Tour of Chilbolton LOFAR Station

Almost two weeks ago, Alejo and myself went up to Chilbolton to make a short video tour of the site explaining the three main components. We've now released the video on You Tube.



Hope you like it.

Final preparations for the LBA build week

Work has been in full swing at Chilbolton, preparing for the LBA build week next week. The grids and ground sheets have now been transported from their initial storage position near the main Chilbolton Observatory building over to the edge of the LBA field (about 1/2 mile away), ready for installation.

It is certainly a long time since they first arrived on the back of a Dutch lorry, all the way back in November 2009. In fact, their arrival was announced in one of the very first posts here.

And here is some never before seen footage taken of the arrival back in November, when some of us went to Chilbolton to witness the arrival. In the movie you'll see/hear Bob Nichol, myself (Karen Masters) and Owain Davies (formerly of STFC) discussing what all the bits are, Owain and Derek McKay checking all the expected parts have arrived, and then Owain, Bob and Alejo Martinez-Sansigre breaking the seal on the RF container to get a first look inside.


And finally some pictures showing how they have just been moved out to the LBA field.

The grids were first chained up by Mark Andrews (pictured) and his crew, ready to be lifted.


Then they are carefully driven across the field using the heavy digger (driven by John Murray). There were two loads of grids, each load weighs just over 1 ton.


After moving all the grids, the ground sheets are then moved over.


Two piles of grids and one pallet-load of ground sheets. All ready on the side of the LBA field in preparation for their installation.

Hydrogen 21 cm transition

A hydrogen atom consists of a proton and an electron, and both of these particles have a quantum property called spin, which is related to their angular momentum. It is a similar concept to rotation around the axis, for example of the Earth. The proton and electron can have a configuration where their spins are pointing in the same direction (parallel) or pointing in opposite directions (anti-parallel). The first configuration has a slightly higher energy than the second configuration.

When a hydrogen atom makes a transition from the parallel to the anti-parallel state, it will emit some electromagnetic radiation with a wavelength of 21 cm (1420 MHz). Alternatively, a hydrogen atom in the anti-parallel state can change to the parallel state by absorbing electromagnetic radiation with a wavelength of 21 cm. This transition is extremely weak, but the masses of hydrogen in galaxies are so large that it can be detected in nearby galaxies. Due to the fact that it occurs at a very specific frequency (or wavelength) this type of emission is known as line emission in contrast to continuum emission, such as free-free and synchrotron radiation.

LOFAR will use this physical process to study the ‘Epoch of Reionisation’. In the early Universe, the vast majority of hydrogen in the Universe was in the form of neutral atoms unlike nowadays, when it is mostly ionised plasma so the protons and electrons are separated. As the first stars switched on, they produced ionising radiation that began to separate neutral hydrogen atoms into electrons and protons (also referred to as ions). We use the prefix ‘Re-‘ in Reionisation because shortly after the Big Bang the Universe was ionised, and it later cooled down and electrons and protons joined to form neutral Hydrogen atoms. Hence the epoch of Reionisation is the second epoch when the Universe was ionised.

The epoch of Reionisation occurred when the scale of the Universe was significantly smaller, approximately one seventh of the present scale or smaller. The radiation at 21 cm has scaled up with the Universe, and reaches us with a wavelength of about 1.5 metres or longer (corresponding to a frequency of 200 MHz or less) and it is therefore observable with LOFAR. Studies of the 21 cm line can yield information on the density of neutral hydrogen and its distribution in the early Universe.

The figure below shows the signal of the 21 cm line from the epoch of reionization which LOFAR is expected to measure. The colour scale shows the difference in the observed intensity caused by regions with a high or low content of neutral hydrogen: light regions have the most neutral hydrogen compared to the average, dark have the least (remember the average changes with epoch, so we are looking at the contrast). The vertical axis shows the physical extent of the regions, the units are megaparsecs, Mpc, which correspond to approximately three million light years. The horizontal axis corresponds to observed frequency (in MHz), with lower frequencies looking at earlier epochs when the Universe was younger. This image was produced by Garrelt Mellema using a simulation of the young Universe by Ilian Iliev.

Against a strong source of radiation, for example synchrotron emission from a background source, the neutral hydrogen will leave an imprint of absorption due to the transition at 21 cm. Below is a simulation of the spectrum of such a strong source. The horizontal axis is frequency, in MHz, the vertical axis is flux density (another measure of intensity). The source is at redshift 10, and at this redshift the 21 cm line appears at 129 MHz. The absorption from neutral hydrogen can be seen as strong dips in the spectrum to the right of 129 MHz. The solid line serves as a guide to the eye, it shows what the spectrum would have looked like without any absorption. Only neutral hydrogen between us and the source can cause absorption, and hence the spectrum is only affected at frequencies higher than 129 MHz. Image credit: C.L. Carilli, N. Gnedin, S. Furlanetto, F. Owen.

Sand People

Now that the HBA cables are in place and patched into the RF-container, the HBA cable grave can be filled in.

First a thick layer of protective sand is spread across the cables. The crew work the cables gently to ensure that the sand penetrates between all of them.


Here is the finished sand cover. This protects the cables against flint and other sharp rock as the spoil is back filled into the grave.


With all cables completely covered, the digger moves in to bulk fill with previously excavated material. And that is it! The HBA cables are fully covered and the crew can walk through the area without the risk of falling into the hole or collapsing a trench wall.


There have been many people involved in the laying of the HBA cables, and here are a just a few of them. From left to right: Dave King, Jon Eastment, Mike Willis, Alejo Martinez-Sansigre and Harry Smith. To these few, and everyone else involved in this stage of the project: thanks and well done!

Disentanglement

One of the consequences of various tasks being ahead or behind the schedule is that the folks at Chilbolton have been forced to deploy all the HBA cables before the RF-Container was ready to accept them. This has been good in that they have been able to get the cables laid into the trenches without incurring further delays. On the other hand, they now need to sort out these cables to feed them in the correct order into the RF-container. Quite a task as you will see!

A bundle of cables... thankfully all electronically labelled. A barcode reader and some home-built software will quickly identify the antenna position associated with each cable.


Stuart Lynn, (our "All in the Gutter" live tweeter) Izabela Bukowska and Cristina Fernandes trace a cable in preparation for its deployment into the final trench to the RF-container.


The first cable pair reach the patch panel (Y & X polarisations for HBA antenna #95).


Alejo Martinez-Sansigre, Dave King and Mike Willis continue work on patching cables into the RF-container


Some more of the day's cable-team (left-to-right): Roger Deane, Cristina Fernandes, Alejo Martinez-Sansigre and Stuart Lynn.

Synchrotron radiation

Another common form of emission is synchrotron radiation. This occurs when an electron moves very close to the speed of light in the presence of a strong magnetic field. The magnetic field will cause the electron to feel a force and to change direction. The electron is being accelerated and it emits radiation. Due to the fact that it is moving close to the speed of light, it focuses the radiation towards the direction that it is travelling and will emit mainly at one frequency.

In astronomical sources of synchrotron emission, we see the summed emission from many electrons with different speeds and moving in different directions. The summed emission of all these individual electrons produces waves with a continuous range of frequencies, known as a continuum. Synchrotron emission does not depend simply on temperature, and it is a case of non-thermal emission.

Synchrotron radiation traces regions with fast-moving electrons and strong magnetic fields, typical of regions where shocks are occurring. Examples of such regions are the supernovae remnants as well as jets produced by neutron stars and black holes, and synchrotron radiation can give information on the energy contained in these regions as well as the strength of the magnetic fields.

Below is a radio map of the centre of the star-forming galaxy M82, which shows two components of synchrotron emission: a diffuse component (the extended red and orange emission) as well as compact emission from individual radio supernovae and supernovae remnants (the bright spots). Image credit: T.W.B. Muxlow, A. Pedlar, E.M. Sanders.

The second image is a composite of the radio galaxy PKS 2356-61. An optical image, in blue and a radio map in red are shown superimposed. The optical emission shows the stellar light, while the radio emission reveals the synchrotron emission from the lobes of a jet emanating from a supermassive black hole. Image credit: A. Koekemoer, R. Schillizi, G. Bicknell and R. Ekers.

Free-free emission

An electron will radiate when it is decelerated, so electromagnetic waves carry energy away from the electron. In space, a free electron that passes near a charge feels a small perturbation, and becomes more stable by emitting an electromagnetic wave. After emitting this wave, the electron will be moving slower, since it has lost energy. This form of emission is often referred to by its German name, Bremsstrahlung, which means braking radiation. Since the electron was free before it emitted the electromagnetic wave (it was not trapped in an atom) and is still free after emission has occurred, it is also known as free-free emission, and we will use this name here.

Below is a sketch of an electron (blue circle) passing near an ion (red circle) and losing energy by free-free emission (green wave).

For free-free emission to occur, a fast electron must pass close to another charged particle. This form of radiation will therefore occur more often in regions of high density, because the electron will have a higher chance to come close to a charged particle that will perturb it. However, for free-free emission to occur, the gas must also be ionised, so that the electrons are free from the protons, rather than bound in a hydrogen atom. The amount of free-free emission depends on the temperature of the ionised gas and it is a type of thermal emission, and because the emission is continuous with frequency, it is described also as continuum emission.

Clouds of ionised gas near regions where young stars are forming will emit free-free radiation. It can therefore be used to trace this gas and to study the surrounding regions to where new stars are born, since it gives information on the temperature and density of the gas. As an example of such a region, below is an image of the Carina nebula, courtesy of the Hubble Heritage.

What physical processes do we observe with LOFAR?

LOFAR stands for LOw-Frequency ARray, and it is a radio telescope. This means it is sensitive to electromagnetic waves whose wavelengths are very long, typically longer than one metre, corresponding to frequencies of 250 MegaHertz (MHz) or less. Other examples of electromagnetic radiation are optical and infrared light, X-rays and gamma-ray radiation. All of these

have much shorter wavelengths (higher frequencies) than radio emission.

The waves detected by LOFAR are of the same type as those used by FM radio. When you tune to 92-95 FM for Radio 4, that's actually tuning the receiver in your radio to 92-95 MHz. In fact LOFAR has a gap between 80-120 MHz. It does not observe at these frequencies since the signal from the FM radio stations overwhelms all other signals, making it virtually impossible to detect any emission from space.

Electromagnetic waves are radiated when a charged particle is accelerated. An example is an electron that is suddenly accelerated by a change in an electric field or a magnetic field. It will radiate some energy away in the form of electromagnetic waves.

In a series of 3 blog posts over the next couple of weeks I will summarise three important physical processes that can be observed with LOFAR to study astrophysical objects outside of our Solar System: free-free emission, synchrotron radiation and the hydrogen 21cm transition.