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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.

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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
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
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
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
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
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
More VLA photos
  • 27 antennas, each 25 m diameter
  • Maximum baseline 36 km
gbt largest steerable rt 110x100 m
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
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
Effelsberg:2nd largest steerable dish
  • 100 m aperture
  • Good for 800 MHz to 96 GHz
some basics of radio telescopes
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
radiographs
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
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
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!
star wind interaction w vlba
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

slide30
W49A: from VLAUltracompact HII regions around newly forming hot stars using 7mm wavelength for high resolution
m17 star forming region w gbt
M17: star forming region w/ GBT

Omega nebula

3.6 cm or 8.4 GHz

image

m33 doppler shifts show rotation
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
3C 227: RG, z=0.086 w/ Polarization Map

From Black et al., MNRAS, 256, 186

slide45
VLBA of 3C279:Apparent Superluminal Motionwith Vapp=3.5c: really V=0.997c at viewing angle of 2 degrees