The Parkes Pulsar Timing Array

1. The Parkes Pulsar Timing Array R. N. Manchester CSIRO Astronomy and Space Science Sydney Australia Summary • Short introduction to pulsar timing basics • Pulsar Timing Arrays (PTAs) • The Parkes Pulsar Timing Array (PPTA) • The extended PPTA data set • Current status and future prospects

2. Spin-Powered Pulsars: A Census • Currently 1984 known (published) pulsars • 1799 rotation-powered disk pulsars • 172 in binary systems • 238 millisecond pulsars (MSPs) • 141 in globular clusters • 8 X-ray isolated neutron stars • 16 AXP/SGR • 20 extra-galactic pulsars Data from ATNF Pulsar Catalogue, V1.43 (www.atnf.csiro.au/research/pulsar/psrcat) (Manchester et al. 2005)

3. . The P – P Diagram Galactic Disk pulsars P = Pulsar period P = dP/dt = slow-down rate . . • For most pulsars P ~ 10-15 • MSPs haveP smaller by about 5 orders of magnitude • Most MSPs are binary, but few normal pulsars are • tc = P/(2P) is an indicator of pulsar age • Surface dipole magnetic field ~ (PP)1/2 . . . Great diversity in the pulsar population!

4. Pulsars as Clocks • Precision of pulse period determination ~ ToA uncertainty/data span • MSPs have ToA uncertainties ~ 100 ns – 1 ms • Data spans ~ 3 - 10 years dP/P ~ 10-7/108 ~ 10-15! • This assumes that spectrum of the residual time series is “white” and the reduced c2 ~ 1 • Neither is quite true in practice, but … For the best MSPs, the 10-yr timing stability is comparable to that of the world’s best atomic clocks

5. What is a Pulsar Timing Array (PTA)? • A PTA is an array of pulsars, widely distributed on the sky, that are timed with high precision at frequent intervals over a long data span • With observations of many pulsars, phenomena which affect all pulsar periods in a correlated way can be separated from phenomena which affect different pulsars differently • For example, a stochastic gravitational wave background can be separated from errors in the time standard because of their different dependence on pulsar sky position. Idea first discussed by Hellings & Downs (1983), Romani (1989) and Foster & Backer (1990)

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

7. Detecting a Stochastic GW Background Hellings & Downs correlation function Simulation of timing-residual correlations among 20 pulsars for a GW background from binary super-massive black holes in the cores of distant galaxies To detect the expected signal, we need ~weekly observations of ~20 MSPs over 5-10 years with TOA precisions of ~100 ns for ~10 pulsars and < 1 s for the rest (Jenet et al. 2005, Hobbs et al. 2009)

8. Major Pulsar Timing Array Projects • European Pulsar Timing Array (EPTA) • Radio telescopes at Westerbork, Effelsberg, Nancay, Jodrell Bank, (Cagliari) • Normally used separately, but can be combined for more sensitivity • High-quality data (rms residual < 2.5 ms) for 9 millisecond pulsars • North American pulsar timing array (NANOGrav) • Data from Arecibo telescope and Green Bank Telescope • High-quality data for 17 millisecond pulsars • Parkes Pulsar Timing Array (PPTA) • Data from Parkes 64m radio telescope in Australia • High-quality data for 20 millisecond pulsars Observations at two or three frequencies required to remove the effects of interstellar dispersion

9. The Parkes Pulsar Timing Array Collaboration • CSIRO Astronomy and Space Science, Sydney Dick Manchester, George Hobbs, Ryan Shannon, Mike Keith, Sarah Burke-Spolaor, Aidan Hotan, John Sarkissian, John Reynolds, Mike Kesteven, Warwick Wilson, Grant Hampson, Andrew Brown, Jonathan Khoo,AnkurChaudhary, (Russell Edwards) • Swinburne University of Technology, Melbourne Matthew Bailes, Willem van Straten,RameshBhat, Stefan Oslowski, Jonathon Kocz, Andrew Jameson • Monash University, Melbourne • Yuri Levin • University of Melbourne • Vikram Ravi(Stuart Wyithe) • University of California, San Diego Bill Coles • University of Texas, Brownsville (Rick Jenet) • MPIfR, Bonn (David Champion), (JorisVerbiest), (KJ Lee) • University of Sydney, Sydney • Daniel Yardley • Southwest University, Chongqing Xiaopeng You • Xinjiang Astronomical Observatory, Urumqi • Wenming Yan, Jingbo Wang • National Space Science Center, Beijing • Xinping Deng

10. The PPTA Project • Using the Parkes 64-m radio telescope at three frequencies, 700 MHz, 1400 MHz and 3100 MHz, to observe 21 MSPs • Observations at 2 - 3 week intervals • Regular good-quality observations since 2005 March • Digital filterbanks and baseband recording systems used • Database and processing pipeline – PSRCHIVE and TEMPO2 • Studying detection algorithms for different types of GW sources • Simulating GW signals and studying implications for galaxy evolution models • Establishing a pulsar-based timescale and investigating Solar system properties • Using PPTA data sets to investigate individual pulsar properties, e.g., pulse polarisation, binary evolution, astrometry etc. Website: www.atnf.csiro.au/research/pulsar/ppta

11. The PPTA Pulsars All (published) MSPs not in globular clusters

12. DM Variations 50cm • Path through ISM changes as pulsar and Earth move • Resulting DM changes affect ToAs: dt ~ n-2 • Observations at three bands: 50cm (700 MHz), 20cm (1400 MHz) and 10cm (3100 MHz) • Solve for DM and frequency-independent variations using Tempo2 10cm 20cm

13. Polarisation Calibration • 20cm feed has significant cross-polar coupling – causes parallactic-angle dependence of pulse profile • Corrected using PSRCHIVE routines (PCM and PAC) • Results in large improvement for highly polarised pulsars, e.g. PSR J1744-1134 • Before PCM correction: • Rms residual = 487 ns • Reduced c2 = 19.0 • After PCM correction: • Rms residual = 195 ns • Reduced c2 = 3.1

14. PPTA Timing Residuals • 6-year data span for most psrs • Offsets between instruments calibrated and removed • Polarisation calibrated and DM-corrected • Residuals after fitting for basic pulsar parameters (F0,F1) • Best rms residuals for: • PSR J0437-4715 – 75 ns • PSR J1909-3744 – 133 ns • (both at 10cm) • Significant “red” noise • Best “white” residuals: • PSR J0437-4715 – 46 ns • PSR J1909-3744 – 61 ns Rms residual 132 ns

15. Current status of PPTA Timing • “Best” band • PCM correction helps half; DM correction important for > half • About half of the sample shows evidence for red noise • Not yet applied: full polarisation (MTM) fitting, frequency-dependant templates Close to target sensitivity but still work to do!

16. Extended PPTA Data Sets • Parkes data from Swinburne timing program for 1994 – 2006 (Verbiest et al. 2008, 2009) added to PPTA data sets • Extended data sets cover up to 17 years • Instrumental offsets measured and fixed or included in fit with Cholesky algorithm to properly deal with red noise

17. Extended PPTA Data Sets • DM and PCM corrected where necessary • Residuals after fitting for astrometric parameters and F0, F1 • Best rms residuals for: • J0437-4715 (190 ns) J1909-3744 (260 ns) • Clear “red” signal for most pulsars

18. Timing-residual Spectra • Cholesky fit of harmonically related sines and cosines to post-fit time series • Dashed line is spectrum of stochastic GW background with amplitude 10-15 • GW spectrum is steeper than red timing residuals – will eventually be detectable!

19. Applications of PPTA (extended) Data Sets • Detection or limiting of the gravitational-wave background in the Galaxy – talks by Ryan and Jingbo this afternoon! • Establishment of a pulsar-based timescale – George’s talk tomorrow! • Investigating DM fluctuations and turbulence in the ISM – Mike’s talk tomorrow! • Improving knowledge of Solar-system planetary masses and detecting currently unknown Solar-system objects (Champion et al. 2010) • Investigation of effects of pulse profile fluctuations on ToAs – Stefan’s talk this afternoon! • Investigating magnetic-field structure in the ISM and Solar wind (You et al. 2007, 2012; Yan et al. 2011) • Studying properties of individual pulsars, e.g., polarisation and mean pulse profiles (Yan et al. 2011) • Related talks by Willem, Xinping, Matthew, et al.

20. The PPTA Now and Future Prospects • In terms of number of pulsars, number and precision of ToAs and data span taken together, the PPTA leads the world! • PPTA data sets and their extension provide the raw material for many investigations as well as inspiration for numerous theoretical studies • So far the PPTA project has generated 32 publications in refereed journals and 25 conference publications, with 6 or 8 in preparation • Realisation of (P)PTA goals will be aided combining PPTA data sets with those from NANOGrav and the EPTA to form the International Pulsar Timing Array (IPTA) – agreements on data sharing, publication policy and data challenges almost in place • Other PTAs based on Meerkat, GMRT, FAST and (hopefully) the proposed Xinjiang 110m telescope will contribute to IPTA data sets • In the longer term, the SKA should enable detailed studies of nanoHertz gravitational waves and their sources as well as greatly improved results for other PTA objectives

21. The Gravitational Wave Spectrum