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  • Slide 1
  • Pulsars and Gravity R. N. Manchester Australia Telescope National Facility, CSIRO Sydney Australia Summary Introduction to pulsars and pulsar timing Parkes pulsar surveys the double pulsar Tests of gravitational theories using pulsars The Parkes Pulsar Timing Array project
  • Slide 2
  • Pulsar Origins MSPs are very old (~10 9 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. Normal Pulsars: Formed in supernova Periods between 0.03 and 10 s Relatively young (< 10 7 years) Mostly single (non-binary) Pulsars are believed (by most people) to be rotating neutron stars Millisecond Pulsars (MSPs): (ESO VLT)
  • Slide 3
  • Spin-Powered Pulsars: A Census Number of known pulsars: 1775 Number of millisecond pulsars: 172 Number of binary pulsars: 134 Number of AXPs: 13 Number of pulsars in globular clusters: 99* Number of extragalactic pulsars: 20 Data from ATNF Pulsar Catalogue, V1.32 (www.atnf.csiro.au/research/pulsar/psrcat; Manchester et al. 2005) * Total known: 137 in 25 clusters (Paulo Freires web page)
  • Slide 4
  • Pulsars as clocks Pulsar periods are incredibly stable and can be measured precisely, e.g. on Jan 16, 1999, PSR J0437-4715 had a period of : 5.757451831072007 0.000000000000008 ms Although pulsar periods are stable, they are not constant. Pulsars lose energy and slow down: dP/dt is typically 10 -15 for normal pulsars and 10 -20 for MSPs Precise pulsar timing parameters are measured by comparing observed pulse times of arrival (TOAs) with predicted TOAs based on a model for the pulsar, then using the timing residuals - deviations from the model - to improve the model parameters and to search for unmodelled effects
  • Slide 5
  • Sources of Pulsar Timing Noise Intrinsic noise Period fluctuations, glitches Pulse shape changes Perturbations of the pulsars motion Gravitational wave background Globular cluster accelerations Orbital perturbations planets, 1 st order Doppler, relativistic effects Propagation effects Wind from binary companion Variations in interstellar dispersion Scintillation effects Perturbations of the Earths motion Gravitational wave background Errors in the Solar-system ephemeris Clock errors Timescale errors Errors in time transfer Receiver noise Instrumental errors Radio-frequency interference and receiver non-linearities Digitisation artifacts or errors Calibration errors and signal processing artifacts and errors
  • Slide 6
  • Discovered at Arecibo Observatory by Russell Hulse & Joe Taylor in 1975 Pulsar period 59 ms, a recycled pulsar Doppler shift in observed period due to orbital motion Orbital period only 7 hr 45 min Maximum orbital velocity 0.1% of velocity of light Relativistic effects detectable! PSR B1913+16: The First Binary Pulsar
  • Slide 7
  • Post-Keplerian Parameters: PSR B1913+16 Periastron advance: 4.226607(7) deg/year M = m p + m c Gravitational redshift + Transverse Doppler: 4.294(1) ms m c (m p + 2m c )M -4/3 Orbital period decay: -2.4211(14) x 10 -12 m p m c M -1/3 Given the Keplerian orbital parameters and assuming general relativity: First two measurements determine m p and m c. Third measurement checks consistency with adopted theory. (Weisberg & Taylor 2005) M p = 1.4408 0.0003 M sun M c = 1.3873 0.0003 M sun Both neutron stars!
  • Slide 8
  • PSR B1913+16 Orbit Decay Energy loss to gravitational radiation Prediction based on measured Keplerian parameters and Einsteins general relativity Corrected for acceleration in gravitational field of Galaxy P b (obs)/P b (pred) = 1.0013 0.0021.. (Weisberg & Taylor 2005) First observational evidence for gravitational waves!
  • Slide 9
  • First discovery of a binary pulsar First accurate determinations of neutron star masses First observational evidence for gravitational waves Confirmation of General Relativity as an accurate description of strong-field gravity Nobel Prize for Taylor & Hulse in 1993 The Hulse-Taylor Binary Pulsar PSR B1913+16
  • Slide 10
  • The Parkes radio telescope has found more than twice as many pulsars as the rest of the worlds telescopes put together.
  • Slide 11
  • Parkes Multibeam Pulsar Survey Principal papers: Covers strip along Galactic plane, -100 o < l < 50 o, |b| < 5 o Central frequency 1374 MHz, bandwidth 288 MHz, 96 channels/poln/beam Sampling interval 250 s, time/pointing 35 min, 3080 pointings Survey observations commenced 1997, completed 2003 Processed on work-station clusters at ATNF, JBO and McGill 740 pulsars discovered, 1015 detected At least 18 months of timing data obtained for each pulsar I: Manchester et al., MNRAS, 328, 17 (2001) System and survey description, 100 pulsars II: Morris et al., MNRAS, 335, 275 (2002) 120 pulsars, preliminary population statistics III: Kramer et al., MNRAS, 342, 1299 (2003) 200 pulsars, young pulsars and -ray sources IV: Hobbs et al., MNRAS, 352, 1439 (2004) 180 pulsars, 281 previously known pulsars V: Faulkner et al., MNRAS, 355, 147 (2004) Reprocessing methods, 17 binary/MSPs VI: Lorimer et al., MNRAS, 372, 777 (2006) 142 pulsars, Galactic population and evolution
  • Slide 12
  • Parkes Multibeam Surveys: P vs P. J1119-6127 New sample of young, high-B, long- period pulsars Large increase in sample of mildly recycled binary pulsars Three new double- neutron-star systems and one double pulsar! J0737-3039
  • Slide 13
  • The first double pulsar! Discovered at Parkes in 2003 One of top ten science break- throughs of 2004 - Science P A = 22 ms, P B = 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! PSR J0730-3039A/B
  • Slide 14
  • PSR J0737-3039B (Lyne et al., Science, 303, 1153, 2004) Double-line binary gives the mass ratio for the two stars strong constraint on gravity theories 0.2 pulse periods Orbital period MSP blows away most of B magnetosphere - dramatic effect on pulse emission (Spitkovsky & Arons 2005)
  • Slide 15
  • Binary pulsars and Gravity Tests of Equivalence Principles Limits on Parameterised Post-Newtonian (PPN) parameters Dipolar gravitational radiation dP b /dt Variation of gravitational constant G dP/dt, dP b /dt Orbit polarisation due to external field orbit circularity Binary pulsars give limits comparable to or better than Solar-system tests, but in strong-field conditions (GM/Rc 2 ~ 0.1 compared to 10 -5 for Solar-system tests)
  • Slide 16
  • PSR J1853+1303 and Nordvedt Effect Long-period binary MSP discovered in Parkes Multibeam Survey P = 4.09 ms, P b = 115 d, Ecc = 0.00002369(9), Min M comp = 0.24 M sun White dwarf companion Test of Strong Equivalence Principle: Differential acceleration in Galactic gravitational field leads to forced eccentricity (Damour & Schaefer 1991) Bayesian analysis with 20 other known low-mass wide binary pulsars | | < 5 x 10 -3 (95% confidence) Comparable to LLR limit but in strong field regime. (Stairs et al. 2005)
  • Slide 17
  • Constraints on Gravitational Theories from PSR J0737-3039A/B Mass functions: sin i < 1 for A and B Mass ratio R = M A /M B Measured value: 1.0714 0.0011 Independent of theory to 1PN order. Strong constraint! Periastron advance : 16.8995 0.0007 deg/yr Already gives masses of two stars (assuming GR): M A = 1.3381 0.0007 M sun M B = 1.2489 0.0007 M sun Star B is a very low-mass NS! Mass Function A Mass function B. (Kramer et al. Science, 314, 97, 2006)
  • Slide 18
  • GR value Measured value Improves as Periast. adv. (deg/yr) - 16.8995 0.0007 T 1.5 Grav. Redshift (ms) 0.3842 0.386 0.003 T 1.5 P b Orbit decay -1.248 x 10 -12 (-1.252 0.017) x 10 -12 T 2.5 r Shapiro range ( s) 6.15 6.2 0.3 T 0.5 s Shapiro sin i 0.99987 0.99974 T 0.5 Measured Post-Keplerian Parameters for PSR J0737-3039A/B.. GR is OK! Consistent at the 0.05% level! (Kramer et al. 2006) Non-radiative test - distinct from PSR B1913+16 +16 -39
  • Slide 19
  • PSR J0737-3039A/B Post-Keplerian Effects R: Mass ratio : periastron advance : gravitational redshift r & s: Shapiro delay P b : orbit decay (Kramer et al. 2006).. Six measured parameters Four independent tests Fully consistent with general relativity (0.05%)
  • Slide 20
  • Orbit Decay - PSR J0737-3039A/B Measured P b = (-1.252 0.017) x 10 -12 in 2.5 years Will improve at least as T 2.5 Not limited by Galactic acceleration! System is much closer to Sun - uncertainty in P b,Gal ~ 10 -16 Main uncertainty is in Shklovskii term due to uncertainty in transverse velocity and distance Scintillation gives V perp = 66 15 km s -1 Timing gives V perp ~10 km s -1 -- correction at 0.02% level VLBI measurements should give improved distance.. Will surpass PSR B1913+16 in ~5 years and improve rapidly!
  • Slide 21
  • PSR J0737-3039: More Post-Keplerian Parameters! Relativistic orbit deformation: e r = e (1 + r ) e = e (1 + ) ~ T 2.5 Should be measurable in a few years Spin orbit coupling: Geodetic precession - precession of spin axis about total angular momentum Changes in pulse profile should give misalignment angle Periastron precession - higher order terms Can give measurement of NS moment of inertia Aberration:x obs = a 1 sin i = (1 + A )x int Will change due to geodetic precession (Damour &

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