Jean-Pierre Macquart

Professor Jean-Pierre Macquart, who preferred to be known as J-P, died suddenly on June 9th, 2020 at the young age of 45. J-P was at the peak of his research career, with an already extensive research and publication record. At the time of this death, he was an Associate Professor at the International Centre for Radio Astronomy Research at Curtin University, Bentley, Western Australia. His most recent achievement was his lead on a paper in Nature that featured extensively in the media, where J-P was described as having solved the long-standing problem of the missing baryons in the Universe.

J-P was an alumnus of the School of Physics at Sydney University, where he commenced his PhD under Professor Don Melrose. He submitted his thesis, "Scintillation and Circular Polarization in Radio Sources" in 2000 and formally graduated with his PhD in 2001. J-P's thesis production was achieved working with the existing subgroup in the Research Centre for Theoretical Astrophysics at Sydney University that was interested in scintillation. Around this time, several observational experiments had shown clear examples of intra-day variability, IDV, that were due to interstellar scintillation, ISS, in the turbulent, ionised inter-stellar medium, ISM, of our Galaxy.

His thesis was directed towards generalizing the then well-established theory for the scintillation of compact radio sources to include the effects of the magnetic field. J-P mastered this generalization and applied it to the interpretation of the observed circular polarization of compact extragalactic sources. His insight into this previously unsolved problem established his international reputation in the field. This was the beginning of J-P's very productive career in exploring the structure of and propagation of radio waves through the turbulent ISM.

Although naturally an excellent theoretician, J-P's career was also shaped by a broad understanding of the observational aspects, where his analytical skills and agile mind stood him in good stead. This was clear even while undertaking his PhD thesis, where he was also involved in the independent observational confirmation of the presence of circular polarisation in Sgr A*, the radio source at the centre of our Galaxy. The wider relationship between circular polarisation and intra-day variability was first revealed through observations of the presence of circular polarisation in both PKS0405-385 and PKS1519-273. This was alongside the publication of theory papers in collaboration with his supervisor, Professor Don Melrose.

Following the completion of his PhD, J-P held a post-doctoral position at the Kapteyn Astronomical Institute, University of Groningen in the Netherlands from 2002-2004. Soon after arrival, he put together and co-edited the proceedings of a conference, "Circular Polarisation from Relativistic Jet Sources", in Amsterdam in July 2002. Clear similarities emerged in the evidence presented between both Galactic jets and jets from active galactic nuclei and showed the emergence of circular polarisation is associated with outburst activity in the nuclei of both. This similarity suggests that the origin of the circular polarisation appears to be identical in both cases.

J-P also held a position as an NRAO Jansky Fellow at Caltech, USA, 2004-2008, before returning to Australia to join the Curtin Institute of Radio Astronomy in 2008. Here he led the research group studying scintillation of ultra-compact radio sources and including the newly discovered fast radio bursts.

In order to stay closely connected to the advances being made across astronomy, J-P made good use of attending international conferences across his broad fields of interest, interest that was reflected in his publication history. This way he made many friends and colleagues across the globe, demonstrating all the necessary qualifications for his many successful international collaborative research papers.

The Micro Arc-second Scintillation Induced Variability, MASIV, Survey made using the VLA at 6 cm wavelength, was J-P's first involvement in a major observational project. The MASIV Survey searched for variability in some 700 flat-spectrum radio sources. MASIV discovered that across the northern sky more than half of the sources surveyed displayed intra-day variability, that is interstellar scintillation, over the course of the year covered by the survey. J-P played a key role in the MASIV project, bringing his in-depth knowledge of scintillation theory and his assistance in data analysis and interpretation. His work on MASIV was to lead to his ground-breaking insights on using Fast Radio Bursts to identify missing matter in the Universe.

The 2007 discovery at Parkes of the short-timescale Lorimer radio transient, proved to be a significant event for J-P, as he began exploring wider questions of the effects of scattering on detection of transient radio sources. Already with an extensive understanding of the physics behind the transmission of radio waves through the turbulent ionized medium of our Galaxy, he was well suited to such an exploration, and the results were of considerable importance. Application of the scattering physics to the Lorimer transient detection at 1.4 GHz, suggested that the short event duration was likely dominated by scattering in the inter-Galactic medium, IGM, and not in the inter-stellar medium, ISM, of our Galaxy. The short timescale observed for the Lorimer burst thus likely represented the first detection of sub-pc scale turbulence in the ionized IGM. Scattering in the IGM was soon to prove itself to be a powerful tool to probe the structure of all the ionized baryons in the IGM in which most of the mass of the Universe resides at redshift <= 7.

J-P then commenced the major development of the Commensal Real-Time ASKAP Fast Transients, CRAFT, Survey, specifically for the ASKAP telescope in Western Australia. ASKAP represented a significant advantage over other survey instruments because of its large collecting area and wide field of view, as well as its interferometric accurate position determination capabilities. Such short-timescale transients are associated with the most energetic and brightest single events in the Universe. Short-timescale transients open new vistas on the physics of high brightness temperature emission, extreme states of matter and the physics of strong gravitational fields.

It was clear that such an instrument could be expected to detect more Lorimer transients while surveying for other fast transients, such as pulsar giant pulses and the episodic and bright emission from Magnetars. Moreover, the detection of many more Lorimer bursts would provide a deep exploration of the IGM. And CRAFT's commensal nature meant that the transients survey would run in parallel with other scheduled ASKAP programs. J-P also realized the potential value of the commensal design concept for high time resolution widefield science for the future of the Square Kilometer Array. While developing the CRAFT design, J-P continued his wide interest in SgrA* and possible nearby pulsars, also in ISS towards pulsars themselves, as well as the MASIV survey analysis. In addition to these he developed a new interest in inter-planetary scintillation.

The design of CRAFT, as led by J-P, focused on its ability to detect transients. At the same time there was a need to determine reliable positions to provide optical identifications and hence redshifts. CRAFT's principal objective was for the detection of extragalactic transients specifically to create an entirely new and extremely sensitive probe on the huge reservoir of baryons present in the IGM. The first ASKAP detection of a Fast Radio Burst, FRB, in January 2017, was a good demonstration of the CRAFT capabilities developed specifically for that radio telescope. Based on pilot wide-field survey observations, FRB 170107 lasting 2.6 msec was detected in multiple beams, and with a dispersion measure of 609 pc cm-3. It was clearly extragalactic.

The next few years then saw a dramatic increase in the number of ASKAP FRBs found through the CRAFT capabilities, with particular emphasis on their "gold standard sample" of 5 FRBs found using CRAFT's full interferometric capabilities. These five FRBs were all found using the same telescope, at the same frequency and with sufficient positional precision to allow highly reliable optical identifications and redshift determination. This was opposed to heterogeneous nature of the searches already conducted by telescopes of different sensitivities at a variety of frequencies, many with no accurate radio positions and hence no redshifts.

From conception to completion, the time was then right to undertake the first direct measurement of the baryon content of the Universe. The paper reported the direct measurement of the baryon density of the Universe using the dispersion of the CRAFT "gold star" FRB sample together with the additional four with published redshifts, utilizing an effect that measures the electron column density along each line of sight and accounts for every ionized baryon. The paper detailing these results was published in Nature on 27 May 2020, with J-P as lead author.

J-P was an exceptional young scientist with an incisive and inquiring mind with a brilliant career unfolding for him. His sudden passing was a shock to all who knew him, and his generous and warm nature, and his outgoing enthusiasm for life is a sad loss for his many colleagues and friends.