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Transcript

Scuola nazionale de Astrofisica Radio Pulsars 2: Timing and ISM Outline • Timing methods • Glitches and timing noise • Binary pulsar timing • Post-Keplerian effects, PSR B1913+16 • Dispersion, pulsar distances • Faraday Rotation – Galactic magnetic field • Scintillation: DISS, RISS 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 • Young pulsars suffer period irregularities and glitches (DP/P <~ 10-6) but these are weak or absent in MSPs Techniques of Pulsar Timing • Need telescope, receiver, spectrometer (filterbank, digital correlator, digital filterbank or baseband system), data acquisition system • Start observation at known time and synchronously average 1000 or more pulses (typically 5 - 10 minutes), dedisperse and sum orthogonal polarisations to get mean total intensity (Stokes I) 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 (tobs) over days – weeks – months – years • TOA rms uncertainty: • Correct observed TOA to infinite frequency at Solar System Barycentre (SSB) tclk: Observatory clock correction to TAI (= UTC + leap sec), via GPS D: dispersion constant (D = DM/(2.41x10-16) s DR: propagation (Roemer) delay to SSB (Uses SS Ephemeris, e.g. DE405) DS: Solar-system Shapiro delay DE: Einstein delay at Earth Timing Techniques (continued) • Have series of TOAs corrected to SSB: ti • Model pulsar frequency by Taylor series, integrate to get pulse phase ( = 1 => P) • Choose t = 0 to be first TOA, t0 • Form residual ri = i - ni, where ni is nearest integer to i • If pulsar model is accurate, then ri << 1 • Corrections to model parameters obtained by making least-squares fit to trends in ri • Timing program (e.g. TEMPO or TEMPO2) does SSB correction, computes ri and improved model parameters • Can solve for pulsar position from error in SSB correction • For binary pulsar, there are additional terms representing Roemer and other (relativistic) delays in binary system Sources of Timing “Noise” Intrinsic noise • Period fluctuations, glitches • Pulse shape changes Perturbations of pulsar 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 Receiver noise Spin Evolution • For magnetic dipole radiation, braking torque ~ 3 • Generalised braking law defines braking index n • n = 3 for dipole magnetic field • Measured for ~8 pulsars Crab: n = 2.515 PSR B1509-58: n = 2.839 • Can differentiate again to give second braking index m, expected value mo • Secular decrease in n observed for Crab and PSR B1509-58 • For PSR B1509-58, mo = 13.26, m = 18.3 2.9 • Implies growing magnetic field (Livingston et al. 2005) Derived Parameters • Actual age of pulsar is function of initial frequency or period and braking index (assumed constant) • For P0 << P, n = 3, have “characteristic age” • If know true age, can compute initial period • From braking equation, can derive B0, magnetic field at NS surface, R = NS radius. Gives value at NS equator; value at pole 2B0 • Numerical value assumes R = 10 km, I = 1045 gm cm2, n=3 • For dipole field, can derive magnetic field at light cylinder • Especially for MSPs, these values significantly modified by “Shklovskii term” due to transverse motion, . e.g. for PSR J0437-4715, 65% of observed P is due to Shklovskii term Pulsar Glitches First Vela glitch (Radhakrishnan & Manchester 1969) (Wang et al. 2000) Probably due to sudden unpinning of vortices in superfluid core of the neutron star transferring angular momentum to the solid crust. Quasi-exponential recovery to equilibrium slowdown rate. Intrinsic Timing Noise • Quasi-random fluctuations in pulsar periods • Noise typically has a very ‘red’ spectrum • Often well represented by a cubic term in timing residuals Stability D8 measured with data span of 108 s ~ 3 years used as a noise parameter Binary pulsars • Some pulsars are in orbit around another star. Orbital periods range from 1.6 hours to several years • Only a few percent of normal pulsars, but more than half of all millisecond pulsars, are binary. • Pulsar companion stars range from very low-mass white dwarfs (~0.01 solar masses) to heavy normal stars (10 - 15 solar masses). • Five or six pulsars have neutron-star companions. • One pulsar has three planets in orbit around it. Keplerian parameters: Pb: Orbital period x = ap sin i: Projected semi-major axis : Longitude of periastron e: Eccentricity of orbit T0: Time of periastron Kepler’s Third Law: (Lorimer & Kramer 2005) PSR B1913+16 From first-order (non-relativistic) timing, can’t determine inclination or masses. Mass function: For minimum mass, i = 90o For median mass, i = 60o PSR B1257+12 – First detection of extra-solar planets A: 3.4 Earth masses, 66.5-day orbit B: 2.8 Earth masses, 98.2-day orbit C: ~ 1 Moon mass, 25.3-day orbit Wolszczan & Frail (1992); Wolszczan et al. (2000) Post-Keplerian Parameters Expressions for post-Keplerian parameters depend on theory of gravity. For general relativity: . : Periastron precession : Time dilation and grav. redshift r: Shapiro delay “range” s: Shapiro delay “shape” . Pb: Orbit decay due to GW emission geod: Frequency of geodetic precession resulting from spin-orbit coupling PSR B1913+16: . . , , Pb measured PSR J0737-3039A/B . . , , r, s, Pb measured Shapiro Delay - PSR J1909-3744 • P = 2.947 ms • Pb = 1.533 d • Parkes timing with CPSR2 • Rms residuals: 10-min: 230 ns Daily (~2 hr): 74 ns • From Shapiro delay: i = 86.58 0.1 deg mc = 0.204 0.002 Msun • From mass function: mp = 1.438 0.024 Msun (Jacoby et al. 2005) Post-Keplerian Parameters: PSR B1913+16 Given the Keplerian orbital parameters and assuming general relativity: • Periastron advance: 4.226607(7) deg/year M = mp + mc • Gravitational redshift + Transverse Doppler: 4.294(1) ms mc(mp + 2mc)M-4/3 • Orbital period decay: -2.4211(14) x 10-12 mp mc M-1/3 First two measurements determine mp and mc. Third measurement checks consistency with adopted theory. Mp = 1.4408 0.0003 Msun Mc = 1.3873 0.0003 Msun Both neutron stars! (Weisberg & Taylor 2005) PSR B1913+16 Orbit Decay • Energy loss 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) PSR B1913+16 The Hulse-Taylor Binary Pulsar • 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 Interstellar Dispersion Ionised gas in the interstellar medium causes lower radio frequencies to arrive at the Earth with a small delay compared to higher frequencies. Given a model for the distribution of ionised gas in the Galaxy, the amount of delay can be used to estimate the distance to the pulsar. Dispersion & Pulsar Distances • For pulsars with independent distances (parallax, SNR association, HI absorption) can detemine mean ne along path. Typical values ~ 0.03 cm-3 • From many such measurements can develop model for Galactic ne distribution, e.g. NE2001 model (Cordes & Lazio 2002) • Can then use model to determine distances to other pulsars Faraday Rotation & Galactic Magnetic Field (Han et al. 2005) Interstellar Scintillation • Small-scale irregularities in the IS electron density deflect and distort the wavefront from the pulsar • Rays from different directions interfere resulting in modulation in space and frequency - diffractive ISS • Motion of the pulsar moves the pattern across the Earth • Larger-scale irregularities cause focussing/defocussing of wavefront - refractive ISS Dynamic Spectra resulting from DISS (Bhat et al., 1999) DISS Secondary Spectrum • Take 2-D Fourier transform of dynamic spectra • Sec spectrum shows remarkable parabolic structures • Not fully understood but main structure results from interference between core and outer rays (Stinebring 2006) ISM Fluctuation Spectrum • Spectrum of interstellar electron density fluctuations • Follows Kolmogorov power-law spectrum over 12 orders of magnitude in scale size (from 10-4 AU to 100 pc) • Mostly based on pulsar observations (Armstrong et al. 1995) End of Part 2 First detection of pulsar proper motion PSR B 1133+16 Derived proper motion: 375 mas yr-1 Manchester et al. (1974)