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The Binary Pulsar PSR B191316
Discovered by Hulse Taylor in 1975
Pulse period 59 ms Orbital Period 7h
45m Double neutron-star system
Velocity at periastron 0.001 of
velocity of light
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Orbit Parameters for PSR B191316
Keplerian
Semi-major axis 2.3417592(19)
s Eccentricity 0.6171308(4) Orbital
period 0.322997462736(7) days Longitude of
periastron 226.57528(6) degrees Time of
periastron 46443.99588319(3) (MJD)
Post-Keplerian (or relativistic)
Periastron advance 4.2226621(11) deg/year Grav.
redshift Transverse Doppler 4.295(2)
ms Orbital period decay -2.422(6) x 10-12
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Neutron-star masses

• PSR B191316
• Periastron advance
• Grav. Redshift
• Orbit decay

First two measurements determine the masses of
the two stars - Both
neutron stars!
(Diagram from C.M. Will, 2001)
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PRS B191316 Orbit Decay

• Prediction based on measured Keplerian
  parameters and Einsteins general relativity
• Corrected for acceleration in gravitational
  field of Galaxy
• Pb(pred)/Pb(obs) 1.0023 /- 0.0046

.
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(Damour Taylor 1991,1992)
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PSR B191316

• First discovery of a binary pulsar
• First observational evidence for gravity waves
• First accurate determinations of neutron star
  masses
• Confirmation of general relativity as an
  accurate description of strong-field
  gravitational interactions

Nobel Prize for Taylor Hulse in 1993
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Einstein was right!
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Parkes High Latitude Pulsar Survey

• Uses multibeam receiver
• Survey region 220o lt gl lt 260o, -60o lt gb lt 60o
• Optimised for MSPs tobs 4 min, tsamp 125
  ms.
• 14 pulsars discovered so far, including 4 MSPs
• PSR J0737-3039 P 22.7 ms, Pb 2.4 h,
  e 0.088, min. companion mass 1.25 Msun gt
  double-neutron-star system!

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PSR J0737-3039
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PSR J0737-3039

• Most highly relativistic binary pulsar known!
• GR precession of periastron 16.86 /- 0.05
  deg/yr, four times as large as for PSR B191316!
• Other GR parameters measurable in 1 year

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Implications for Gravitational Wave Detectors

• Coalescence time 85 Myr about 1/3 191316
  value
• Luminosity 1/6 value for 191316 gt many
  similar systems in Galaxy
• Implies an increase in the Galactic merger rate
  by about factor of eight
• Increases predicted detection rate for LIGO from
  about one per century to one every few years.

(Burgay et al., Nature, in press.)
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Precision timing of PSR J0437-4715
PSR J0437-4715 is a binary millisecond pulsar
discovered at Parkes in 1993. It is the closest
and strongest MSP known.
Timing observations at Parkes over the past two
years using a baseband recording system have
given the best-ever pulsar timing precision.
Collaborative project Swinburne University,
Caltech and ATNF
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PSR J0437-4715
Parameters from timing observations
R.A. (2000) 04h 37m 15.s7865145(7) Dec.
(2000) -47o 15 08.461584(8) P
5.757451831072007(8) ms Pb 5.741046(3)
days Eccentricity 0.000019186(5) Proper
motion 140.892(9) mas/year Parallax 7.19(14)
mas Orbit inclination 42.75 deg.
(van Straten et al., Nature, July 12 2001)
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Shapiro delay of PSR J0437-4715
Shape predicted from measured parameters
Mcompanion 0.236 /- 0.017 Msun
Mpulsar 1.58 /- 0.18
Msun
RMS Residual 35 ns!
Independent test of predictions of general
relativity!
(van Straten et al., Nature, July 12 2001)
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A Pulsar Timing Array

• Combine precision timing observations of many
  millisecond pulsars widely distributed on
  celestial sphere
• Solve for all pulsar parameters as well as
  global terms which affect all pulsars
• Can in principle detect effects of gravitational
  waves passing over the Earth could be first
  direct detection of gravitational waves!
• Can detect long-term irregularities in
  terrestrial timescale establish a pulsar
  timescale!
• Can improve knowledge of Solar system
  properties, e.g. masses and orbits of outer
  planets and asteroids.

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Clock error
Same for pulsars in all directions
Earth
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Error in Earth Velocity
Opposite sign in opposite directions - Dipole
Earth
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Gravitational Wave passing over Earth
Opposite sign in orthogonal directions -
Quadrupole
Earth
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Summary

• Recent large-scale radio pulsar searches at
  Parkes have more than doubled the number of known
  pulsars
• New population of high-B pulsars and new SNR
  associations
• Globular clusters contain many millisecond
  pulsars
• Precision timing of binary millisecond pulsars
  measures many properties of binary stars and
  tests general relativity.
• Discovery of highly relativistic binary pulsar
  significantly increases predicted rate of LIGO
  detections of merger events.
• A millisecond pulsar timing array can establish
  a pulsar timescale and may detect gravitational
  waves.

Thank you!