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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 The sound of a pulsar The Vela pulsar Located in the Vela supernova remnant Pulse period = 89 ms Age = 11,000 years 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’. 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. 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 Distribution of Pulsar Periods Total number known ~ 1500 • ‘Normal’ pulsars: 0.1 - 8.5 seconds • ‘Millisecond’ pulsars: 1.5 - 25 ms. About 80 known. 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. 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. 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. 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. 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. The Parkes radio telescope has found more than twice as many pulsars as the rest of the world’s telescopes put together. 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. Parkes multibeam pulsar surveys Distribution of Pulsar Periods Distribution of Dispersion Measures Distribution of pulsars on the Galactic Plane . P- P Diagram • 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 PSR J1119-6127 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) Pulsar – SNR Associations Cumulative Distribution by Year of Discovery 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) Positions of 47 Tuc Pulsars • Positions from pulse timing observations typical uncertainty < 1 milliarcsecond • All pulsars lie within central region of cluster Camilo et al. (2000) 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) 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) Millisecond pulsars in other clusters NGC 6266 PSR J1701-30 P 5.24 ms Pb 3.81 d Mc >0.19 Msun d 6.7 kpc NGC 6397 PSR J1740-53 NGC6544 PSR J1807-24 NGC 6752 PSR J1910-59 3.65 ms 3.06 ms 1.35 d (eclipse) 0.071 d (1.7 h) >0.18 Msun >0.009 Msun (10 MJup) 2.2 kpc 2.5 kpc 3.27 ms 0.86 d >0.19 Msun 3.9 kpc D’Amico et al. (2001) The Binary Pulsar PSR B1913+16 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 Orbit Parameters for PSR B1913+16 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 Orbital period decay 4.295(2) ms -2.422(6) x 10-12 Neutron-star masses PSR B1913+16: • 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) PRS B1913+16 Orbit Decay • Prediction based on measured Keplerian parameters and Einstein’s general relativity • Corrected for acceleration in gravitational field of Galaxy . . • Pb(pred)/Pb(obs) = 1.0023 +/- 0.0046 (Damour & Taylor 1991,1992) PSR B1913+16 • 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 Einstein was right! Parkes High Latitude Pulsar Survey • Uses multibeam receiver • Survey region: 220o < gl < 260o, -60o < gb < 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 => double-neutron-star system! PSR J0737-3039 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 B1913+16! • Other GR parameters measurable in ~ 1 year Implications for Gravitational Wave Detectors • Coalescence time 85 Myr – about 1/3 1913+16 value • Luminosity ~1/6 value for 1913+16 => 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.) 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 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) 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) 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. Clock error Same for pulsars in all directions Earth Error in Earth Velocity Opposite sign in opposite directions - Dipole Earth Gravitational Wave passing over Earth Opposite sign in orthogonal directions - Quadrupole Earth 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!