1. 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.

2. 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).

3. 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.

4. 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”.

5. 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.

6. 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

7. 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 dd

8. Merja Tornikoski Metsähovi Radio Observatory B observe the radiation dd 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

9. 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

10. 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

11. 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!!!)

12. 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).

13. 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”.

14. 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  44 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

15. 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.

16. 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.

17. 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.

18. 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

19. 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.

20. Merja Tornikoski Metsähovi Radio Observatory Resolution Millimetri-VLBI, 2mm /D Degr Single dish radio Ground-based optical Interfermometry arrays Intercontinental

21. Merja Tornikoski Metsähovi Radio Observatory Atmosphere Attenuattion. Refraction. Scattering. Atmospheric emission. ”Sky noise”.

22. 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 -  ’ )

23. 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”).

24. 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  )

25. 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

26. 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

27. 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”).

28. 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!

29. 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”.

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