Detection of Gravitational Waves with Pulsar Timing

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1 Detection of Gravitational Waves with Pulsar Timing R. N. Manchester Australia Telescope National Facility, CSIRO Sydney Australia Summary Detection of gravitational waves Pulsar Timing Array (PTA) projects Current status and future prospects

2 Gravitational Waves Prediction of general relativity and other theories of gravity Generated by acceleration of massive object(s) Astrophysical sources: Fluctuations in inflation era Cosmic strings BH formation in early Universe Binary black holes in galaxies Coalescing neutron-star binaries Compact X-ray binaries (K. Thorne, T. Carnahan, LISA Gallery)

3 Orbital Decay in Double-Neutron-Star Systems Rapid orbital motion of two stars generates gravitational waves Energy loss causes slow decrease of orbital period First observed in Hulse-Taylor binary system PSR B ; exactly in accordance with GR prediction! First observational evidence for gravitational waves! Now detected in *** DNS systems In a few years, the Double Pulsar (PSR J A/B) will give the most precise determination (Kramer et al. 2006) PSR B Orbit Decay (Weisberg & Taylor 2005)

4 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, VIRGO, LISA LIGO Two sites in USA Perpendicular 4-km arms Spectral range Hz Initial phase now operating Advanced LIGO ~ 2014 LISA Orbits Sun, 20 o behind the Earth Three spacecraft in triangle Arm length 5 million km Spectral range Hz Planned launch ~2020

5 Limiting the GW Background with Pulsars Observed pulsar periods modulated by gravitational waves in Galaxy With observations of just a few pulsars, can only put a limit on strength of the stochastic GW background Best limits are obtained for GW frequencies ~ 1/T where T is length of data span Current best limit based on Kaspi et al. (1994) Arecibo data plus Parkes data for seven pulsars: GW energy density relative to closure density Ω gw (1/8 yr) < Consistent with all current SMBH evolutionary models (Jaffe & Backer 2003; Wyithe & Loeb 2003, Enoki et al. 2004, Sesana et al. 2008) Limits EOS of matter in inflation era and tension in cosmic strings (Grishchuk 2005; Damour & Vilenkin 2005) (Jenet et al. 2006)

6 A Pulsar Timing Array (PTA) 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 Requires observations of ~20 MSPs over 5 10 years; 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 asteroids Idea first discussed by Hellings & Downs (1983), Romani (1989) and Foster & Backer (1990)

7 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

8 Detecting a Stochastic GW Background Simulation of timingresidual correlations among 20 pulsars for a GW background from binary super-massive black holes in the cores of distant galaxies Hellings & Downs correlation function 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 µs for the rest (Jenet et al. 2005, Hobbs et al. 2009)

9 Sky positions of all known MSPs suitable for PTA studies In the Galactic disk (i.e. not in globular clusters) Short period and relatively strong circle radius ~ S 1400 /P ~60 MSPs meet criteria, but only ~30 good candidates

10 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 µs) for 9 millisecond pulsars North American pulsar timing array (NANOGrav) Data from Arecibo 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

11 The Parkes Pulsar Timing Array Project Using the Parkes 64-m radio telescope to observe 20 MSPs ~25 team members principal groups: Swinburne University (Melbourne; Matthew Bailes), University of Texas (Brownsville; Rick Jenet), University of California (San Diego; Bill Coles), ATNF (Sydney; RNM) Observations at 2 3 week intervals at three frequencies: 685 MHz, 1400 MHz and 3100 MHz New digital filterbank systems and baseband recorder system Regular observations commenced in mid-2004 Timing analysis PSRCHIVE and TEMPO2 GW simulations, detection algorithms and implications, galaxy evolution studies

12 PSR J at 10cm with PDFB2 Rms timing residual 56 ns!!

13 PPTA Pulsars: 1.5 years of PDFB2 data Timing data at 2-3 week intervals at 10cm or 20cm TOAs from 64-min observations (except J , J , J , each 32 min) Uncorrected for DM variations Solve for position, F0, F1, Kepler parameters if binary Four pulsars with rms timing residuals < 200 ns, eleven < 1 µs Best results on J (80 ns), J (110 ns), J (170ns) Approaching our goal but not there yet! Name Period (ms) DM (cm -3 pc) Orbital period (d) Band Rms Residual ( s) J cm 0.08 J cm 0.54 J cm 1.27 J cm 1.80 J cm 1.06 J cm 1.59 J cm 0.28 J cm 0.96 J cm 0.94 J cm 0.20 J cm 1.62 J cm 2.89 J cm 0.41 J cm 1.95 J cm 0.45 J cm 0.11 J cm 0.17 J cm 2.86 J cm 1.49 J cm 0.36

14 The Stochastic GW Background Super-massive binary black holes in the cores of galaxies formed by galaxy mergers GW in PTA range when orbital period ~10 years Strongest signal from galaxies with z ~ 1 BH masses ~ M 8 nhz 100 nhz Expect detectable signal with current PTAs! (Sesana, Vecchio & Colacino 2008)

15 GW from Formation of Primordial Black-holes Black holes of low to intermediate mass can be formed at end of the inflation era from collapse of primordial density fluctuations difficult to form at later times Intermediate-mass BHs (IMBH) proposed as origin of ultra-luminous X-ray sources; lower mass BHs may be dark matter Collapse to BH generates a spectrum of gravitational waves depending on mass Pulsar timing already rules out formation of Black Holes in mass range M! (Saito & Yokoyama 2009)

16 Sensitivity Single-source Detection Localisation with PPTA PPTA SKA Predicted merger rates for 5 x 10 8 M binaries (Wen et al. 2009, Sesana et al. 2009) PPTA can t detect individual binary systems - but SKA will! (Anholm et al. 2008) Need better sky distribution of pulsars - international PTA collaborations are important!

17 Measuring Planetary Masses Search for planet-mass errors in Solarsystem ephemeris used for baryctr correction Jupiter is best candidate 11-year orbit Using DE421 with: B (Arecibo, Effelsberg & Parkes, 22 years); J , J , J (Parkes, 6-14 years) Best result (so far) with just Parkes data on three pulsars: M Jupiter = (10) x 10-4 M Sun Much better than best published result; ~ 6 times worse than unpublished Galileo value used in DE421, but consistent with it More pulsars, more data span, should give best available value! (Champion et al., in prep)

18 A Pulsar Timescale Terrestrial time defined by a weighted average of caesium clocks at time centres around the world Correction of TAI to give TT(BIPMXX) each year Revisions of TT(BIPM) show variations of up to 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 (σ z ) comparable to differences between best atomic timescales 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

19 Summary Precision timing of pulsars is a great tool which has given the first observational evidence for the existence of gravitational waves We are now approaching the level of TOA precision that is required to achieve the main goals of PTA projects Good chance that detection of nanohertz GW will be achieved with a further 5-10 years of data if current predictions are realistic Major task is to eliminate all sources of systematic error - good progress, but not there yet Progress toward all goals will be enhanced by international collaboration - more (precise) TOAs and more pulsars are better! Current efforts will form the basis for detailed study of GW and GW sources by future instruments with higher sensitivity, e.g. SKA

20 The Gravitational Wave Spectrum

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22 Timing Stability of MSPs 10-year data span for 20 PPTA MSPs Includes 1-bit f/b, Caltech FPTM and CPSR2 data σ z : frequency stability at different timescales τ For white timing residuals, expect σ z ~ τ -3/2 Most pulsars roughly consistent with this out to 10 years Good news for PTA projects! 10 µs 100 ns (Verbiest et al. 2009)

23 DFB for 10cm/20cm CPSR2 for 50cm About 6 yr data span At 20cm, DM of 10-4 cm -3 pc corresponds to t = 210 ns Will be applied to pipeline processing Algorithm development by Xiaopeng You, George Hobbs and Stefan Oslowski Dispersion Corrections

24 PTA Pulsars: Timing Residuals 30 MSPs being timed in PTA projects world-wide Circle size ~ (rms residual) MSPs being timed at more than one observatory