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Merja Tornikoski Metsähovi 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.
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Merja Tornikoski Metsähovi 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).
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Merja Tornikoski Metsähovi 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.
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Merja Tornikoski Metsähovi 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”.
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Merja Tornikoski Metsähovi Radio Observatory ”Flux”? Object emits radiation L [W/Hz] L L L d 0 L = ∫ L d [W] luminosity ”flux” energy flux ”flux” Total flow of energy outward from a body per unit time over all wavelengths. Flow of energy at a certain frequency.
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Merja Tornikoski Metsähovi Radio Observatory Radiation propagates and is diluted by the distance r F F = L 4 r 2 isotropic Hz m 2 W [] or: S flux density ”flux” apparent brightness flux r [ W m2m2 ] 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
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Merja Tornikoski Metsähovi Radio Observatory B (surface) brightness intensity flux per unit solid angle B W Hz m 2 sr [ ] flux density: integrate over the source F = ∫ B d source B F does not depend on the distance 1/r 2 Note: 1. ”Flux” can mean several different things! 2. For flux density: 1 jansky, Jy = 10 -26 W Hz -1 m -2 dd
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Merja Tornikoski Metsähovi Radio Observatory B observe the radiation dd dA P direction of incoming radiation: surface A gathers the radiation power through A: dW = B cos d dA d E = ∫ ∫ ∫ ∫ ∫ B cos d dA d dt tA Source: B ( ) Telescope:∫ ∫ ∫ ∫ A t directivity bandwidth surface area integration time
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Merja Tornikoski Metsähovi Radio Observatory Black body radiation Ideal absorber and emitter, in thermal equilibrium. Planck formula: B (T)= 2 h 3 / (c 2 (e h /kT -1) ) For low frequencies: Rayleigh-Jeans approximation: B (T)= 2 k T 2 / c 2 = 2 k T / 2
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Merja Tornikoski Metsähovi Radio Observatory Brightness temperature T B = 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 power P d = k T d
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Merja Tornikoski Metsähovi Radio Observatory Source brightness temperature T S = B 2 k (Rayleigh-Jeans) approximately equal to T fys, if a black body not equal to T fys otherwise! (Blazars!!!)
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Merja Tornikoski Metsähovi 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).
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Merja Tornikoski Metsähovi 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 A e < A geom, power gathering properties depend on the radiation pattern P n ( ). 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”.
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Merja Tornikoski Metsähovi Radio Observatory... antennae Aperture efficiency η = A e / A g A = 2 / A e Main beam solid angle: M Minor lobe solid angle: m = A - M A = ∫ ∫ P n ( ) sin d d 44 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 A e / 2
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Merja Tornikoski Metsähovi 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 sidelobe levels (in the on-axis system diffraction, reflection & blockage from the secondary mirror). Allows for larger prime-focus instruments.
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Merja Tornikoski Metsähovi 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.
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Merja Tornikoski Metsähovi Radio Observatory... surface accuracy Phase error, rad Affects 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 ≈ η 0 e –(4 ) 2 Gain G = η 4 A e / 2 Determination and adjustment: holographic measurements. Some examples of surface accuracy: Metsähovi 13.7 m dish: 0.1 mm rms SEST 15m dish: 70 m rms. Should be ~ 1/20 of the wavelength.
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Merja Tornikoski Metsähovi 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, T A. Not (directly) related to the physical temperature within the antenna structure! P = kT A [W/Hz]. The observed flux density (point source in the beam) S o = 2kT A / A e
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Merja Tornikoski Metsähovi Radio Observatory... Antenna temperature There are some second order effects to T A from physical temperature! A e : Heat expansion A e decreases, increases. Heat deformation η A e P n : Heat deformation. T sys : T rx includes losses from the waveguides & transmission lines, may depend on the physical temperature.
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Merja Tornikoski Metsähovi Radio Observatory Resolution Millimetri-VLBI, 2mm /D Degr Single dish radio Ground-based optical Interfermometry arrays Intercontinental
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Merja Tornikoski Metsähovi Radio Observatory Atmosphere Attenuattion. Refraction. Scattering. Atmospheric emission. ”Sky noise”.
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Merja Tornikoski Metsähovi 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: T atm I,atm = S (T atm )(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 (T atm )(1-e - ’ )
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Merja Tornikoski Metsähovi Radio Observatory... atmosphere In terms of the brightness temperature: T B,obs = T B ( ) e - + T atm (1-e - ’ ) he antenna temperature from the atmosphere: T sky (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”).
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Merja Tornikoski Metsähovi Radio Observatory Signal & noise Note: optical ”background” ~ radio ”noise” optical ”noise” ~radio ”noise fluctuations” Detecting a signal: Observe changes in T sys (i.e. changes in the power P = k T sys ). 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 )
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Merja Tornikoski Metsähovi Radio Observatory... signal & noise The smallest observable change: T sys = T sys c rec / sqrt( B) c rec : depends on the type of the receiver, Total power receiver: c rec = 1 Dicke-system c rec = 2 A point source produces a change in the antenna temperature: T A = A e S /( 2 k) must be ≥ T sys, otherwise will be lost in the noise. smallest observable flux: Note: usually we want S/N > 4 or 5 (or more ) S min = 2 k AeAe T sys sqrt ( B) c rec
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Merja Tornikoski Metsähovi Radio Observatory Detecting a weak signal... The signal is ”noise within noise” T rec e.g. 1000 K bkg. source T rec e.g. 100 K bkg. source1 source2
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Merja Tornikoski Metsähovi Radio Observatory What we want... Large surface area A e (”big & good antenna”). Small system temperature T sys (”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”).
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Merja Tornikoski Metsähovi Radio Observatory Examples 1 2 Large gains are needed: Tsys ~ 100 K B ~ 500 MHz power P = k Tsys B ~ 10 -14 W Detector needs P ~ 10 mW signal amplification ~ 10 12 times (120 dB) ! Weak signals are detected: Antenna A e ~ 50 m 2 Typical blazar S ~ 1 Jy We need to detect the rise in antenna temperature T A = A e S / (2 k) ~ 0.02 K The signal is about 1/10000 of the noise!
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Merja Tornikoski Metsähovi 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”.
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Merja Tornikoski Metsähovi Radio Observatory... How to proceed? 1. 2. 3. Protect, Suppress Filter, Clean ”I’m outa here, man!”
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