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.

Video of ionospheric scintillation

Recently, we ran a small article on ionospheric scintillation and how dramatically it can change depending on atmospheric conditions. A few days later we showed two more interesting plots, both to the same scale, which demonstrated how substantial the different ionospheric scintillation conditions can be.

However, there is nothing quite like a video to really show how the scintillation can affect the signal. So today, we have just that!

What we did was make all-sky images for each of the two extremes shown in that last post we made on the topic.

The observing frequency was the same in both cases 58.594 MHz), 1 second integration per frame, 195.3 kHz bandwidth. The sequence is sped up somewhat (5 minutes data in about 30 seconds of video). As the time of day was the same in both instances, so the radio sky is in roughly the same orientation.

On the 25.12.2013 (left), the ionosphere exhibited extreme scintillation. The two bright sources (Cas A and Cyg A) are flickering quite dramatically. However, exactly two days later (27.12.2013, right), the ionosphere was relatively stable and the brightnesses of the two strong radio sources remain steady.

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!

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.

The "A" sources

Some of the stars that we see with our own eyes have names, but many are catalogued in some systematic way. One of the first major attempts to do this listed each star according to its brightness within a given constellation. The brightest would be assigned the Greek letter alpha, then the next brightest would be beta, then gamma, and so on. Then, just to mix things up, this would be followed by the constellation's Latin name (in the genitive case, of course!). Thus we have stars such as alpha Orionis or beta Ursae Minoris. Since then, there have been many optical star catalogues made, with combinations of letters, numbers, etc., referring to them by position, brightness or some other criteria.

In the early days of radio astronomy, it was noted that, like its optical counterpart, there were distinct objects in the radio sky. These discreet sources were also labelled along similar lines. The brightest "radio star" in a given constellation would be named by the constellation and the Latin letter, starting with "A" for the brightest, then "B", and so forth.

Because radio astronomy advanced so quickly, this system was only used briefly, before naming radio objects with catalogue numbers or positions, such as 3C273 or PSR B1919+21. However, those first few remain highly significant and are, of course, the brightest. So radio astronomers continue to refer to them by their historic names. Because it is usually only the brightest ones that get this treatment, they are usually just the "A" sources from each constellation. Collectively, they are referred to as the "A-sources" or the "A-team".

We use the positions of the A-sources a lot, so we may as well make the list available to everyone else too. The following parametersare the Right Ascensions and Declinations in decimal degrees and radians for each of these historic radio sources.


Name    RA(deg)     Dec(deg)    RA(rad)     Dec(rad)
Cas A    350.8500     58.8150    6.1234877     1.0265154
Cen A    201.3651    -43.0191    3.5144833    -0.7508252
Cyg A    299.8682     40.7339    5.2336866     0.7109409
For A     50.6730    -37.2093    0.8844107    -0.6494249
Her A    252.7839      4.9926    4.4119122     0.0871371
Hyd A    139.5235    -12.0956    2.4351453    -0.2111072
Ori A     83.8221     -5.3911    1.4629713    -0.0940926
Per A     49.9507     41.5117    0.8718036     0.7245158
Per B     69.2682     29.6705    1.2089587     0.5178480
Pic A     79.9572    -45.7788    1.3955169    -0.7989909
Pup A    126.0292    -42.9967    2.1996239    -0.7504334
Sex A    152.7521     -4.6914    2.6660268    -0.0818802
Sgr A    266.4168    -29.0078    4.6498510    -0.5062818
Tau A     83.6331     22.0145    1.4596727     0.3842255
Vir A    187.7059     12.3911    3.2760865     0.2162659

Coordinates are J2000 equinox; J2000.0 epoch.               

Sources:

• Baars et al., The absolute spectrum of CAS A - an accurate flux density scale and a set of secondary calibrators, A&A, 61, 99, 1977
• Bolton, J.G., Stanley, G.J, and Slee, O.B., Galactic Radiation at Radio Frequencies - VIII, Aust.J.Phys, 7, 1, 109-129, 1953.
• Roger, R.S., et al., The radio emission from the Galaxy at 22 MHz, A&A.sup, 137, 7-19, 1999.
• Scaife, A.M.M, & Heald, G.H., A broadband flux-scale for low-frequency raio telescopes, MNRAS, 423, 1, pp. L30-L34, 2010. 
• Stanley, G.J., and Slee, O.B., Galactic Radiation at Radio Frequencies - II, Aust.J.Phys, 1949.
• van der Tol, S., Bayesian Estimation for Ionospheric Calibration in Radio Astronomy, PhD Thesis, 2009
• SIMBAD online catalogue, http://simbad.u-strasbg.fr/
• LOFAR system catalogue, ASTRON, 2011.