Strange radio sky

A rather odd all-sky image appeared in the last hour. As you can see, the dominant flux seems to be coming from the Galactic plane, but away from the Galactic centre.

I am guessing that strong scintillation caused Cas A and Cyg A to particularly fade at that point and the autoscaling has thus highlighted the rest of the Galactic plane. In any case, Cyg A is pretty low to the horizon at this point.

Or perhaps there is a riometric event causing a sweep of absorption in the northwest (top-right, due to the sky-view orientation). After all, there has been some strong Solar activity in the last couple of days.

Still, it is an interesting all-sky image. It might make for a neat little project to investigate the data around this particular time.

Radio bright sun

On 20 March 2014, the Sun was particularly active. As it passed over the southern horizon, KAIRA detected strong radio emission. We had the station correlator running at the time and managed to capture this all-sky image sequence.

The video covers the period from approx. 10:05 until 10:15 UTC. Each frame is a 1-second integration, however it has been sped up to reduce the duration. The date/time (UTC) is shown in the filename at the top-left of the image sequence. The observing frequency is subband 300 (= approx. 58.6 MHz, with 195 kHz bandwidth).

The object near the centre of the image is Cas A and to its right is Cyg A. These are normally very bright radio sources. However, the sun (on the lower edge) easily outshines them... especially around 10:09 UTC (about 0:24 in the video). For the purposes of making this sequence, we really had to turn down the contrast to prevent complete saturation.

When the sun is radio-bright, strange artefacts can be seen in other parts of the image. These are the "sidelobe" responses of the telescope and are not real sources.

It says something about the strength of the solar radio bursts, when the sidelobe response can outshine even Cas A!

Changes in frequency

As our regular readers will know, KAIRA is constantly monitoring the radio sky as part of its routine observations. We automatically update the image on the right-hand side of the web log showing a recent view (updates are typically made each hour).

Recently we made a change to the experiment which meant the image looks a bit more smeared out. This change was noticed by some of our regular viewers, who asked us why.

The reason is that the new observations use a different frequency. In fact, it is nearly half that of the old observation set. The resolution of a radio telescope is roughly governed by the size of the telescope and the wavelength. Very roughly, this is:

resolution (in radians) = wavelength  /  diameter

where 1 radian = approx 57 degrees, and the wavelength and diameter are measured in the same units.

The wavelength of the radio waves is related to the frequency:

wavelength = speed-of-light  /  frequency

If the frequency is in hertz, and the speed of light is in metres per second (specifically 3 x 108 m/s), then this gives the wavelength in metres.

The numbers are not too important. What is, though is that as the frequency gets higher, the wavelength gets shorter. And, as the wavelength gets shorter, the resolution gets finer.

We've made an observation to show this...




Watch for the frequency in the lower corner. Wait until it gets to about 18 MHz, at which point the main sources of "Cas A" and "Cyg A" snap into view. Then, watch as the frequency goes up, our determination of their positions improves. In other words, and the frequency goes up, the resolution gets better!

The very start and very end of the video are because there is insufficient signal to noise. However, between 18 and 80 MHz, it is mostly okay (apart from the odd burst of radio frequency interference).

Riometric beams

When we talk about riometry, there are several different varieties. Obviously, there is single-beam, integrated all-sky riometry ("old school"), but the modern variety makes positionally resolved measurements by putting multiple "beams" on the sky.

KAIRA has been doing this now for quite a while, in addition to some single-beam work and new advanced techniques that are now being developed. However, to get an idea "where" on the sky these beams are, today we are featuring an all-sky image from earlier this year, with the beam centres superimposed over it.

Note that the beam "circles" are not to any scale. Also, the large loop is not a single beam, but rather the path that the "Cas A beam" follows as Cas A tracks across the sky. The slight discrepancy in the Cas A source and beam position is due to the fact that the displayed data is not calibrated.

Sunshine in the data

'Tis "Data Monday" again. Today, we've an example of where the radio-bright sun is showing up in one of the all-sky images.

Click on the plot to enlarge it.Note that this is an orthographic projection. Also, this is a "sky-view"... in other words, it is as if you are lying on your back in the middle of the array looking up. A few markers indicate some bright sources (Cas A and Cyg A) and directions (such as the direction to the VHF transmitter in Tromsø).

However, the one to look for is the very bright object at the bottom of the plot: radio emission from the sun.

Freeze frame

On the right hand side of this web log there is an all-sky image. It is always the latest image that was taken with KAIRA, updated every minute or so in order to always be up-to-date. Apart from being an interesting curiosity, it is useful for the researchers, as it lets us keep tabs on the data flow and notice if something stops. But it also lets us monitor the radio sky for interesting events, whether an active Sun or pesky radio-frequency interference (RFI).

However, some you may have noticed that it hasn't updated recently. It is still set at this image, taken on the evening of the 9th July 2013.

The last "regular" all-sky image... at least for a little while, while we complete the
visualisation software for the new correlator mode that we are now running.

The reason for the stop at this point has nothing to do with a failure of the system. In fact, KAIRA is still working; still observing 24-hours a day. What has happened is that we have changed our correlator mode.

At KAIRA, we are always experimenting with new and interesting ways of using the LOFAR technology. From the now-popular 357 mode, through to wide bandwidth low-bit observing, we pride ourselves in the novel usage of the system to visualise and discover new things about the natural universe.

Currently, we are at it again. This time we're experimenting with a new station correlator configuration. This will allow us to carry out a different type of radio astronomy observation, and it will also be useful for our ionospheric scintillation work too. And while the data system is working fine, we haven't yet written the software to convert this new visibility data into an online image. Yes, we'll get there, but it is going to take a little while.

So, given that we switched on the new correlator mode on 09-Jul-2013 at 23:00 UTC, the above image is the most recent processed image we have.

Updates to follow soon!

Opening display

In exactly one week from today, KAIRA will be formally opened. Although we have been operating for some time now, all the auxiliary tasks have now been completed and routine operations are now, just that. The event has been timed to exploit the hot Arctic summer, and provide a lead-in to the 100th anniversary of Sodankylä Geophysical Observatory. There are lots of preparations to be done, but also a few interesting things along the way which we'll be sharing.

Today's offering is from a new all-sky display that will feature at the opening ceremony.

The radio sky image is the one that is already familiar to our readers; it often appears on the right-hand side and updates every ten minutes or so. It shows the radio intensity across the sky in false colour. However the foreground of this display is a fish-eye view of the KAIRA site.

Imagine lying on the HBA array, looking up, with full hemispheric vision. Then this is the surrounding landscape you would see. North to the top, East to the right. Saana is to the lower left, the VHF is aligned with the tile join, off to the top right. The panorama was made from a series of images taken by Thomas Ulich a while back. We stitched approx. 40 individual frames together to get the landscape panorama, which was then projected and overlaid over the incoming all-sky images.

The final display has more to it than this, but we're saving that for the opening! It updates continuously, providing a dynamic display of the invisible radio universe overhead.

Routine display of all-sky images

For those of you who have been wondering about the strange plots on the right-hand side of our weblog page, here is the explanation.

These are all-sky radio images, taken using KAIRA. Our array is capable of imaging the entire sky instantaneously. In some ways, you can think of it as a fish-eye lens for radio astronomy. Because of the typical observation mode, we can taken these images regularly and put them on the web for everyone to see. In fact, we are the only LOFAR-based station that we know of that does this on a regular basis. Typically, the images are updated every 9 minutes or so. Here's a recent example:

Along the top of the image is the date (top left) and time (top right). The times are given in Coordinated Universal Time (UTC). Along the bottom are some details of the observation. The "mode" is the "RCU mode" (RCU = Receiver Unit) which specifies which filters are being used and which antenna array is selected. Typically, this is Mode=3 for our low-band antenna array observations. "Sb" is the subband (or receiver channel). Each receiver unit splits the signal up into 512 subbands which are sampled and processed. These all-sky observations are only one subband. The equivalent frequency of this subband is shown at the bottom right.

Around the edge of the plot are the cardinal points (North, East, South, West). Although it might seem that East and West are incorrect, this is actually what you expect when you look up. Imagine lying on your back, looking up. If North is above your head, then East is on your left and West to the right. This is also what you see on conventional star maps.

Because the images are regularly updated, you can watch the radio sources change position with time. The sequence below shows four images, separated by approximately one hour each. As you can see, the position of the radio objects move. This is because the Earth is rotating in the opposite direction. As a result, they appear to be moving around the north celestial pole.

The amount of time it takes for the sources to complete one full circuit is one sidereal day (approx. 23 hours 56 minutes). This is due to the mix between the rotation of the Earth and the orbit of the Earth around the Sun. This also means that for a given time of day, the radio sky will appear at a different position at different times of the year.

First all-sky image with KAIRA

Yesterday we managed to take our first all-sky image with KAIRA. The observing frequency is 59.6 MHz and we used the entire LBA array.

First all-sky image with KAIRA. A 1-second integration at 59.6 MHz.

The image is not calibrated (in fact, we still have a long way to go). However, there are still a number of features visible: The three red areas on the left are (from bottom to top) the galactic plane, Cyg A and Cas A. The amber-coloured patch just to the right of Cas A is probably a calibration effect.

The next step is to acquire calibration data and determined better solutions for the amplitude, phase and delay errors.

Getting this far has been a lot of work, so thanks to everyone who has helped achieve this.