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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 VLA in Closest Array More VLA photos • 27 antennas, each 25 m diameter • Maximum baseline 36 km VLBA: 10 25m dishes, 8000km baseline 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 Arecibo: 305m fixed dish 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 Tycho’s SN remnant Crab SNR and Pulsar W50, SNR home of microquasar SS433 Cas A: SN1680?: Inner ejecta crossing swept up shell SN 1993J in M81 from some VLBA+ VLA+ EVN+ NASA SN 1993J from VLBA Pulsars in Globular Cluster M62 “The Duck”, pulsar moving at ~500 km/s Sco X-1: jets from pulsar in binary: VLBA + APT + EVN SS 433: bullets at 0.26c 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+105 Apparent v = 1.25 c from v = 0.92 c BH 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 VLA Ultracompact 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 Atomic H in Our Galaxy: GBT et al. 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 3C31: FR I Radio Galaxy 3C 130 & 3C 449: FR I’s 3C75 in A400: Two Merging Cores of cD M87 Jet to Bubble Montage Compact Symmetric Source: 4C31.04 Canonical FR II: Cygnus A Quasar: 3C 175 3C 227: RG, z=0.086 w/ Polarization Map From Black et al., MNRAS, 256, 186 Quasars 3C215 (weird) & 3C263 (normal) 3C353: Peculiar FR II VLBA + Space antenna HALCA: 1156+295 VLBA of 3C279: Apparent Superluminal Motion with Vapp=3.5c: really V=0.997c at viewing angle of 2 degrees