Single-dish blazar radio astronomy

1. Single-dish blazar radio astronomy • First lecture: Fundamentals of radio astronomy. • Second lecture: Blazar observing techniques. • Third lecture: Radioastronomical blazar data into blazar science.

2. Radio astronomy • Wavelength range ca. 100m – 100 mm (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).

3. Radio astronomical observations • Obvious benefits of radio astronomy:Observations can be made during daytime,+ during cloudy weather (depending on n). • Note: possible Sun limits. • Atmospheric transmission. • Humidity, clouds, wind, moisture/snow on the telescope/radome.

4. 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 n-domains. • Dense sampling possible (daytime obs. etc.). • Natural part of the ”big picture”.

5. ”Flux”?  L = ∫ Lndn [W] 0 luminosity ”flux” Object emits radiation Total flow of energy outward from a body per unit time over all wavelengths. Ln Ln energy flux ”flux” Ln [W/Hz] L Flow of energy at a certain frequency. n dn

6. ] [ W m2  Fn r r Ln isotropic Fn = 4 p r2 Hz m2 [ ] W Radiation propagates and is diluted by the distance apparent brightness flux 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 density ”flux” flux per unit bandwidth point source or: S

7. (surface) brightness intensity flux per unit solid angle B ] [ W Bn dW Hz m2 sr source flux density: integrate over the source Fn = ∫ Bn dW W Bn does not depend on the distance µ 1/r2 Fn Note: 1. ”Flux” can mean several different things! 2. For flux density: 1 jansky, Jy = 10-26 W Hz-1 m-2

8. observe the radiation Bn dW Q direction of incoming radiation: Q surface A gathers the radiation power through A: dA dW = Bn cosQ dW dA dn P Bn cosQ dW dA dn dt E = ∫ ∫ ∫ ∫ ∫ Q W A n t Source: Bn (Q,f) Telescope: ∫ directivity Q ∫ bandwidth n ∫ surface area A ∫ integration time t

9. Black body radiation • Ideal absorber and emitter, in thermal equilibrium. • Planck formula:Bn(T)= 2 h n3/ (c2 (ehn/kT-1)) • For low frequencies: Rayleigh-Jeans approximation:Bn(T)= 2 k T n2/ c2 = 2 k T / l2

10. Brightness temperature • TB = the temperature that the source would have in order to produce the observed Bn. • 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 Pn dn = k T dn

11. Source brightness temperature l Bn TS = (Rayleigh-Jeans) 2 k approximately equal to Tfys, if a black body not equal to Tfys otherwise! (Blazars!!!)

12. 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).

13. ... antennae • The radiation pattern determines the beam width of the telescope ≈ resolution. Main lobe ≈ l / 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 (Q, f). • Beam solid angle WA”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”.

14. ... antennae WA = ∫ ∫ Pn (Q, f) sin Q dQ df 4p Transmits to the direction Q,f the power P(Q,f). Aperture efficiency η= Ae / Ag WA = l2 / Ae Main beam solid angle: WM Minor lobe solid angle: Wm = WA - WM Beam efficiency eM = WM / WA Stray factor em = Wm / WA Directivity D = 4p / WA Gain G = k D = k 4 p Ae / l2

15. ... 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.

16. 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.

17. ... surface accuracy • Phase error, f radAffects the power in the main beam: e-f2Gaussian distribution over the whole surface. • Surface deviation (surface error), e rms (e.g. l/20) phase error 4 p e / l. • Surface efficiencyη = ηsurf ≈ η0 e –(4pe/l)2 • Gain G = η 4 p Ae / l2 • Determination and adjustment: holographic measurements. • Some examples of surface accuracy:Metsähovi 13.7 m dish: 0.1 mm rmsSEST 15m dish: 70 mm rms. • Should be ~ 1/20 of the wavelength.

18. 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! • Pn = kTA [W/Hz]. • The observed flux density (point source in the beam)So = 2kTA / Ae

19. ... 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.

20. Resolution Millimetri-VLBI, 2mm Degr Single dish radio l/D Ground-based optical Interfermometry arrays Intercontinental Intercontinental

21. Atmosphere • Attenuattion. • Refraction. • Scattering. • Atmospheric emission. • ”Sky noise”.

22. ... atmosphere • Source intensity In, optical depth towards the source tOptical depth  the distance travelled in the atmosphere does not need to be known.Attenuation: e-tThe observed intensity: In(o) = In(t) e-tRadiation from the atmosphere integrated over the optical depth: In,atm = ∫ Sn(T(t’))e-t’dt’nThe effective temperature of the atmosphere: Tatm In,atm = Sn(Tatm)(1-e-t’)The observed intensity: the sum of the source intensity attenuated by the atmosphere and the ”noise” from the atmosphere:In,obs = In(t) e-t + Sn(Tatm)(1-e-t’)

23. ... atmosphere • In terms of the brightness temperature:TB,obs = TB(t) e-t + Tatm(1-e-t’) • The 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:t(el) = t0/sin(el) • Note: approximation (homogeneous, plane-parallel) not always feasible: pay attention to conditions (temporal and spatial fluctuations, ”sky noise”).

24. 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 TsysDn). • Tsys ~ random event • Bandwidth B  coherence time 1/B • In one second B random events. • In t seconds tB random events. • Statistical noise sqrt(tB). • Since the input noise is random, the relative uncertainty DT in the measurement of the noise temperature Tsys at the input of the detector:DT = Tsys / sqrt(Bt)

25. ... signal & noise 2 k Tsys crec Smin = sqrt (t B) Ae • The smallest observable change:DTsys = Tsys crec / sqrt(t 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 ≥ DTsys , otherwise will be lost in the noise. •  smallest observable flux:Note: usually we want S/N > 4 or 5 (or more  )

26. Detecting a weak signal... The signal is ”noise within noise” source Trec e.g. 1000 K bkg. bkg. source1 Trec e.g. 100 K source2 bkg. bkg. bkg.

27. 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 t(”plenty of observing time”). • Minimal attenuation & scatter, small skynoise effects(”perfect weather”).

28. Examples • 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 ~ 1012 times (120 dB) ! 1 Weak signals are detected: Antenna Ae ~ 50 m2 Typical blazar S ~ 1 Jy We 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! 2

29. 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”.

30. ... How to proceed? 1. 2. 3. Protect, Suppress ”I’m outa here, man!” Filter, Clean