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 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 • 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
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)
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!
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
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
More VLA photos • 27 antennas, each 25 m diameter • Maximum baseline 36 km
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
GMRT: largest collecting area • Mesh design, good enough for long wavelengths • 30 telescopes, 45 m aperture, maximum baseline: 25 km
Effelsberg:2nd largest steerable dish • 100 m aperture • Good for 800 MHz to 96 GHz
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: 410-22 W/m2/Hz @ 100 MHz 510-22 W/m2/Hz @ 10 GHz • SNR: Cas A: 210-22 W/m2/Hz @ 100 MHz • 1 Jansky = Jy = 10-26 W/m2/Hz =10-23 erg/s/cm2/Hz
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
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
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!
Microquasar GRS 1915+105Apparent v = 1.25 c from v = 0.92 cBH mass about 16 Suns
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
W49A: from VLAUltracompact HII regions around newly forming hot stars using 7mm wavelength for high resolution
M17: star forming region w/ GBT Omega nebula 3.6 cm or 8.4 GHz image
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
3C 227: RG, z=0.086 w/ Polarization Map From Black et al., MNRAS, 256, 186
VLBA of 3C279:Apparent Superluminal Motionwith Vapp=3.5c: really V=0.997c at viewing angle of 2 degrees