A rough guide to radio astronomy and its use in lensing

studiesSimple stuff other lecturers may

assume you know (and probably do)

Basic Radio Information

• Resolution. θ=λ/d– Increase resolution by increasing d with interferometer

arrays but loose surface brightness sensitivity.

– Increase resolution by decreasing λ but miss low surface brightness extended emission because this has a steep spectrum.

– Dilemma for lensing studies because want resolution but also to see extended emission for many lines of sight through the lensing potential

Basics

• Steep spectrum => extended low brightness jet or lobe emission.

• Flat spectrum => compact high brightness emission most like from the core of the AGN

• Most radio sources have a flat spectrum core, a one-sided jet and twin lobes.

Types of radio source.

• Classical double, e.g. 3C175– Steep spectrum

– Extended

– Double

– Non-variable

– A quasar

• Difficult to recognize when lensed and no use for Hubble constant because lack of variability

3C273

• Flat spectrum, core-dominated quasar– Variable in both optical and radio

– Compact superluminal core/jet

– More extended jet

– A very bright quasar

• This type of source, if lensed, would be good for Hubble const., because of its variability and jets which could be radio-imaged.

2352+495

• Compact symmetric object (CSO)– Flat spectrum– Mini double– Non-variable– Optically very faint– Good for mass modelling but not for Hubble

constant because of lack of variability and and difficulty of getting a redshift for the lensed object.

Sensitivity• Flux sensitivity given by:

Where Ae is the effective area of the telescope, Tsys is the system noise, B the receiver bandwidth and τ the integration time. The system noise comes from:

• Sky

• Atmosphere

• Reciever

• Tsys are within ~2 of perfect so increase B (fibres) or increase area– e-MERLIN, EVLA, e-VLBI in pipeline (B)

– SKA being planned (A)

BT

AS sys

erms

1

Weather• Weather can mess up radio observations• Clouds/rain/snow can attenuate the signal and,

more importantly, increase Tsys by a large factor. Time delay monitoring puts the most stringent demands on calibration.

• Interferometers rely on knowing the relative phases of the signals. Even in clear air the troposphere can introduce path random differences that compromise interferometer array imaging capability.

• The ionosphere can also mess up the phases at low frequencies. (To most radio astronomers a low frequency is less < 5GHz and a high frequency >22GHz.)

How to make life Easy!Observe between 1.4 and 15GHz

• Sky background is minimum• Atmospheric attenuation/emission

minimum.• Receiver noise performance optimum• At lower frequencies ionospheric phase

instability is a problem• At higher frequencies the tropospheric

phase instabilities become a problem

Calibration• Amplitude

– Observe regularly standard sources• Atmosphere• Telescope pointing• Receiver gain drift

• Phase– Observe point sources of known position regularly

• Tropospheric and ionospheric phase• Receiver drifts.

• Polarization– Observe a source of known polarization position angle

• Position angles calibration

– Observe a compact unpolarized source.• Calibrate the instrumental polarization.

Calibration practicalities

• Different sources required for flux density, phase and polarization calibration.– Flux calibrators need to be non-variable and these are

usually somewhat extended sources and there are only a small number.

– Phase calibrators need to be compact, and you need lots of them so that there is one nearby your target so that you can point at it every few minutes. Compact sources are often variable and are not good flux density standards.

Typical calibration procedure for interferometer arrays

• Do one or two observations of a flux standard. (The flux standard can often be used as the polarization position angle calibrator.)

• Make frequent observations of one or more compact phase calibration sources near the target for phase calibration and secondary flux calibration (assume that flux does not change for the duration of the observations). The phase calibrator can often be used to calibrate the polarization residuals. – For lens monitoring one wants to go to a secondary

calibrator every few minutes.

Radio instruments for gravitational lensing studies

• VLA 0.2 arcsec 0.5mJy

• MERLIN 0.05 arcsec 2.0mJy

• GMRT 1 arcsec 1.0mJy

• AT 1arcsec 2.0mJy

• EVN 0.005 arcsec 1.0mJy

• VLBA 0.0005 arcsec 2.0mJy

• Global VLBI 0.0005 arcsec 1.0mJy

The number of radio lens systemsThere are not many more to find with current sensitivities

• There are ~million catalogued radio sources with flux densities >1mJy.– Many are too weak or too extended to make it

worthwhile looking for multiple images.– There are, say, 200,000 useful targets and more than

20,000 of these have been looked at and these are the easy ones.

– If we really worked hard we might find ~200 systems with current instruments.

• Many sources intrinsically double so need a lot of hard work to distinguish these from double-image lens systems.

Finding Lens systems

• Use the VLA – can map 1000 sources per day.

• Need to confirm with more extensive observations, e.g. MERLIN, VLBI, optical.

Propagation Effects

• Scattering can change surface brightness

• Scattering and/or free-free absorption can change the spectrum of one image and not another

• Faraday rotation can rotate polarization position angles and even depolarise.

Time delay monitoring

• The VLA is best but has limited resolution even in its highest resolution configuration and it is only in this configuration for 4 months every 16.

• MERLIN has more resolution but the calibration is not so good. It can monitor for more than 4 months in a row (Andy Biggs talk).

Conclusions

• Radio observations offer powerful ways to:– Find lens systems– Monitor their variability– Obtain detailed maps for model constraints

• Careful calibration is vital

• Propagation effects can modify lensed image properties.