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Radio Astronomy

Radio Astronomy. Bhal Chandra Joshi. National Centre for Radio Astrophysics - T I F R. Constellations. Galileo. Cartwheel Galaxy. Crab Supernova at many l. X-rays. Optical. Infra-red. Radio. Vela Supernova. Karl Gunther Jansky. Jansky’s observations

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Radio Astronomy

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  1. Radio Astronomy Bhal Chandra Joshi National Centre for Radio Astrophysics - T I F R

  2. Constellations

  3. Galileo

  4. Cartwheel Galaxy

  5. Crab Supernova at many l X-rays Optical Infra-red Radio

  6. Vela Supernova

  7. Karl Gunther Jansky Jansky’s observations - The direction of arrival of the hiss moved systematically round the sky everyday - The source position in the sky advanced by 15’’ every day - The source coincided with the center of our galaxy The hiss was the radio waves Generated in the region of Galactic center

  8. Diffraction Optical Images are characterized by wealth of details - resolution - ability to resolve two closely spaced points in sky What determines resolution ? - any telescope is an aperture - single slit - diffraction - interference between two point sources in the aperture - circular Airy’s pattern (Sir George Biddel Airy 1801-1892) - Rayleigh criterion ~ l/D (Lord Rayleigh – John William Strutt 1842 – 1919) Human eye – 5 mm -- 20” Optical telescope -- 1 m -- 0.1” Radio telescope -- 45 m -- 40’

  9. Engineering and Radio Astronomy - I Resolution ~ l/D Moderate Optical Telescope ~ 0.1 “ arc Radio Telescope (300m) @ l = 1m ~ 10’ arc Discoveries of pulsars, quasars, HI gas clouds in radio  Large antenna  Mechanical, structural & control system design

  10. Interferometry Intereferometry - angular resolution by measuring spatial frequencies for several pairs of antenna spread over a large area = earth rotation aperture synthesis Giant Meterwave Radio Telescope

  11. 45 m Dish - G M R T

  12. Tornado Nebula - GMRT

  13. Most Significant discoveries in Radio Astronomy • - Cosmic Microwave Background Radiation - 1963 - Quasars - 1963 • - Radio Pulsars - 1967 • Noble Prizes in Radio Astronomy • - Sir Martin Ryle & Sir Anthony Hewish 1974 • Interferometry & Pulsars • - Arnio Penzias & Robert Wilson 1978 • CMBR • - Joseph Taylor & Hulse 1993 • Binary Pulsar

  14. Penzias and Wilson

  15. Science with Radio Telescope • Study of Hydrogen in the Universe • - HI Line 1420 MHz • - rotation of Galaxies - cosmology • - Distances • -Radio Galaxies and Quasars • - Supermassive Black holes • -Gamma ray Bursts • -Supernova Remnants • -Neutron Stars and Pulsars • -Sun

  16. Radio Pulsars - A quick introduction • Radio emission in the form of a train of narrow pulses

  17. Radio Pulsars - A quick introduction • Radio emission in the form of a train of narrow pulses • rapidly rotating highly magnetized neutron stars • Pulsars are celestial lighthouses !

  18. Radio Pulsars - A quick introduction • Radio emission in the form of a train of narrow pulses • rapidly rotating highly magnetized neutron stars • Pulsars are celestial lighthouses ! • Pulsar periods are highly stable ~ 1.5 ms to 8.5 s • Pulsar period typically increases by 1 part in 1015 • Pulsars are compact stars • – D ~ 10 Km, r ~ 1014 g/cm3, I ~ 1045 gm cm2 • Pulsars have high magnetic and Electric fields • - B ~ 1012 G • A star of extremes !!

  19. How and Why of Radio Pulsars • 1. How are Neutron stars (pulsars) born ? • 2. How do Neutron stars survive collapse ? • 3. How do we know radio pulsars are rotating NS ? • 4. Why is the pulsar period so stable ? • 5. How we know that pulsars have very high magnetic fields

  20. How are Neutron star born ? In stars, pressure support is provided by nuclear burning The energy release ends when Fe nuclei reached Death of star in a spectacular expolsion - Supernova Relic core - Neutron star

  21. How NS survive eventual collapse • - Degeneracy Pressure of Neutrons • - Quantum effects at high density • - Pauli’s exclusion principle • - Electron capture and Neutron drip - r ~ 107 g/cc

  22. How do we know radio pulsars are rotating NS ? • Typical pulse widths ~ few ms implies size of 1000 Km- compact objectWhite Dwarf or NS • Periodic pulses can be emitted by • - Oscillations of star: periodic changes in size • - Orbital motion in binary system • : one star eclipsing another • - Rotation

  23. Pulsar - NS ? Oscillating star • 1. Oscillations • Restoring force - gravity • P ~ ( G r ) –1 s • White Dwarf: r ~ 107 g/cc P > 1 s • NS : r ~ 1014 g/cc P < 4 ms • Discovery ofCrab Pulsar: P - 33 ms • Vela Pulsar: P - 89 ms • Increase in pulsar period

  24. Pulsar - NS ? Binary system • Kepler’s Law : • R = 1500 M01/3 P2/3 • White Dwarf: P ~ 1 s - satellite grazing surface • Smaller periods cannot be explained • NS : P > 1 ms possible • Substantial Gravitational Radiation • Large magnitude of increase in P • Tidal disruption of companion

  25. Pulsars - Rotating NS ? • Stellar Rotation is the only viable model • : WD will be disrupted at ms P • - only NS survives • Association with Supernova Remnants - Crab Nebula • Polarization observations of Vela pulsar • “Rapidly rotating Neutron star with a strong dipolar magnetic field would act as a very energetic electric generator, which could provide a source of energy for radiation from the surrounding nebula, such as Crab Nebula” • Pacini 1967

  26. Why is the pulsar period so stable ? • Pulsars are dense compact stars • I ~ 1045 g cm2 ( 1033 for earth ) • Huge rotational kinetic energy resorvoir • Very hard brakes required • Dipolar radiation acts as a break but not sufficient

  27. How we know that pulsars have very high magnetic fields ? • Principle of conservation of energy • Slow down <-> Loss of energy • Loss of energy <-> Radiation • Estimation using “astrophysical approximation”

  28. Pulsar Research • 1. Search for new pulsars • 2. Pulsars as tools - probe of ISM • 3. Studying pulsars for pulsar’s sake • - how does a pulsar shine ? • 4. Pulsars as tools - internal structure of NS • 5. Pulsars as tool • - a laboratory for testing gravity theories

  29. Pulsar search • more than 1700 pulsars discovered till date • 800 pulsars in Parkes multibeam survey • “Normal”, “millisecond” and long period pulsars • Binary pulsars - • Double Pulsar • (Burgay et al. 2003; Lyne et al 2004)

  30. Pulsar search • Freire, Gupta, Ransom et al. 2004 McLaughlin, Joshi et al. 2005

  31. Pulsars as probe of ISM Ahuja, Gupta, Mitra et al. 2005

  32. Pulse emission • Drifting subpulses • Vivekanand & Joshi 1999 Gupta, Gil, Kijak et al. 2004

  33. Pulse Emission • Nulling Giant Pulses • Joshi & Vivekanand 2000 Joshi et al. 2004

  34. Internal structure • Pulsar glitchesKrawczak et al. 2003

  35. Pulsars as gravitational laboratory • Detection of Gravitational Radiation – PSR B1913+16(Hulse & Taylor 1975, Taylor et al. 1979, Taylor & Weisberg 1989) • Discovery of Double Pulsar - PSR J0737-3039 • - J0737-3039A ~ 23 ms • - J0737-3039B ~ 2.7 s Joshi et al. 2005

  36. Gravity testing with J0737-3039 Shapiro delay Constraint on GTR 1.337 pm 0.005 (A) 1.250 pm 0.005 (B) (Lyne et al. 2004)

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