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1st of July 2022
...and MAVERIC
by Tudor et al.
Globular clusters are stable, tightly bound clusters of tens of thousands to millions of stars. They are associated with all types of galaxies and within our Galaxy they are found in the halo and the bulge surrounding the Galactic Centre. But as most stars are not strong radio emitters, Globular clusters are not generally detected as radio sources. It had been thought that few stellar-mass black holes would reside in present-day clusters, as most of them would have been ejected due to mutual interactions, but recent theoretical work now suggests that a significant fraction of black holes may be retained in clusters. Radio continuum observations offer a new window on compact objects in globular clusters compared to typical X-ray or optical studies.

As part of the MAVERIC survey, Tudor et al. have used the Australia Telescope Compact Array to carry out a deep (noise level ≈ 4 µJy/beam) radio continuum survey of 26 southern globular clusters at central frequencies of 5.5 and 9.0 GHz. They have just published a catalogue of 1285 radio continuum sources in the fields surrounding these 26 clusters. Considering the surface density of background sources, we find significant evidence for a population of radio sources in seven of the 26 clusters, and also identify at least 11 previously known compact objects (6 pulsars and 5 X-ray binaries). Many of the unclassified radio sources near the centres of the clusters are likely to be true cluster sources, and multi-wavelength follow-up will be necessary to classify these objects and better understand the demographics of accreting compact binaries in globular clusters. The image above shows the 5.5 GHz image of the NGC 6652 field. The core and half-light radius of the cluster are shown in red dotted circles, and the detected sources are highlighted with green circles. The radio source within the half-light radius is associated with a candidate transitional millisecond pulsar first discovered at X-ray energies.

Yesterday's ADAP ("The Goose") noted how astronomers are imaginative in the names they give to pulsar wind nebulae: today's ("MAVERIC") celebrates the acronyms and abbreviations they give to their projects -- MAVERIC stands for Milky-way ATCA/VLA Exploration of Radiosources in Clusters. (And together, Goose and MAVERIC recall the Tom Cruise movie Top Gun and its current sequel!)

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