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The Parkes Pulsar Timing Array Project

The Parkes Pulsar Timing Array Project. R. N. Manchester. Australia Telescope National Facility, CSIRO Sydney Australia. Summary. Brief introduction to pulsars and pulsar timing Gravitational waves and pulsars The Parkes Pulsar Timing Array project. Pulsar Origins.

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The Parkes Pulsar Timing Array Project

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  1. The Parkes Pulsar Timing Array Project R. N. Manchester Australia Telescope National Facility, CSIRO Sydney Australia Summary • Brief introduction to pulsars and pulsar timing • Gravitational waves and pulsars • The Parkes Pulsar Timing Array project

  2. Pulsar Origins Pulsars are believed (by most people) to be rotating neutron stars Normal Pulsars: • Formed in supernova • Periods between 0.03 and 10 s • Relatively young (< 107 years) • Mostly single (non-binary) (ESO – VLT) Millisecond Pulsars (MSPs): • MSPs are very old (~109 years). • Mostly binary • 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 binary system.

  3. 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)

  4. . P - P Diagram • Millisecond pulsars have very low P and are very old • Most MSPs are binary • MSPs are formed by ‘recycling’ an old pulsar in an evolving binary system • ‘Normal’ pulsars have significant period irregularities, but MSP periods are very stable . J0737-3039

  5. Measurement of pulsar periods • Start observation at known time and average 1000 or more pulses to get mean pulse profile. • Cross-correlate this with a standard template to give the arrival time at the telescope of a fiducial point on profile, usually the pulse peak – the pulse time-of-arrival (TOA). • Measure a series of TOAs over days – weeks – months – years. • Compare observed TOAs with predicted values from a model for pulsar using TEMPO - differences are called timing residuals. • Fit the observed residuals with functions representing errors in the model parameters (pulsar position, period, binary period etc.). • Remaining residuals may be noise – or may be science!

  6. Model timing residuals • Period: DP = 5 x 10-16 s • dP/dt: DP = 4 x 10-23 • Position: Da = 1 mas • Proper motion: Dm = 5 mas/yr • Parallax: Dp = 10 mas .

  7. Sources of Pulsar Timing “Noise” • Intrinsic noise • Period fluctuations, glitches • Pulse shape changes • Perturbations of the pulsar’s motion • Gravitational wave background • Globular cluster accelerations • Orbital perturbations – planets, 1st order Doppler, relativistic effects • Propagation effects • Wind from binary companion • Variations in interstellar dispersion • Scintillation effects • Perturbations of the Earth’s motion • Gravitational wave background • Errors in the Solar-system ephemeris • Clock errors • Timescale errors • Errors in time transfer • Instrumental errors • Radio-frequency interference and receiver non-linearities • Digitisation artifacts or errors • Calibration errors and signal processing artifacts and errors • Receiver noise

  8. Gravitational Waves (NASA GSFC) • Prediction of general relativity and other theories of gravity • Generated by acceleration of massive object(s) • Astrophysical sources: • Inflation era • Cosmic strings • SN, BH formation in early Universe • Binary black holes in galaxies • Coalescing neutron-star binaries • Compact X-ray binaries (Stuart Wyithe’s talk) (K. Thorne, T. Carnahan, LISA Gallery)

  9. PSR B1913+16 Orbit Decay • Discovered at Arecibo Observatory by Russell Hulse & Joe Taylor in 1975 • Double-neutron-star system, orbital period 7.75 hours • Binary system loses energy to gravitational radiation • Prediction based on measured Keplerian parameters and Einstein’s general relativity • Corrected for acceleration in gravitational field of Galaxy • Pb(obs)/Pb(pred) = 1.0013  0.0021 . . First observational evidence for gravitational waves! (Weisberg & Taylor 2005)

  10. Detection of Gravitational Waves • Huge efforts over more than four decades to detect gravitational waves • Initial efforts used bar detectors pioneered by Weber • More recent efforts use laser interferometer systems, e.g., LIGO, LISA LIGO LISA • Two sites in USA • Perpendicular 4-km arms • Spectral range 10 – 500 Hz • Initial phase now commissioning • Advanced LIGO ~ 2011 • Orbits Sun, 20o behind the Earth • Three spacecraft in triangle • Arm length 5 million km • Spectral range 10-4 – 10-1 Hz • Planned launch 2015 - 2020

  11. Limits on GW Background using Pulsars • Observed pulse periods affected by presence of gravitational waves in Galaxy • For stochastic GW background, effects at pulsar and Earth are uncorrelated • With observations of one or two pulsars, can only put limit on strength of stochastic GW background - effect can’t be greater than observed residuals • Best limits are obtained for GW frequencies ~ 1/T where T is length of data span • Analysis of 8-year sequence of Arecibo observations of PSR B1855+09 gives Wg = rGW/rc < 10-7(Kaspi et al. 1994, McHugh et al.1996) • Extended Arecibo data set gives better limit - Andrea’s talk Timing residuals for PSR B1855+09

  12. Individual Black-Hole Binary Systems • Most (maybe all) galaxies have a super-massive black hole at their core • Galaxy mergers are common, so binary black holes will exist in many galaxies • Dissipative effects will result in spiral-in and eventual merger of BH pair • For binary BH with orbital periods of order years, can (in principle) detect the signature of the emitted GW in pulsar timing data • Limits placed on binary mass ratio for six nearby galaxies containing a central BH assuming orbital period ~2000 days from Arecibo observations of three pulsars (Lommen & Backer 2001) • Based on VLBI measurements, proposed that there is a 1010 Msun binary BH with 1-year period in 3C66B (z=0.02) (Sudou et al. 2003) • Using Kaspi et al. (1994) Arecibo timing of PSR B1855+09, existence ruled out at 98% confidence level (Jenet et al. 2004) Expected timing signature for 3C66B binary BH

  13. A Pulsar Timing Array • With observations of many pulsars widely distributed on the sky can in principle detect a stochastic gravitational wave background • Gravitational waves passing over the pulsars are uncorrelated • Gravitational waves passing over Earth produce a correlated signal in the TOA residuals for all pulsars • Pulsar timing could give the first direct detection of gravitational waves! • A timing array can detect instabilities in terrestrial time standards – establish a pulsar timescale • Can improve knowledge of Solar system properties, e.g. masses and orbits of outer planets and trans-Neptunian objects • Idea first discussed by Foster & Backer (1990)

  14. Clock errors All pulsars have the same TOA variations: monopole signature • Solar-System ephemeris errors Dipole signature • Gravitational waves Quadrupole signature Can separate these effects provided there is a sufficient number of widely distributed pulsars

  15. Detecting a Stochastic GW Background Simulation using Parkes Pulsar Timing Array (PPTA) pulsars with GW background from binary black holes in galaxies (Rick Jenet, George Hobbs)

  16. The Parkes Pulsar Timing Array Project Collaborators: • Australia Telescope National Facility, CSIRO Dick Manchester, George Hobbs, Russell Edwards, John Sarkissian, John Reynolds, Mike Kesteven, Grant Hampson, Andrew Brown • Swinburne University of Technology Matthew Bailes, Ramesh Bhat, Joris Verbiest, Albert Teoh • University of Texas, Brownsville Rick Jenet, Willem van Straten • University of Sydney Steve Ord • National Observatories of China, Beijing Xiaopeng You • Peking University, Beijing Kejia Lee • University of Tasmania Aidan Hotan

  17. The PPTA Project (Observing Systems) • Using the Parkes 64-m radio telescope to make precision timing observations of 20 MSPs at 2 - 3 week intervals • Observations at three frequencies, 685 MHz (bandwidth 64 MHz), 1400 MHz (256 MHz) and 3100 MHz (1 GHz) to allow investigation of and correction for propagation effects (George’s talk) • Either 20cm Multibeam centre beam or HOH receiver for 1400 MHz, dual-band 10cm/50cm coaxial receiver for 685 and 3100 MHz. All systems receive dual linear polarisation • Backend systems: Wideband correlator (WBC, 1 GHz, 2-bit sampling), Prototype digital filterbank (PDFB1, 256 MHz, 8-bit sampling), Swinburne baseband recorder (CPSR2, 2 x 64 MHz, 2-bit sampling). All systems give full Stokes parameters

  18. Sky Distribution of Millisecond Pulsars P < 20 ms and not in globular clusters

  19. PPTA Pulsars • 20 MSPs - all in Galactic disk except J1824-2452 (B1821-24) in M28 • Two years of timing data at 2 -3 week intervals and at three frequencies • Uncorrected for DM variations and polarisation calibration - routine script processing • Except for J1937+2134 (B1937+21), fit only  and  and basic binary parameters • Half the sample has rms residual less than 1 s • Expect factor of two improvement with fine tuning for each pulsar .

  20. Simulations of the PPTA show that to reach the major goals we need ~weekly observations of ~20 MSPs over at least five years with TOA precisions of ~100 ns for ~10 pulsars and < 1 s for rest (Jenet et al. 2005) Still have a way to go!

  21. The PPTA Project (current and future) • Investigation of properties of potential GW sources (talks by Ray Norris, Stuart Wyithe) • Development of detection algorithms (talks by Rick Jenet, Yuri Levin, Sergei Kopeikin) • Development of improved data analysis methods, e.g., TEMPO2: New timing analysis program, systematic errors in TOA corrections < 1 ns, graphical interfaces, predictions and simulations (Hobbs et al. 2006, Edwards et al. 2006) • Investigation of and correction for propagation effects - both interstellar and Solar system (Xiaopeng You, George’s talk) • Development of new observing systems: • PDFB2: 1 GHz bandwidth, higher frequency and time resolution, real-time RFI mitigation (Andrew Brown talk and lab visit) • APSR: New generation baseband system, up to 1 GHz bandwidth, 8-bit sampling, PDFB2 as a front-end • International collaboration is vital to reaching our goals: Collaboration established with European Pulsar Timing Array (EPTA) project (Michael Kramer, tomorrow). Discussions continuing with North American project team (Andrea’s talk)

  22. RFI Mitigation • RFI becoming more of a problem • Increased number of sources • Increased receiver sensitivity and bandwidth • Serious issue for 50cm (700 MHz) at Parkes Digital TV at 700 MHz • Developing techniques for removing RFI from signal • Successful tests with short CPSR2 observations with off-line processing and real-time using NTD antennas (Mike Kesteven, Ludi de Souza) • Real-time mitigation (both impulsive and narrow-band) will be integrated into PDFB2 (Kesteven et al. 2004)

  23. A Pulsar Timescale • Terrestrial time defined by a weighted average of caesium clocks at time centres around the world • Comparison of TAI with TT(BIPM03) shows variations of amplitude ~1 s even after trend removed • Revisions of TT(BIPM) show variations of ~50 ns • Pulsar timescale is not absolute, but can reveal irregularities in TAI and other terrestrial timescales • Current best pulsars give a 10-year stability (z) comparable to TT(NIST) - TT(PTB) • Full PPTA will define a pulsar timescale with precision of ~50 ns or better at 2-weekly intervals and model long-term trends to 5 ns or better (Petit 2004)

  24. Solar System • We use a planetary ephemeris based primarily on planetary radar observations (currently JPL ephemeris DE405) to correct observed TOAs to an inertial frame - the Solar System barycentre • Errors in this will result in systematic timing residuals which can be solved for to improve the ephemeris • With data from international collaborators, can improve on published mass for Jupiter - accuracy better than 10-9 M(Edwards et al. 2007) • Precision better than 10-10 M with further 5-10 years of timing data • Potential to detect currently unknown trans-Neptunian objects (TNOs) Timing signatures for 3 x 10-9 M error Jupiter Saturn

  25. Current and Future Limits on the Stochastic GW Background • Arecibo data for PSR B1855+09 (Kaspi et al. 1994) and recent PPTA data • Monte Carlo methods used to determine detection limit for stochastic background described by hc = A(f/1yr) (where  = -2/3 for SMBH, ~ -1 for relic radiation, ~ -7/6 for cosmic strings) • Current limit: gw(1/8 yr) ~ 2  10-8 • For full PPTA (100ns, 5 yr): ~ 10-10 • Currently consistent with all SMBH evolutionary models (e.g., Jaffe & Backer 2003; Wyithe & Loeb 2003, Enoki et al. 2004) • If no detection with full PPTA, all current models ruled out • Already limiting EOS of matter in epoch of inflation (w = p/ > -1.3)and tension in cosmic strings (Grishchuk 2005; Damour & Vilenkin 2005) Timing Residuals 10 s (Jenet et al. 2006)

  26. The Gravitational Wave Spectrum

  27. Summary • Pulsars are extraordinarily good clocks and provide highly sensitive probes of a range of gravitational effects • Direct detection of gravitational waves (GW) is a major goal of current astrophysics - it will open a new window on the Universe • A pulsar timing array can detect long-period GW from astrophysical sources (or rule out most current source models) • Parkes Pulsar Timing Array (PPTA) timing 20 MSPs since mid-2004. Goal is ~100 ns rms residuals on at least half of sample. • Need to improve by a factor of 2 -5 on current performance to have reasonable chance of detection gravitational waves: improved observing systems, data analysis methods and international collaboration • A pulsar-based timescale will have better long-term stability than current best terrestrial timescales • Can improve on some Solar System parameters and potentially detect TNOs

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