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Radio Astronomy

Radio Astronomy. The 2nd window on the Universe: The atmosphere is transparent in the centimeter & meter bands < 5 mm mostly absorbed by molecular bands >15 m or so, absorbed or reflected by the ionosphere. Summary History of Radio Astronomy.

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Radio Astronomy

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  1. Radio Astronomy The 2nd window on the Universe: The atmosphere is transparent in the centimeter & meter bands < 5 mm mostly absorbed by molecular bands >15 m or so, absorbed or reflected by the ionosphere

  2. Summary History of Radio Astronomy • Karl Jansky @ Bell Labs was researching noise in “short wave” radio communication. • Aside from thunderstorms, he found (1932) a steady hiss, peaking with sidereal, not solar, time • Localized to Sagittarius (center of galaxy) 20.5 MHz • Grote Reber -- working at home, made a dish antenna @ 160 MHz: confirmed Milky Way origin • Also detected the Sun and Jupiter • WWII led to development of radar; afterwards many of these physicists and electrical engineers became • RADIO ASTRONOMERS: US, England, Netherlands, Australia, Germany & Russia

  3. Astronomical Emitters of Radio Waves • Symbiotic stars (LR/LO < 10-6 for most stars!) • “Microquasars”: some X-ray binaries • Pulsars • Supernova Remnants • Radio Galaxies • Quasars (and other AGN)

  4. Big Advantages of Radio Astronomy • Can observe DAY & NIGHT • Can penetrate clouds • Only stopped by strong winds, thunderstorms and snow! • Radio interferometry can produce better resolution than optical astronomy!

  5. Disadvantages of Radio Astronomy • Powers received are very low, since each photon has a small h •  need big collectors (dishes) • Angular resolution is poor: /d • Optical: to get ~0.5 arcsec, =500nm •  d~50 cm (but can’t do much better w/o AO or optical interferometry) • Radio: to get ~0.5 arcsec, =5cm •  d~50 km • Thus, radio astronomers need interferometers

  6. Radio Telescopes • NRAO Very Large Array • NRAO Very Long Baseline Array • NRAO Green Bank Telescope • TIFR Giant Metrewave Radio Telescope • MPIfRA Effelsberg Radio Telescope • NAIC Arecibo Radio Dish

  7. VLA in Closest Array

  8. More VLA photos • 27 antennas, each 25 m diameter • Maximum baseline 36 km

  9. VLBA:10 25m dishes, 8000km baseline

  10. GBT:largest steerable RT: 110x100 m • Asymmetric design keeps feeds off to side: no struts and diffaction from them • Works from 3m down to 3mm • Best for pulsar studies and molecular lines

  11. GMRT: largest collecting area • Mesh design, good enough for long wavelengths • 30 telescopes, 45 m aperture, maximum baseline: 25 km

  12. Effelsberg:2nd largest steerable dish • 100 m aperture • Good for 800 MHz to 96 GHz

  13. Arecibo: 305m fixed dish

  14. Some Basics of Radio Telescopes • Key considerations: • Effective area  Gain (so antenna patterns are important) • Beam width  Resolution • Bandwidth, : different feeds at different  • Wider  gives stronger signal, but narrower gives better spectral resolution • Antenna temperature: TA = P / (kB ) • Sizes of sources compared to beams • Fluxes: Sun: 410-22 W/m2/Hz @ 100 MHz 510-22 W/m2/Hz @ 10 GHz • SNR: Cas A: 210-22 W/m2/Hz @ 100 MHz • 1 Jansky = Jy = 10-26 W/m2/Hz =10-23 erg/s/cm2/Hz

  15. Radiographs • Colors usually indicate fluxes: red is brightest • Images of supernova remnants • Pulsars and nearby shocks and jets • Black holes: jets in microquasars • Star forming regions • Galactic structure • Radio galaxies • Quasars

  16. Tycho’s SN remnant

  17. Crab SNR and Pulsar

  18. W50, SNR home of microquasar SS433

  19. Cas A: SN1680?: Inner ejecta crossing swept up shell

  20. SN 1993J in M81 from some VLBA+ VLA+ EVN+ NASA

  21. SN 1993J from VLBA

  22. Pulsars in Globular Cluster M62

  23. “The Duck”, pulsar moving at ~500 km/s

  24. Sco X-1: jets from pulsar in binary: VLBA + APT + EVN

  25. SS 433: bullets at 0.26c

  26. X-ray Nova GRO J1655-40: microquasar Apparent v=1.3 c from actual speed of about 0.9c Approaching jet also has Doppler enhanced flux

  27. Superluminal Motion? • Vapp=Vsin/[1-(V/c)cos] • =1/(1-2)1/2 , with =V/c • =1/ (1- cos) • Sobs=Sem n+ , with n=2 for smooth jet and n=3 for knot or shock • For large  and small  (~1/ ) this boosting factor can be > 10000!

  28. Microquasar GRS 1915+105Apparent v = 1.25 c from v = 0.92 cBH mass about 16 Suns

  29. Star Wind Interaction w/VLBA Both O star and Wolf-Rayet star (evolved O star) eject strong winds and when they collide they form a curved hot region which radiates and accelerates charged particles

  30. W49A: from VLAUltracompact HII regions around newly forming hot stars using 7mm wavelength for high resolution

  31. M17: star forming region w/ GBT Omega nebula 3.6 cm or 8.4 GHz image

  32. Atomic H in Our Galaxy: GBT et al.

  33. M33: Doppler shifts show rotation • Used VLA measuring H 21cm spin-flip line to map atomic hydrogen, with spatial resolution of 10” • Color coded to blue approaching and red receding: velocity resolution - 1.3 km/s, • Includes Westerbork data for total intensity

  34. 3C31: FR I Radio Galaxy

  35. 3C 130 & 3C 449: FR I’s

  36. 3C75 in A400: Two Merging Cores of cD

  37. M87 Jet to Bubble Montage

  38. Compact Symmetric Source: 4C31.04

  39. Canonical FR II: Cygnus A

  40. Quasar: 3C 175

  41. 3C 227: RG, z=0.086 w/ Polarization Map From Black et al., MNRAS, 256, 186

  42. Quasars 3C215 (weird) & 3C263 (normal)

  43. 3C353: Peculiar FR II

  44. VLBA + Space antenna HALCA: 1156+295

  45. VLBA of 3C279:Apparent Superluminal Motionwith Vapp=3.5c: really V=0.997c at viewing angle of 2 degrees

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