Pulsar Timing Arrays
Download
1 / 32

Pulsar Timing Arrays - PowerPoint PPT Presentation


  • 237 Views
  • Uploaded on

Pulsar Timing Arrays. R. N. Manchester. CSIRO Astronomy and Space Science Sydney Australia. Image: Swinburne Astronomy. Pulsar Timing Arrays ( PTAs). A PTA consists of many pulsars widely distributed on the sky with frequent high-precision timing observations over a long data span

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' Pulsar Timing Arrays' - umay


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

Pulsar Timing Arrays

R. N. Manchester

CSIRO Astronomy and Space Science Sydney Australia

Image: Swinburne Astronomy


Pulsar Timing Arrays (PTAs)

  • A PTA consists of many pulsars widely distributed on the sky with frequent high-precision timing observations over a long data span

  • Can in principle detectgravitational waves (GW)

    • GW passing over the pulsars are uncorrelated

    • GW passing over Earth produce a correlated signal in the TOA residuals for all pulsars

  • Most likely source of GW detectable by PTAs is a stochastic background from super-massive binary black holes in distant galaxies

  • Requires observations of ~20 MSPs over ~10 years; could give the first direct detection of gravitational waves!

  • A timing array can also detect instabilities in terrestrial time standards – establish a pulsar timescale

  • Idea first discussed by Hellings & Downs (1983), Romani (1989) and Foster & Backer (1990)


    Orbital Decay in PSR B1913+16

    • Orbital motion of two stars generates gravitational waves

    • Energy loss causes slow decrease of orbital period

    • Predict rate of orbit decay from known orbital parameters and masses of the two stars using GR

    • Ratio of measured value to predicted value = 0.997 +/- 0.002

    PSR B1913+16 Orbit Decay

    • Confirmation of general relativity!

    • First observational evidence for gravitational waves!

    (Weisberg , Nice & Taylor 2010)


    Correlated Signals in a PTA

    • Clock errors

      All pulsars have the same TOA variations: Monopolesignature

    • Solar-System ephemeris errors

      Dipole signature

    • Gravitational waves

      Quadrupolesignature

    Hellings & Downs GW correlation function

    Can separate these effects provided there is a sufficient number of widely distributed pulsars

    (Hobbs et al. 2009)


    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

      • Timing 23 millisecond pulsars, data spans 5 - 18 years

    • North American pulsar timing array (NANOGrav)

      • Data from Arecibo and Green Bank Telescope

      • Timing 40 millisecond pulsars, data spans 1 - 29 years

    • Parkes Pulsar Timing Array (PPTA)

      • Data from Parkes 64m radio telescope in Australia

      • Timing 24 millisecond pulsars, data spans 3 – 19 years


    The Parkes Pulsar Timing Array Collaboration

    • CSIRO Astronomy and Space Science, Sydney

      George Hobbs, Dick Manchester, Ryan Shannon, Matthew Kerr, Aidan Hotan, John Sarkissian, John Reynolds, (Mike Keith)

    • Swinburne University of Technology, Melbourne

      Matthew Bailes, Willem van Straten, (Stefan Oslowski)

    • Monash University, Melbourne

      • Yuri Levin

    • University of Melbourne

      • Vikram Ravi(Stuart Wyithe)

    • University of Western Australia, Perth

    • Linqing Wen, Xingjiang Zhu

    • Curtin University, Perth

    • Ramesh Bhat

    • University of California, San Diego

      Bill Coles

    • Jet Propulsion Laboratory, Pasadena

      Sarah Burke-Spolaor

    • NSSC/NAOC, Beijing

    • Xinping Deng, Shi Dai

    • Xinjiang Astronomical Observatory, Urumqi

      • Jingbo Wang

    • Southwest University, Chongqing

      XiaopengYou

    Manchester et al. (2013)www.atnf.csiro.au/research/pulsar/ppta


    The PPTA Pulsars

    All (published) MSPs not in globular clusters


    Dispersion Corrections

    • Uncorrected DM variations add noise to timing data

    • Contribute power to unmodelled signals, e.g. gravitational waves

    Correction for these variations is essential!

    DDM = 10-4 cm-3 pc

    DM (10-4 cm-3 pc)

    • PPTA observes all pulsars in three bands: 50cm, 20cm, 10cm

    • Best band after DM corrections used for further analysis

    DtDM = 212 ns at 1.4 GHz

    (Keith et al. 2013)

    MJD


    PPTA Timing Residuals

    • Timing data for 22 pulsars

    • Data spans to 19 years

    • Best 1-year rms timing residuals about 40 ns, most < 1ms

    • Low-frequency (red) variations significant in about half of sample – some due to uncorrected DM variations in early data, e.g., J1045-4509

    • Several of best-timing pulsars nearly white (e.g., J0437-4715, J1713+0747, J1909-3744)


    Gravitational Wave Background

    • Most likely detectable source of GW for PTAs is background from binary supermassive black holes in distant galaxies

    • Simple parameterisation of characteristic strain for cosmological distribution of circular BH binaries:

    • Induced modulation of timing residuals has spectrum ~ f -13/3– PTAs most sensitive to signals with f ~ 1/Tspan ~ nHz

    • GW spectrum may be dominated by a strong individual source

    (Sesana 2012)


    Limiting the GW Background

    • Can use auto-correlations –pulsar terms contribute power to ACF

    • Detection statistic:

    Weights:

    Spectral model:

    GW signal:

    Expected signal for A95

    • Used six best PPTA pulsars (fPPTA=2.8 nHz)

    • 95% confidence limit on GW signal in data:

    Observed spectrum

    M

    W

    Fitted GW signal (A=1.2x10-15)

    A95 < 2.4 x 10-15

    Rel. energy density

    WGW(fPPTA) < 1.3 x 10-9

    (Shannon et al. 2013)


    Current PPTA Limit

    95% Confidence Limits

    Gaussian

    Probability of a GW signal in the PPTA data

    Non-gaussian

    Millennium

    First observational challenge to physical models for the GW background!

    Hydro sim.

    (Kuiler et al. 2013)

    (Sesana 2013)

    (McWilliams et al. 2012)

    (Shannon et al., 2013, Science)


    TT(PPTA2011) – Relative to TAI

    • PPTA extended data set, 19 pulsars

    • Timing referenced to TT(TAI)

    • Clock term sampled at ~1 yr intervals

    • Constrained to have no quadratic or annual terms

    • Compared with BIPM2010 – TAI with quadratic removed

    PPTA

    BIPM2010

    First realisation of a pulsar timescale with stability comparable to that of current atomic timescales!

    (Hobbs et al. 2012)


    The International Pulsar Timing Array

    • The IPTA is a consortium of consortia, formed from existing PTAs

    • Currently three members: EPTA, NANOGrav and PPTA

    • Aims are to facilitate collaboration between participating PTA groups and to promote progress toward PTA scientific goals

    • There is a Steering Committee which sets policy guidelines for data sharing, publication of results etc.

    • The IPTA organises annual Student Workshops and Science Meetings – 2013 meetings were in Krabi, Thailand June 17-28

    • The IPTA has organisedData Challenges for verification of GW detection algorithms


    IPTA Data Sets: 50 MSPs

    Bands

    Black: 70cm

    Red: 50cm

    Green: 35cm

    Blue: 20cm

    Aqua: 15cm

    Red: 10cm

    1984

    2013


    Future Prospects

    • Continuing searches will increase number of known MSPs

    • Very high sensitivity of FAST and SKA should allow timing of 100 – 200 weaker MSPs

    • Smaller telescopes will continue to have an important role – because of jitter noise, minimum useful observation time ~ 30 min

    Combined IPTA data sets should give a GW detection within the next 10 years

    FAST

    SKA Mid-Frequency Array

    2017

    2020+


    A Pulsar Census

    • Currently 2302 known (published) pulsars

    • 2132 rotation-powered disk pulsars

    • 224 in binary systems

    • 317 millisecond pulsars

    • 142 in globular clusters

    • 8 X-ray isolated neutron stars

    • 21 magnetars (AXP/SGR)

    • 28 extra-galactic pulsars

    Data from ATNF Pulsar Catalogue, V1.48 (www.atnf.csiro.au/research/pulsar/psrcat) (Manchester et al. 2005)


    Detection of Gravitational Waves

    • Generated by acceleration of massive objects in Universe, e.g. binary black holes

    • 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

    eLISA

    • Two sites in USA

    • Perpendicular 4-km arms

    • Spectral range 10 – 500 Hz

    • Initial phase now operating

    • Advanced LIGO ~ 2016

    • Orbits Sun, 20o behind the Earth

    • Two or three spacecraft

    • Arm length 5 million km

    • Spectral range 10-4 – 10-1 Hz

    • Planned launch ~2028


    The Isotropic GW Background

    • Strongest source of nHz GW waves is background from orbiting super-massive black holes in distant galaxies

    Rate of energy loss to GW:

    Chirp mass:

    Frequency in source frame:

    • Number of mergers per comoving volume based on model for galaxy evolution (e.g. Millennium simulation) plus model for formation and evolution of SMBH in galaxies

    (Sesana 2013)


    • distSimpleparameterisation of characteristic strain for cosmological ributionof circular BH binaries:

    A is the characteristic strain at a GW frequency f = 1/1 yr

    • GW modulates observed pulsar frequency

    • Modulation spectrum of observed timing residuals:

    GW power ~ f -13/3

    • Pulsar timing arrays are most sensitive to GW signals with frequency f ~ 1/Tspan~ few nanoHertz

    • Strength of GW background often expressed as a fraction of closure energy density of the Universe:

    (Phinney 2001; Jenet et al. 2006)


    Stochastic GW Background: Distribution of SMBBH

    • Most of background from SMBBH in galaxies at z = 1-2

    • Biggest contribution from largest BH masses: 108 – 109Msun

    (Sesana et al. 2008)


    Localisation of GW Sources

    • Fits quadrupolar signature to arbitrary waveforms for multiple pulsars – good for continuous or burst sources

    • Strong GW source injected into PPTA data sets

    • Grid search over sky to measure detection significance as function of position

    • “Blind” search: independent software for injection and detection

    Source detected at close to correct position

    (George Hobbs and Ryan Shannon)



    DM Correction

    • Observed ToAs are sum of frequency-independent “common-mode” terms tCM(e.g., clock errors, GW, etc) and interstellar delays tDM – assume ~ l2

    • The interstellar term tDM is noise – want to minimise it

      • Observe at ~zero wavelength, i.e., X-ray or g-ray

      • Observe at two or more wavelengths, l1 and l2 (with l1 > l2)

    • Can then solve for tDM and tCM:


    Effect of CM Term

    No CM

    Pre-fit (Wh+GW+DM)

    • If CM term not included in fit, power is extracted from freq-independent variations and coupled into DM variations

    • With CM term included, all freq-independent power (e.g., GW signal, clock errors) is contained in CM values

    Pre-fit (Wh+GW)

    Post-fit

    1 yr-1

    10 yr-1

    CM incl.

    (Keith et al. 2013)

    Power spectra of timing residuals


    MSP Polarisation

    • Grand-average profiles for PPTA pulsars, very high S/N

    • New profile features, complex polarisation properties

    (Yan et al. 2010)


    Pulsars as Clocks

    • Because of their large mass and small radius, NS spin rates - and hence pulsar periods – are extremely stable

    • For example, in 2001, PSR J0437-4715 had a period of :

    5.7574519243621370.000000000000008 ms

    • Although pulsar periods are very stable, they are not constant

    • Pulsars are powered by their rotational kinetic energy

    • They lose energy to relativistic winds and low-frequency electromagnetic radiation (the observed pulses are insignificant)

    • Consequently, all pulsars slow down (in their reference frame)

    • Typical slowdown rates are less than a microsecond per year

    • For millisecond pulsars, slowdown rates are ~105 smaller


    Measurement of pulsar periods

    • Start observation at a known time and average 103 - 105 pulses to form a 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 ToAsover days – weeks – months – years

    • Transfer ToAsto an inertial frame – the solar system barycentre

    • Compare barycentricToAswith predicted values from a model for the pulsar – the 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!


    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

    Pulsars are powerful probes of a wide range of astrophysical phenomena

    • 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


    PSR J0730-3039A/B

    The first double pulsar!

    • Discovered at Parkes in 2003

    • One of top ten science break-throughs of 2004 - Science

    • PA = 22 ms, PB = 2.7 s

    • Orbital period 2.4 hours!

    • Periastron advance 16.9 deg/yr!

    (Burgay et al., 2003; Lyne et al. 2004)

    Highly relativistic binary system!


    Measured Post-Keplerian Parameters for PSR J0737-3039A/B

    GR value Measured value Improves as

     Periast. adv. (deg/yr) - 16.8995  0.0007 T1.5

     Grav. Redshift (ms) 0.3842 0.386  0.003 T1.5

    Pb Orbit decay -1.248 x 10-12 (-1.252  0.017) x 10-12 T2.5

    r Shapiro range (s) 6.15 6.2  0.3 T0.5

    s Shapiro sin i 0.99987 0.99974 T0.5

    .

    .

    +16 -39

    GR is OK! Consistent at the 0.05% level!

    Non-radiative test - distinct from PSR B1913+16

    (Kramer et al. 2006)


    ad