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A LOFAR project for Dan

Submitter: Raffaella Morganti
Description: On the 6th of December last year, many of us received the sad news of the passing of Dan Harris. The life and career of Dan has followed the development of radio technology and the improvement of radio telescopes. He was there when the Northern Cross was built in Italy and he was here, in the Netherlands, in the highly productive first decade of the WSRT, as one of the first users of this instrument.

The study of radio sources and radio jets has been the "leitmotif " of Dan's research. He used the powerful combination of radio and X-ray observations to unveil many of the complicated aspects of the physics of radio jets. Dan has been always aware of this “power” and, in particular, he was aware that things would even be better by extending the radio data to the lowest possible frequencies.

Perhaps counter-intuitively, the low-energy electrons which emit at low radio frequencies are key for understanding the processes driving the X-ray emission. These low-energy electrons are also long-lived (hundreds of thousands of years), and over this timescale they not only encode the history of the radio source, but also produce inverse Compton scattering with cosmic microwave background (CMB) photons and originate X-ray emission.

Thus, it was not surprising to see Dan's great interest for the LOFAR radio telescope! Dan was interested in LOFAR from the early stages and, actually, even before the start of operations. Dan's idea was to use LOFAR to resolve the debate about the dominant emission mechanism for the X-ray emission of powerful radio jets. The low radio frequencies covered by LOFAR would tell us whether the X-ray emission from radio jets is the extrapolation of Inverse Compton/CMB models or not. But to be able to answer this question, the high spatial resolution of International LOFAR was required. Therefore, he has been anxiously waiting for the long-baseline capabilities of LOFAR, asking about the status and the progress in the calibration and imaging at every occasion I would meet him. Only with an International LOFAR observation one could match the resolution at higher frequencies and resolve the jet structure: no other low-frequency radio telescope could provide this!

The pilot study we proposed was targeting one of Dan's favourite objects, 4C19.44. This object has a bright jet with knots observed both in radio and X-ray. With the help of some of the LOFAR long-baseline experts at ASTRON (Javier Moldon, Adam Deller and Raymond Oonk) an impressive sub-arcsecond image was obtained at 150 MHz, using 8 international stations in combination with the Dutch array (picture on the left). The great quality of the image is even more evident when compared with the high-frequency VLA image; see the overlay on the right between the high-frequency (4.8 GHz) VLA image and the LOFAR one.
This perfect coincidence between the structures was exactly what Dan needed for his experiment; he was delighted to see this result.

Unfortunately he did not have the time to complete the full analysis of these data, but we have made a commitment to complete the 4C19.44 long-baseline paper; both for his memory and because of the lot of work he has dedicated to this project. This is what he would have liked to see! Hopefully, we will also be able to expand this pilot project to more objects, as Dan was planning to do.

A Memorial for Dan Harris is held in Boston on Saturday 19th March and we hope this short contribution will complete the picture of Dan's active scientific life until the last day, as we would have expected from him!

Copyright: ASTRON
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