The Square Kilometre Array (SKA)

Abstract:

Accretion onto black holes often leads to the launch of outflows that substantially influence their surrounding environments. The two primary forms of these outflows are X-ray disk winds—hot, ionized gases ejected from the accretion disk—and relativistic jets, which are collimated streams of particles often expelled along the rotational axis of the black hole. While previous studies have revealed a general association between spectral states and different types of outflow, the physical mechanisms governing wind and jet formation remain debated. Here, using coordinated NICER and MeerKAT observations of the recurrent black hole X-ray binary 4U 1630–472, we identify a clear anti-correlation between X-ray disk winds and jets: during three recent outbursts, only one type of outflow is detected at a time. Notably, this apparent exclusivity occurs even as the overall accretion luminosity remains within the range expected for a standard thin disk, characteristic of the canonical soft state. These results suggest a competition between outflow channels that may depend on how the accretion energy is partitioned between the disk and the corona. Our findings provide observational constraints on jet and wind formation in X-ray binaries and offer a fresh perspective on the interplay between different modes of accretion-driven feedback.

Abstract:

This paper presents a comprehensive wideband study of FRB 20240619D focusing on its hyperactivity, rotation measure evolution, and the search for an associated persistent radio source. Using data from the MeerKAT, Murriyang, and Lovell telescopes, we analysed the spectral, temporal, and polarimetric properties of 1539 bursts. Our observations reveal a remarkably high burst rate of 161 bursts per hour in early August above a fluence value of 1.6 Jy ms as well as significant secular variations in rotation measure and diverse polarization characteristics, including high linear polarization fractions and occasional circular polarization. The burst activity also showed frequency dependence with approximately 61 per cent of the total number of bursts detected between 1300 and 1800 MHz. The burst activity of FRB 20240619D ceased abruptly after a period of intense activity lasting approximately 80 d, suggesting an episodic behaviour. Follow-up observations with MeerKAT and Australia Telescope Compact Array did not reveal an associated compact persistent radio source. Altogether, our results highlight the importance of continued long-term monitoring and multiwavelength observations in understanding the emission mechanisms and diversity of progenitor populations of fast radio bursts.

Abstract:

ABSTRACT Radio haloes are diffuse synchrotron sources that trace the turbulent intracluster medium (ICM) of galaxy clusters. However, their origin remains unknown. Two main formation models have been proposed: the hadronic model, in which relativistic electrons are continuously injected by cosmic-ray protons; and the leptonic turbulent re-acceleration model, where cluster mergers re-energize electrons in situ. A key discriminant between the two models would be the existence of ultra-steep spectrum radio haloes (USSRHs), which can only be produced through turbulent re-acceleration. Here, we report the discovery of an USSRH in the galaxy cluster SPT-CLJ2337–5942 at redshift $z = 0.78$ in the MeerKAT-South Pole Telescope 100 deg$^2$ UHF (0.58–1.09 GHz) survey. This discovery is noteworthy for two primary reasons: it is the highest redshift USSRH system to date; and the close correspondence of the radio emission with the thermal ICM as traced by Chandra X-ray observations, further supporting the leptonic re-acceleration model. The halo is underluminous for its mass, consistent with a minor merger origin, which produces steep-spectrum, lower luminosity haloes. This result demonstrates the power of wide-field, high-fidelity, low-frequency ($\lesssim 1$ GHz) surveys like the MeerKAT-SPT 100 deg$^2$ programme to probe the origin and evolution of radio haloes over cosmic time, ahead of the Square Kilometre Array.