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AARTFAAC's first transient detection

Submitter: Stefan Wijnholds
Description: Yesterday we reported the successful correlation of 288 antennas on the LOFAR superterp. Peeyush Prasad (University of Amsterdam) and I have been digging into this data and found that it contains a nice surprise: a solar burst, i.e., the LOFAR all-sky transient detection facility detected its first transient in its first observation!

We are also learning a lot from these data from a data reduction process, which is demonstrated by this dailyimage. The top left image shows an uncalibrated all-sky image based on a 15-kHz 10-s snapshot of raw data near 60 MHz. It shows Cas A, Cyg A, Vir A and the solar burst, that makes the Sun the brightest source in the image. The Galactic plane, the north polar spur and loop III from the Haslam survey. The power of the point source is scattered around due to imperfect inter-station calibration.

That problem was tackled by treating the 288 dipoles as a single station using the standard LOFAR station calibration extended with source position estimation to deal with direction dependent ionospheric effects. This results in well calibrated array responses towards the point sources as shown in the top right image. After subtraction of extended emission, we obtain an image in which the power is dominated by the point sources (bottom left). Thanks to careful calibration, Cas A, Cyg A and Vir A can easily be subtracted as well. The Sun proved to be a bit more challenging since it can not be regarded as a simple point source at the resolution provided by the AARTFAAC system, although solar size is still smaller than the AARTFAAC half power beam width. We have got the reasonable, but sitll not perfect, result shown in the bottom right panel by using a 3-component point source model. The brightest source in this image is Hercules A and also standard LOFAR array level calibration sources like 3C196 and 3C295 are clearly visible. The noise level in this image is about 10 Jy, giving a dynamic range of 2300 in this 15-kHz 10-s snapshot.
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