merja tornikoski metsähovi radio observatory single-dish blazar radio astronomy first lecture:...
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Merja TornikoskiMetsähovi Radio Observatory
Single-dish blazar radio astronomy
• First lecture: Fundamentals of radio astronomy.
• Second lecture: Blazar observing techniques.
• Third lecture: Radioastronomical blazar data into blazar science.
Merja TornikoskiMetsähovi Radio Observatory
Radio astronomy
• Wavelength range ca. 100m – 100 m (MHz – THz).(Microwave/millimetre/submillimetre sub-regions).
• Broad frequency range: different kinds of antennae, receivers & technology!
• No (direct) images.• Signal usually << noise
emphasis on receiver technology and measurement methods.
• Terminology often differs from / contradicts with the terminology used in optical astronomy! (Historical and practical reasons).
Merja TornikoskiMetsähovi Radio Observatory
Radio astronomical observations
• Obvious benefits of radio astronomy:Observations can be made during daytime,+ during cloudy weather (depending on ).
• Note: possible Sun limits.• Atmospheric transmission.• Humidity, clouds, wind,
moisture/snow on the telescope/radome.
Merja TornikoskiMetsähovi Radio Observatory
Radio astronomy in blazar science
• Dynamical events relatively close to the central engine (1-10 pc) radio flux monitoring, multifrequency radio data, multifrequency data.– Reasons for activity.– Energy production.– Reprocessing of energy.
• Flux data for larger source samples: unification models etc.
• Advantages:– Radio emission mechanism is relatively well understood
(synchrotron radiation from the jet/shock) helps in constraining/testing models also in other -domains.
– Dense sampling possible (daytime obs. etc.).– Natural part of the ”big picture”.
Merja TornikoskiMetsähovi Radio Observatory
”Flux”?
Object emitsradiation
L [W/Hz]L
L
L
d
0
L = ∫ Ld [W]
luminosity”flux”
energy flux”flux”
Total flow of energy outward from a body per unit timeover all wavelengths.
Flow of energy at a certain frequency.
Merja TornikoskiMetsähovi Radio Observatory
Radiation propagates and is diluted by the distance
r
F
F = L4 r2
isotropicHz m2
W[ ]
or: S
flux density”flux”
apparent brightnessflux
r
[ Wm2
]
point source
amount of energy, measured over all wavelengths, collected per unit time crossing the unit surface area of a detector that is normal to the direction of the radiation
flux per unit bandwidth
Merja TornikoskiMetsähovi Radio Observatory
B
(surface) brightnessintensityflux per unit solid angle
BW
Hz m2 sr[ ]
flux density: integrate over the source F = ∫ B d
source
B
F
does not depend on the distance
1/r2
Note: 1. ”Flux” can mean several different things!2. For flux density: 1 jansky, Jy = 10-26 W Hz-1 m-2
d
Merja TornikoskiMetsähovi Radio Observatory
B
observe the radiation
d
dA
P
direction of incoming radiation: surface A gathers the radiationpower through A:
dW = B cos d dA d
E = ∫ ∫ ∫ ∫ ∫ B cos d dA ddt tA
Source: B ()
Telescope: ∫
∫
∫
∫
A
t
directivity
bandwidth
surface area
integration time
Merja TornikoskiMetsähovi Radio Observatory
Black body radiation
• Ideal absorber and emitter, in thermal equilibrium.• Planck formula:
B(T)= 2 h 3/ (c2 (eh/kT-1))• For low frequencies: Rayleigh-Jeans approximation:
B(T)= 2 k T 2/ c2 = 2 k T / 2
Merja TornikoskiMetsähovi Radio Observatory
Brightness temperature
• TB = the temperature that the source would have in order to produce the observed B.
• Does not need to be the physical temperature!
• Nyquist’s theorem: the corresponding derviation for the noise power flowing in a single-mode transmission line connected to a black body at temperature T leads to the one-dimensional analogue of the Planck law.
• Observing a black body or the sky/source:we observe the powerP d = k T d
Merja TornikoskiMetsähovi Radio Observatory
Source brightness temperature
TS = B
2 k(Rayleigh-Jeans)
approximately equal to Tfys, if a black bodynot equal to Tfys otherwise! (Blazars!!!)
Merja TornikoskiMetsähovi Radio Observatory
Radio telescope, antennae
• Radio telescopes are not limited by ”seeing”, but by the radiation pattern of the telescope.
• Radiation properties determined by refraction/reflection of electromagnetic radiation.
• Reciprocity principle:antenna’s transmission and reception properties are identical.
• Typically anisotropic.• Radiation pattern:
Main lobe,side + back lobes (= minor lobes).
Merja TornikoskiMetsähovi Radio Observatory
... antennae• The radiation pattern determines the beam width of the
telescope ≈ resolution. Main lobe ≈ / D.Resolution of single-dish radio telescopes poor in comparison to the optical telescopes!
• HPBW (Half-power beamwidth).
• Effective aperture Ae < Ageom,power gathering properties depend on the radiation pattern Pn ().
• Beam solid angle A ”the angle through which all the power from a transmitting antenna would stream if the power were constant over this angle and equal to the maximum value”.
Merja TornikoskiMetsähovi Radio Observatory
... antennae
Aperture efficiency η = Ae / Ag
A = 2 / Ae
Main beam solid angle: M
Minor lobe solid angle: m = A - M
A = ∫ ∫ Pn () sin d d4
Transmits to the direction the power P().
Beam efficiency M = M / A
Stray factor m = m / A
Directivity D = 4 / A
Gain G = k D = k 4 Ae / 2
Merja TornikoskiMetsähovi Radio Observatory
... antennae
• Cassegrain type:Parabolic main reflector, hyperbolic secondary reflector.Receiver at (near) the secondary focus,housed within the main telescope structure.
• Off-axis Gregorian type:Elliptical secondary.Better beam efficiency and sidelobelevels (in the on-axis system diffraction,reflection & blockage from the secondarymirror).Allows for larger prime-focus instruments.
Merja TornikoskiMetsähovi Radio Observatory
Surface accuracy/irregularities
• Good reflective characeristics.• Uniform shape over the entire area.• Uniform shape in different elevations.
• In reality, the shape is never perfect!– Gravitational forces.– Wind.– Heat: solar + other, panels + support structure.– Unevenness: panel installation, wearing out with
time, etc.
Merja TornikoskiMetsähovi Radio Observatory
... surface accuracy
• Phase error, radAffects the power in the main beam: e-2
Gaussian distribution over the whole surface.• Surface deviation (surface error), rms (e.g./20)
phase error 4 / .• Surface efficiency
η = η surf ≈ η0 e –(4)2
• Gain G = η 4 Ae / 2
• Determination and adjustment: holographic measurements.• Some examples of surface accuracy:
Metsähovi 13.7 m dish: 0.1 mm rmsSEST 15m dish: 70 m rms.
• Should be ~ 1/20 of the wavelength.
Merja TornikoskiMetsähovi Radio Observatory
Antenna temperature
• Antenna ”sees” a region of radiation through its directional pattern, the temperature of the region within the antenna beam determines the temperature of the radiation resistance. = Antenna temperature, TA.
• Not (directly) related to the physical temperature within the antenna structure!
• P = kTA [W/Hz].• The observed flux density (point source in the
beam)So = 2kTA / Ae
Merja TornikoskiMetsähovi Radio Observatory
... Antenna temperature
• There are some second order effects to TA from physical temperature!
• Ae: Heat expansion Ae decreases, increases. Heat deformation η Ae
• Pn: Heat deformation.
• Tsys: Trx includes losses from the waveguides & transmission lines, may depend on the physical temperature.
Merja TornikoskiMetsähovi Radio Observatory
Resolution
Millimetri-VLBI, 2mm
/D
Degr
Single dish radio
Ground-based optical
Interfermometry arrays
Intercontinental
Intercontinental
Merja TornikoskiMetsähovi Radio Observatory
Atmosphere
• Attenuattion.• Refraction.• Scattering.• Atmospheric emission.• ”Sky noise”.
Merja TornikoskiMetsähovi Radio Observatory
... atmosphere
• Source intensity I, optical depth towards the source Optical depth the distance travelled in the atmosphere does not need to be known.Attenuation: e-
The observed intensity: I(o) = I() e-
Radiation from the atmosphere integrated over the optical depth: I,atm = ∫ S(T(’))e-’d’The effective temperature of the atmosphere: Tatm
I,atm = S(Tatm)(1-e-’)he observed intensity: the sum of the source intensity attenuated by the atmosphere and the ”noise” from the atmosphere:I,obs = I() e- + S(Tatm)(1-e-’)
Merja TornikoskiMetsähovi Radio Observatory
... atmosphere
• In terms of the brightness temperature:TB,obs = TB() e- + Tatm(1-e-’)
he antenna temperature from the atmosphere: Tsky
(dominates the background at short wavelengths)• Atmosphere can be approximated as a plane parallel
the optical depth depends on the elevation and the optical depth in the zenith:(el) =0/sin(el)
• Note: approximation (homogeneous, plane-parallel) not always feasible: pay attention to conditions (temporal and spatial fluctuations, ”sky noise”).
Merja TornikoskiMetsähovi Radio Observatory
Signal & noise
• Note: optical ”background” ~ radio ”noise” optical ”noise” ~radio ”noise fluctuations”
• Detecting a signal: Observe changes in Tsys
(i.e. changes in the power P = k Tsys ).• Tsys ~ random event
– Bandwidth B coherence time 1/B– In one second B random events.– In seconds B random events.– Statistical noise sqrt(B).– Since the input noise is random, the relative
uncertainty T in the measurement of the noise temperature Tsys at the input of the detector:T = Tsys / sqrt(B)
Merja TornikoskiMetsähovi Radio Observatory
... signal & noise
• The smallest observable change:Tsys = Tsys crec / sqrt( B) crec : depends on the type of the receiver,Total power receiver: crec = 1Dicke-system crec = 2
• A point source produces a change in the antenna temperature: TA = Ae S /( 2 k)must be ≥ Tsys , otherwise will be lost in the noise.
smallest observable flux:
Note: usually we want S/N > 4 or 5 (or more )Smin =
2 kAe
Tsys
sqrt ( B)crec
Merja TornikoskiMetsähovi Radio Observatory
Detecting a weak signal...The signal is ”noise within noise”
Trec e.g. 1000 K
bkg. bkg.
source
Trec e.g. 100 K
bkg. bkg. bkg.
source1
source2
Merja TornikoskiMetsähovi Radio Observatory
What we want...
• Large surface area Ae (”big & good antenna”).
• Small system temperature Tsys (”good, preferably cooled, receiver”).
• Broad-band receiver B(”continuum receiver, no sideband rejection”).
• Long integration time (”plenty of observing time”).
• Minimal attenuation & scatter, small skynoise effects(”perfect weather”).
Merja TornikoskiMetsähovi Radio Observatory
Examples
1
2
Large gains are needed:
Tsys ~ 100 KB ~ 500 MHzpower P = k Tsys B ~ 10-14 WDetector needs P ~ 10 mW signal amplification ~ 1012 times (120 dB) !
Weak signals are detected:
Antenna Ae ~ 50 m2
Typical blazar S ~ 1 JyWe need to detect the rise in antenna temperature TA = Ae S / (2 k) ~ 0.02 K The signal is about 1/10000 of the noise!
Merja TornikoskiMetsähovi Radio Observatory
Future of radio astronomy?
• Radio frequencies are a ”natural resource” that must be ”conserved”!
• Radioastronomical use: passive use, active use means interference for us!
• < 30 GHz: 0.7% for ”primarily passive use”.
• 30-275 GHz: 3.0% for ”primarily passive use”.