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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
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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
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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: 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
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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?
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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