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1. Tackling the Unique Challenges of Low-frequency Solar Polarimetry with the Square Kilometre Array Low Precursor: The Algorithm

   https://doi.org/10.3847/1538-4357/ac6758  

   Kansabanik, Devojyoti ; Oberoi, Divya ; Mondal, Surajit ( June 2022 , The Astrophysical Journal)

   Coronal magnetic fields are well known to be one of the crucial parameters defining coronal physics and space weather. However, measuring the global coronal magnetic fields remains challenging. The polarization properties of coronal radio emissions are sensitive to coronal magnetic fields. While they can prove to be useful probes of coronal and heliospheric magnetic fields, their usage has been limited by technical and algorithmic challenges. We present a robust algorithm for precise polarization calibration and imaging of low-radio frequency solar observations and demonstrate it on data from the Murchison Widefield Array, a Square Kilometre Array (SKA) precursor. This algorithm is based on theMeasurement Equationframework, which forms the basis of all modern radio interferometric calibration and imaging. It delivers high-dynamic-range and high-fidelity full-Stokes solar radio images with instrumental polarization leakages <1%, on par with general astronomical radio imaging, and represents the state of the art. Opening up this rewarding, yet unexplored, phase space will enable multiple novel science investigations and offer considerable discovery potential. Examples include detection of low-level circular polarization from thermal coronal emission to estimate large-scale quiescent coronal fields; polarization of faint gyrosynchrotron emissions from coronal mass ejections for robust estimation of plasma parameters; and detection of themore »first-ever linear polarization at these frequencies. This method has been developed with the SKA in mind and will enable a new era of high-fidelity spectropolarimetric snapshot solar imaging at low radio frequencies.

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2. Synthesizing Observations and Theory to Understand Galactic Magnetic Fields: Progress and Challenges

   https://doi.org/10.3390/galaxies8010004  

   Beck, Rainer ; Chamandy, Luke ; Elson, Ed ; Blackman, Eric G. ( March 2020 , Galaxies)

   Constraining dynamo theories of magnetic field origin by observation is indispensable but challenging, in part because the basic quantities measured by observers and predicted by modelers are different. We clarify these differences and sketch out ways to bridge the divide. Based on archival and previously unpublished data, we then compile various important properties of galactic magnetic fields for nearby spiral galaxies. We consistently compute strengths of total, ordered, and regular fields, pitch angles of ordered and regular fields, and we summarize the present knowledge on azimuthal modes, field parities, and the properties of non-axisymmetric spiral features called magnetic arms. We review related aspects of dynamo theory, with a focus on mean-field models and their predictions for large-scale magnetic fields in galactic discs and halos. Furthermore, we measure the velocity dispersion of H i gas in arm and inter-arm regions in three galaxies, M 51, M 74, and NGC 6946, since spiral modulation of the root-mean-square turbulent speed has been proposed as a driver of non-axisymmetry in large-scale dynamos. We find no evidence for such a modulation and place upper limits on its strength, helping to narrow down the list of mechanisms to explain magnetic arms. Successes and remaining challenges ofmore »dynamo models with respect to explaining observations are briefly summarized, and possible strategies are suggested. With new instruments like the Square Kilometre Array (SKA), large data sets of magnetic and non-magnetic properties from thousands of galaxies will become available, to be compared with theory.« less
3. Multiscale magnetic fields in the central molecular zone: inference from the gradient technique

   https://doi.org/10.1093/mnras/stac159  

   Hu, Yue ; Lazarian, A. ; Wang, Q. Daniel ( January 2022 , Monthly Notices of the Royal Astronomical Society)

   The central molecular zone (CMZ) plays an essential role in regulating the nuclear ecosystem of our Galaxy. To get an insight into magnetic fields of the CMZ, we employ the gradient technique (GT), which is rooted in the anisotropy of magnetohydrodynamic turbulence. Our analysis is based on the data of multiple wavelengths, including molecular emission lines, radio 1.4 GHz continuum image, and Herschel $70\, {\mu }{\rm m}$ image, as well as ionized [Ne ii] and Paschen-alpha emissions. The results are compared with the observations of Planck 353 GHz and High-resolution Airborne Wideband Camera Plus (HWAC+) $53\, {\mu }{\rm m}$ polarized dust emissions. We map the magnetic fields orientation at multiple wavelength across the central molecular zone, including close-ups of the Radio Arc and Sagittarius A West regions, on multiscales from ∼0.1 pc to 10 pc. The magnetic fields towards the central molecular zone traced by the GT are globally compatible with the polarization measurements, accounting for the contribution from the galactic foreground and background. This correspondence suggests that the magnetic field and turbulence are dynamically crucial in the galactic center. We find that the magnetic fields associated with the Arched filaments and the thermal components of the Radio Arc are in good agree withmore »the HAWC+ polarization. Our measurement towards the non-thermal Radio Arc reveals the poloidal magnetic field components in the galactic center. For Sagittarius A West region, we find a great agreement between the GT measurement using [Ne ii] emission and HWAC+ $53\, {\mu }{\rm m}$ observation. We use the GT to predict the magnetic fields associated with ionized Paschen-alpha gas down to scales of 0.1 pc.

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4. Flux variability from ejecta in structured relativistic jets with large-scale magnetic fields

   https://doi.org/10.1051/0004-6361/202039654  

   Fichet de Clairfontaine, G. ; Meliani, Z. ; Zech, A. ; Hervet, O. ( March 2021 , Astronomy & Astrophysics)

   Context. Standing and moving shocks in relativistic astrophysical jets are very promising sites for particle acceleration to large Lorentz factors and for the emission from the radio up to the γ -ray band. They are thought to be responsible for at least part of the observed variability in radio-loud active galactic nuclei. Aims. We aim to simulate the interactions of moving shock waves with standing recollimation shocks in structured and magnetized relativistic jets and to characterize the profiles of connected flares in the radio light curve. Methods. Using the relativistic magneto-hydrodynamic code MPI-AMRVAC and a radiative transfer code in post-processing, we explore the influence of the magnetic-field configuration and transverse stratification of an over-pressured jet on its morphology, on the moving shock dynamics, and on the emitted radio light curve. First, we investigate different large-scale magnetic fields with their effects on the standing shocks and on the stratified jet morphology. Secondly, we study the interaction of a moving shock wave with the standing shocks. We calculated the synthetic synchrotron maps and radio light curves and analyze the variability at two frequencies 1 and 15.3 GHz and for several observation angles. Finally, we compare the characteristics of our simulated light curvesmore »with radio flares observed from the blazar 3C 273 with the Owens Valley Radio Observatory and Very Long Baseline Array in the MOJAVE survey between 2008 and 2019. Results. We find that in a structured over-pressured relativistic jet, the presence of the large-scale magnetic field structure changes the properties of the standing shock waves and leads to an opening in the jet. The interaction between waves from inner and outer jet components can produce strong standing shocks. When crossing such standing shocks, moving shock waves accompanying overdensities injected in the base of the jet cause very luminous radio flares. The observation of the temporal structure of these flares under different viewing angles probes the jet at different optical depths. At 1 GHz and for small angles, the self-absorption caused by the moving shock wave becomes more important and leads to a drop in the observed flux after it interacts with the brightest standing knot. A weak asymmetry is seen in the shape of the simulated flares, resulting from the remnant emission of the shocked standing shocks. The characteristics of the simulated flares and the correlation of peaks in the light curve with the crossing of moving and standing shocks favor this scenario as an explanation of the observed radio flares of 3C 273.« less
5. A broadband radio view of transient jet ejecta in the black hole candidate X-ray binary MAXI J1535–571

   https://doi.org/10.1017/pasa.2021.38  

   Chauhan, Jaiverdhan ; Miller-Jones, J. C. ; Anderson, G. E. ; Paduano, A. ; Sokolowski, M. ; Flynn, C. ; Hancock, P. J. ; Hurley-Walker, N. ; Kaplan, D. L. ; Russell, T. D. ; et al ( January 2021 , Publications of the Astronomical Society of Australia)

   Abstract We present a broadband radio study of the transient jets ejected from the black hole candidate X-ray binary MAXI J1535–571, which underwent a prolonged outburst beginning on 2017 September 2. We monitored MAXI J1535–571 with the Murchison Widefield Array (MWA) at frequencies from 119 to 186 MHz over six epochs from 2017 September 20 to 2017 October 14. The source was quasi-simultaneously observed over the frequency range 0.84–19 GHz by UTMOST (the Upgraded Molonglo Observatory Synthesis Telescope) the Australian Square Kilometre Array Pathfinder (ASKAP), the Australia Telescope Compact Array (ATCA), and the Australian Long Baseline Array (LBA). Using the LBA observations from 2017 September 23, we measured the source size to be $34\pm1$ mas. During the brightest radio flare on 2017 September 21, the source was detected down to 119 MHz by the MWA, and the radio spectrum indicates a turnover between 250 and 500 MHz, which is most likely due to synchrotron self-absorption (SSA). By fitting the radio spectrum with a SSA model and using the LBA size measurement, we determined various physical parameters of the jet knot (identified in ATCA data), including the jet opening angle ( $\phi_{\rm op} = 4.5\pm1.2^{\circ}$ ) and the magnetic field strengthmore »( $B_{\rm s} = 104^{+80}_{-78}$ mG). Our fitted magnetic field strength agrees reasonably well with that inferred from the standard equipartition approach, suggesting the jet knot to be close to equipartition. Our study highlights the capabilities of the Australian suite of radio telescopes to jointly probe radio jets in black hole X-ray binaries via simultaneous observations over a broad frequency range, and with differing angular resolutions. This suite allows us to determine the physical properties of X-ray binary jets. Finally, our study emphasises the potential contributions that can be made by the low-frequency part of the Square Kilometre Array (SKA-Low) in the study of black hole X-ray binaries.« less