Pulsars are Cool. Seriously. Scott Ransom National Radio Astronomy Observatory / University of Virginia

Transcription

1 Pulsars are Cool. Seriously. Scott Ransom National Radio Astronomy Observatory / University of Virginia

2 Neutron Stars Spin rates up to 716 Hz Solar masses km radii Central densities several times nuclear Surface temp ~106 K Luminosity up to 10,000x the Sun's! Detailed emission mechanisms unknown Surface gravity ~1011 times Earth's Magnetic field (Gauss): Millisecond: Normal : Magnetar:

3 Neutron Stars Spin rates up to 716 Hz Solar masses km radii Central densities several times nuclear Detailed emission mechanisms unknown Surface gravity ~1011 times Earth's These are exotic objects Surface temp ~106 K Luminosity up to 10,000x the Sun's! Magnetic field (Gauss): Millisecond: Normal : Magnetar:

4 The Discovery of Pulsars PhD student Jocelyn Bell and Prof. Antony Hewish Initially Little Green Men Hewish won Nobel Prize in 1974

5 What are their radio properties? Continuum sources Typically somewhat to highly linearly polarized Steep radio spectra (index of -1 to -3, typical obs freqs GHz) Point sources Special ISM effects (freq dependent) Highly time variable Wide variety of timescales Very faint average flux density ~mjy

6 Confusion? None for pulsars! Pulsars separated via time (or spin frequency!) rather than spatially. Gain variations? Who cares?! Observations are continually on and off source. Large beam? Doesn't matter! Sub-arcsec positions come from pulsar timing. Timing solns for 33 Ter5 MSPs (VLA contours in green)

7 Fundamental Physics with Pulsars Gravitational wave detection (e.g. high precision timing) Physics at nuclear density (e.g. neutron star interiors) Strong-field gravity tests (e.g. binary pulsar dynamics) Also many others: Plasma physics (e.g. magnetospheres, pulsar eclipses) Astrophysics (e.g. stellar masses and evolution) Fluid dynamics (e.g. supernovae collapse) Magnetohydrodynamics (e.g. pulsar winds) Relativistic electrodynamics (e.g. pulsar magnetospheres) Atomic physics (e.g. NS atmospheres) Solid state physics (e.g. NS crust properties)

8 Basic Physical Information from Pulsars Rotating dipole magnet in a vacuum (I = 1045 g cm2): radiates energy and therefore spins-down (p-dot) Surface magnetic field strength (B) Spin-down luminosity (E-dot) Age (T) and Characteristic Age ( c) (braking index: n ~ 3)

9 P-Pdot Diagram Pulsar HertzsprungRussell Diagram HR Diagram: Temp (color) vs Luminosity P-Pdot Diagram Period vs Spindown rate

10 Pulsar Flavors Young High B Young (high B, fast spin, very energetic) Old Low B

11 Crab Nebula SN1054AD Pulsar rotates 30 times per second! Anasazi Indian cave pictogram, Chaco Canyon, NM

12 The Crab is visible at all energies! Red = Radio Green = Optical Blue = X-ray

13 Pulsar Flavors Young High B Young (high B, fast spin, very energetic) Pulsars move down and right across the diagram as they lose energy (assuming that the magnetic field doesn't change...) Old Low B

14 Pulsar Flavors Young High B Young (high B, fast spin, very energetic) Normal (average B, slow spin) Old Low B

15 Science with normal pulsars Used to: study the unknown pulsar emission mechanism probe the interstellar medium (scattering, scintillation, rotation measures, electron distribution) Scintillation Walker et al 2008 Measure PSR distances (HI absorption) Drifting Sub-pulses Bhattacharyya et al 2007

16 Pulsar Flavors Young High B Young (high B, fast spin, very energetic) Normal (average B, slow spin) Eventually they slow down so much that there is not enough spin to generate the electric fields which produce emission. Low B Their lifetimes are Myrs. Old

17 Pulsar Flavors Young High B Young (high B, fast spin, very energetic) Normal (average B, slow spin) Old Millisecond (low B, very fast, very old, very stable spin, best for basic physics tests) Low B

18 1982: Enigmatic bright, steep-spectrum, polarized, and scintillating radio source... Using Arecibo: 1.558ms pulsar (640 Hz)! (6 pulses) 21x faster than Crab! ~half an octave above Concert A! Courtesy Bob Rood

19 Millisecond Pulsars: via Recycling Supernova produces a neutron star Red Giant transfers matter to neutron star Alpar et al 1982 Radhakrishnan & Srinivasan 1984 Millisecond Pulsar emerges with a white dwarf companion Picture credits: Bill Saxton, NRAO/AUI/NSF

20 Pulsar Flavors Young High B Young (high B, fast spin, very energetic) Normal (average B, slow spin) ng! Old (low B, very fast, very old, very stable spin, best for basic physics tests) Re cy cli Millisecond Low B

21 The Primary Pulsar Telescopes Arecibo GBT Parkes Jodrell Bank

22 New All-Sky Pulsar Surveys All major radio telescopes are conducting all-sky pulsar surveys We know of only about 5% of the total pulsars in the Galaxy! Generate lots of data (~50MB/s!): 1000s of hrs, 1000s of channels, khz sampling: Green Bank Telescope gives more than a Petabyte! Requires huge amounts of high performance computing Processing 2 min of GBT data requires 2 days on a fast CPU! Millions of false positives

23 Dispersion Lower frequency radio waves are delayed with respect to higher frequency radio waves by the ionized interstellar medium t DM -2 High Freq Low Freq (DM = Dispersion Measure) Coherent Dedispersion exactly removes this effect, but is very computationally difficult

24 Scattering and Pulse Broadening -4.4 Multipath propagation causes frequency dependent pulse broadening.

25 Searching for New Pulsars Pulsars are: Very weak radio sources Binary pulsars show Doppler effects Often distant (therefore weaker and high DM) Predominantly found in the Galactic Plane (ISM effects) Sensitivity (A /Ttot) (tint BW)1/2 Computations Fspin3 tint2 Solutions: Use large telescopes and sensitive receivers Use longer integration times Use advanced algorithms to adaptively remove interference Use advanced algorithms to optimize sensitivity to weak binary MSPs (the hardest PSRs to detect)

26 Basic Radio Pulsar Search Recipe Step (% of CPU Time) 1. Interference identification and removal (1%) 2. De-dispersion of the raw data (5%) 3. Normal FFT search (slow pulsars) (15%) 4. Acceleration search (binary MSPs) (60%) 5. Single-pulse search (15%) 6. Sifting of candidates (<1%) 7. Folding of candidates (3%) Processing a single ~2-min pointing takes ~2 days! Big surveys have ~105 pointings, therefore 5+ CPU centuries!

27 Ter5 A (4th harm) Ter5 N (3rd harm)

28 Single Pulse Searches Some pulsars have highly variable pulse amplitudes or shut off completely (i.e. nulling) RRATs Look for dispersed individual pulses (e.g. McLaughlin & Cordes, 2003, ApJ, 596, 982) New PALFA Pulsar J

29 New Millisecond Pulsars Numbers have: quadrupled in last 10 yrs doubled in last ~3 years Why? Rise in computing capability, sensitive new radio surveys, Fermi! Year

30 Currently ~70 new Radio/gamma-ray MSPs because of Fermi! ~10% of them look like they will be good timers Courtesy: Paul Ray

31 Millisecond Pulsars are Very Precise Clocks PSR J At 12:40PM PST February : P = ms +/ ms The last digit changes by 1 every 2 minutes! This digit changes by 1 every ~4000 years! This extreme precision is what allows us to use pulsars as tools to do unique physics!

32 Pulsar Timing: Unambiguously account for every rotation of a pulsar over years Pulse Measurements (TOAs: Times of Arrival) Observation 1 Obs 2 Pulses Model (prediction) Obs 3 Time Measurement - Model = Timing Residuals Single day at telescope Time in days Predict each pulse to ~200 ns over 2 yrs!

33 Does it work? PSR J ~3yrs of Fermi gamma-ray data ~3000 photons (~3/day) ~560 binary orbits ~24 billion rotations of MSP Perfectly lined up from radio pulsar timing 2 Pulse Rotations

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35 Demorest et al. 2010, Nature

36 Ask the right question... Highly circular orbit has a radius of ~3.4 million km (~5 x Solar radius or ~9 x Earth-Moon distance)...get a spectacular answer! The measured difference between the semi-major and semi-minor axes is: 2.8 +/- 0.2 mm! Demorest et al. 2010, Nature

37 The Binary Pulsar: B First binary pulsar discovered at Arecibo Observatory by Hulse and Taylor in 1974 NS-NS Binary Ppsr = ms Porb = hrs a sin(i)/c = lt-s e = ω = 4.2 deg/yr Mc = (7) M Mp = (7) M

38 Post-Keplerian Orbital Parameters Besides the normal 5 Keplerian parameters (Porb, e, asin(i)/c, T0, ω), General Relativity gives: (Orbital Precession) (Grav redshift + time dilation) (Shapiro delay: range and shape ) where: T GM /c3 = μs, These are only functions of: M = m1 + m2, and s sin(i) - the (precisely!) known Keplerian orbital parameters Pb, e, asin(i) - the mass of the pulsar m1 and the mass of the companion m2

39 Post-Keplerian Orbital Parameters Besides the normal 5 Keplerian parameters (Porb, e, asin(i)/c, T0, ω), General Relativity gives: Need eccentric orbit and time for precession (Orbital Precession) (Grav redshift + time dilation) Need compact orbit and a lot of patience Need high precision, Inclination, and m2 (Shapiro delay: range and shape ) where: T GM /c3 = μs, These are only functions of: M = m1 + m2, and s sin(i) - the (precisely!) known Keplerian orbital parameters Pb, e, asin(i) - the mass of the pulsar m1 and the mass of the companion m2

40 The Binary Pulsar: B Three Relativistic Observables: ω, γ, Porb Indirect detection of Gravitational Radiation In 1993, Russell Hulse and Joseph Taylor were awarded the Nobel Prize for their work on PSR B ! From Weisberg & Taylor, 2003

41 The Double Pulsar: J Faster spin, more compact orbit, edge on system, 6 relativistic observables, 2 pulsars! Overall, much better than HulseTaylor binary PSR. Currently GR tests to ~0.01%! Measured vs Predicted Relativistic Shapiro Delay Kramer et al., 2006, Science, 314, 97

42 Shapiro Delay NRAO / Bill Saxton Irwin Shapiro 1964 Shapiro et al. 1968, 1971

43 J : Incredible Shapiro Delay Signal Full Shapiro Signal No General Relativity Mwd = 0.500(6) M Mpsr = 1.97(4) M! Inclination = 89.17(2) deg! Full Relativistic Solution Demorest et al. 2010, Nature, 467, 1081D see Ozel et al. 2010, ApJL, 724, 1990

44 An MSP in a Triple Stellar System Recently with GBT: a stellar triple system!

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46 Direct Gravitational Wave Detection (Pulsar Timing Array) Looking for nhz freq gravitational waves from super massive black hole binaries Need good MSPs: Significance scales with the number of MSPs being timed Must time 20+ pulsars for 10+ years at precision of ~100 nanosec! Bill Saxton (NRAO/AUI) For more information, see nanograv.org Australia Europe North America

47 Where do these GWs come from? Coalescing Super-Massive Black Holes Basically all galaxies have them Masses of Solar Masses Galaxy mergers lead to black hole mergers When BHs within 1pc, GWs are main energy loss For nearby very massive binaries, we can get 10s of nano-second timing residuals Potentially measurable with a single MSP, but much better using an array of MSPs.

48 What about the future? We only know of about 2,000 out of ~50,000+ pulsars in the Galaxy! Many of them will be Holy Grails Sub-MSP, PSR-Black Hole systems, MSP-MSP binary Several new huge telescopes... We need them because we are sensitivity limited! MeerKAT (64 dishes, SA) FAST (500m, China))

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50 Summary Pulsars are Cool. Seriously.