Transcript Otsikko

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 Tornikoski
Metsähovi Radio Observatory
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).
Merja Tornikoski
Metsähovi Radio Observatory
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.
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 n-domains.
– Dense sampling possible (daytime obs. etc.).
– Natural part of the ”big picture”.
Merja Tornikoski
Metsähovi Radio Observatory
”Flux”?
Object emits
radiation

L = ∫ Ln dn
0
luminosity
”flux”
[W]
Total flow of energy outward from a body per unit time
over all wavelengths.
Ln
Ln
energy flux
”flux”
L
dn
Merja Tornikoski
Metsähovi Radio Observatory
Ln [W/Hz]
Flow of energy at a certain frequency.
n
Radiation propagates and is diluted by the distance
apparent brightness
flux
r

flux density
”flux”
flux per unit bandwidth
r
Merja Tornikoski
Metsähovi Radio Observatory
Ln
4 p r2
or: S
2
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
Fn
Fn =
]
[W
m
point source
isotropic
[
W
Hz m2
]
(surface) brightness
intensity
flux per unit solid angle
B
Bn
d
[
flux density: integrate over the source
Bn
W
Hz m2 sr
]
source
Fn = ∫ Bn d

does not depend on the distance
Fn 1/r2
Note:
1. ”Flux” can mean several different things!
2. For flux density: 1 jansky, Jy = 10-26 W Hz-1 m-2
Merja Tornikoski
Metsähovi Radio Observatory
observe the
radiation
Bn
Q
d
direction of incoming radiation: Q
surface A gathers the radiation
power through A:
dA
dW = Bn cosQ d dA dn
P
E=∫ ∫ ∫ ∫ ∫
Q A n t
Bn cosQ d dA dndt
Source: Bn (Q,f)
Telescope:
Q
∫
directivity
∫
n
bandwidth
∫
surface area
∫
t
integration time
A
Merja Tornikoski
Metsähovi Radio Observatory
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
Merja Tornikoski
Metsähovi Radio Observatory
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
Merja Tornikoski
Metsähovi Radio Observatory
Source brightness temperature
TS =
lBn
(Rayleigh-Jeans)
2k
approximately equal to Tfys, if a black body
not equal to Tfys otherwise! (Blazars!!!)
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).
Merja Tornikoski
Metsähovi Radio Observatory
... 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 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 Tornikoski
Metsähovi Radio Observatory
... antennae
Transmits to the direction Q,fthe power P(Q,f).
A = ∫ ∫ Pn (Q,f) sin Q dQ df
4p
Aperture efficiency η = Ae / Ag
A = l2 / Ae
Main beam solid angle: M
Minor lobe solid angle: m = A - M
Beam efficiency eM = M / A
Stray factor em = m / A
Directivity D = 4p / A
Gain G = k D = k 4 p Ae / l2
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.
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.
Merja Tornikoski
Metsähovi Radio Observatory
... surface accuracy
• Phase error, f rad
Affects the power in the main beam: e-f2
Gaussian distribution over the whole surface.
• Surface deviation (surface error), e rms (e.g. l/20)
 phase error 4 pe / l.
• Surface efficiency
η = η surf ≈ 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 rms
SEST 15m dish: 70 mm rms.
• Should be ~ 1/20 of the wavelength.
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, 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
Merja Tornikoski
Metsähovi 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 Tornikoski
Metsähovi Radio Observatory
Resolution
Degr
Single dish radio
l/D
Ground-based optical
Interfermometry arrays
Intercontinental
Intercontinental
Merja Tornikoski
Metsähovi Radio Observatory
Millimetri-VLBI, 2mm
Atmosphere
•
•
•
•
•
Attenuattion.
Refraction.
Scattering.
Atmospheric emission.
”Sky noise”.
Merja Tornikoski
Metsähovi Radio Observatory
... atmosphere
• Source intensity In, optical depth towards the source t
Optical depth  the distance travelled in the atmosphere does not
need to be known.
Attenuation: e-t
The observed intensity: In(o) = In(t) e-t
Radiation from the atmosphere integrated over the optical depth:
In,atm = ∫ Sn(T(t’))e-t’dt’n
The 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’)
Merja Tornikoski
Metsähovi Radio Observatory
... 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”).
Merja Tornikoski
Metsähovi 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 Dn).
• 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)
Merja Tornikoski
Metsähovi Radio Observatory
... signal & noise
• The smallest observable change:
DTsys = Tsys crec / sqrt(t B)
crec : depends on the type of the receiver,
Total power receiver: crec = 1
Dicke-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:
Smin =
2k
Ae
Tsys
crec
sqrt (t B)
Note: usually we want S/N > 4 or 5 (or more  )
Merja Tornikoski
Metsähovi Radio Observatory
Detecting a weak signal...
The signal is ”noise within noise”
source
bkg.
Trec e.g. 1000 K
bkg.
source1
Trec e.g. 100 K
bkg.
source2
bkg.
bkg.
Merja Tornikoski
Metsähovi 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 t
(”plenty of observing time”).
• Minimal attenuation & scatter, small skynoise effects
(”perfect weather”).
Merja Tornikoski
Metsähovi Radio Observatory
Examples
Large gains are needed:
1
2
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) !
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!
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”.
Merja Tornikoski
Metsähovi Radio Observatory
... How to proceed?
1.
Protect, Suppress
3.
”I’m outa here, man!”
2.
Filter, Clean
Merja Tornikoski
Metsähovi Radio Observatory