SETI on TV

One of our plans for the LOFAR-UK station is to perform some SETI (Search for Extraterrestrial Intelligence) experiments (hopefully some as part of the remaining Project Dorothy dates). You'll hear more about this in the future.

This is just a quick post to point out that this week on the BBC there was a program about the Drake Equation (still available on iPlayer: http://www.bbc.co.uk/iplayer/episode/b00wltbk/The_Search_for_Life_The_Drake_Equation/) there was also a rerun of a Horizon Program on SETI from 2008 (also on iPlayer at: http://www.bbc.co.uk/iplayer/episode/b0094cym/Horizon_20072008_Are_we_Alone_in_the_Universe/). 

First Image from eMERLIN

Today is an exciting day for UK radio astronomy as eMERLIN has released its first image. This dramatic image shows the Double Quasar. In the image, light from a quasar billions of light years away is bent around a foreground galaxy by the curvature of space. A quasar is a galaxy powered by a super-massive black hole, leading to the ejection of jets of matter moving at almost the speed of light - one of which can be seen arcing to the left in the image.

This is a composite of the new e-MERLIN radio image of the Double Quasar and an earlier Hubble Space Telescope (HST) optical image. The radio emission generated by the black hole as seen with e-MERLIN is visible as the compact bright region superimposed on the (yellow-green) optical emission seen by HST.  
The e-MERLIN image is shown in false-colour with a colour table ranging from blue through red to white, where the colours represent the brightness of the radio emission. The HST image is made from WFPC2 images through two filters: the F555W filter (V-band) is coloured green and the F814W filter (I-band) is coloured red.
Credit: Jodrell Bank Centre for Astrophysics, University of Manchester 

e-MERLIN is an array of seven radio telescopes, spanning 217km, connected by a new optical fibre network to Jodrell Bank Observatory.

 As a radio telescope array eMERLIN of course has many similarities to LOFAR, but to readers familiar with LOFAR there are also several big differences. To start with eMERLIN observes at much higher frequences (shorter wavelengths) than LOFAR. The frequency (or equivalently wavelength) of electromagnetic radiation which can be detected using radio technology stretches all the way from sub-mm radiation (at many GHz) down to the limit set by the ionosphere at 30MHz (many metres in wavelength). eMERLIN detects radiation in three radio bands at roughly 1.5, 5 and 22 GHz, while LOFAR has two bands at much lower frequency (LBA at 30 - 80 MHz and the HBA at 120 - 240 MHz). This change in frequency means that the tecnhology for the antennas is much different. LOFAR as you know uses many dipole antennas all connected together by software for each "station". This would not work for the frequencies observed by eMERLIN which requires each point in the array to be a "traditional" radio antenna (as illustrated above). 

This e-MERLIN image demonstrates the successful transmission of wide-bandwidth digitised signals from all the telescopes remote from Jodrell Bank over the optical fibre network. This initial image, taken at a frequency of roughly 6.5 GHz, has an angular resolution of 50 milli-arcseconds, similar to the resolution of the Hubble Space Telescope. The new system is already approaching 3 times the sensitivity of the previous radio-linked MERLIN telescope. This will result in a very substantial (around a factor 5) further increase in sensitivity. Operations at full sensitivity, (achieved by including the Lovell telescope and upgrades of the bandwidths in the data links) are expected in 2011.

For more details see the press release at Jodrell Bank.

Good moon rising

On the evening of the 24th August 2010, one of the site security cameras caught this photograph of the moon rising over the LOFAR field. There seem to be three containers in the photograph. The raised one on the left is an WWII ex-RAF water tower. The middle one, with the white box on the side of it, is the RF-container. This is where the bulk of the LOFAR signal processing occurs. The white box is the air-conditioning unit. The battered-looking container on the right is the construction skip, where the surplus and waste materials are temporarily stored.

LOFAR Videos on YouTube

A couple of nice videos relating to LOFAR which were made for the opening in the Netherlands are now available on YouTube.

The first two are a collection of soundbites from famous radio astronomers along with images.



(part two start with a quote from Steve Rawlings from Oxford).


Then there is this "Introduction to LOFAR".

Why have International Stations?

There's a nice item out online right now (see the Astronomy Now version) illustrating the benefit international stations like the one we're building at Chilbolton add to LOFAR. The article is describes the improvement in resolution seen between an image of a distant quasar taken by just the LOFAR core, and one taken using the core connected to two stations in Germany.

Here's the image.

Radio image of quasar 3C 196 at 4-10 metre wavelengths (30-80MHz frequency). Left: Data from the LOFAR stations in the Netherlands only. Right: Greater detail is revealed when the German stations are added to the array. Image: Olaf Wucknitz, Bonn University.

We're looking forward to seeing the first images taken including the Chilbolton station. But first we have to build it!

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.

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.

LOFAR-UK on Radio Solent

For your listening pleasure, you can download here (.WAV) or here (.MP3) the interview I did with BBC Radio Solent presenter Matt Treacy during the test installation of LOFAR Low Band Arrays at Chilbolton last month.

Photo of the team after construction:

LOFAR UK on BBC Radio Solent

An interview I recorded with BBC Radio Solent reporter Matt Treacy last week during the LOFAR LBA test installation at Chilbolton aired this morning on the local Hampshire radio. It will be available for the next 7 days on the BBC iPlayer at this location: iPlayer link (about 45 minutes into the show).

What will LOFAR see?

Courtesy of BrentJens on You Tube, a sneak peak of the radio sky above LOFAR.



(link to movie.)

Explanation from BrentJens:

All sky movie of the radio sky above the LOFAR prototype near Exloo, The Netherlands, at a frequency of 50 MHz. The data were taken at 29 and 30 April 2008. The edge of the circle is the horizon. North is up, East towards the left. The movie shows Cas A and Cyg A twinkling (scintillating) like stars, as well as the Galactic centre rising and setting. Near the end of the movie, there are thunderstorms over Germany visible towards the east. The total duration of the observation was 24 hours.