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 dndt
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 =
lBn
(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,fthe 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 pe / 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