Probing Magnetic Fields with Square Kilometre Array and its Precursors

Abstract

Origin of magnetic fields, its structure and effects on dynamical processes in stars to galaxies are not well understood. Lack of a direct probe has remained a problem for its study. The first phase of Square Kilometre Array (SKA-I), will have almost an order of magnitude higher sensitivity than the best existing radio telescope at GHz frequencies. In this contribution, we discuss specific science cases that are of interest to the Indian community concerned with astrophysical turbulence and magnetic fields. The SKA-I will allow observations of a large number of background sources with detectable polarization and measure their Faraday depths (FDs) through the Milky Way, other galaxies and their circum-galactic mediums. This will probe line-of-sight magnetic fields in these objects well and provide field configurations. Detailed comparison of observational data (e.g., pitch angles in spirals) with models which consider various processes giving rise to field amplification and maintenance (e.g., various types of dynamo models) will then be possible. Such observations will also provide the coherence scale of the fields and its random component through RM structure function. Measuring the random component is important to characterise turbulence in the medium. Observations of FDs with redshift will provide important information on magnetic field evolution as a function of redshift. The background sources could also be used to probe magnetic fields and its coherent scale in galaxy clusters and in bridges formed between interacting galaxies. Other than FDs, sensitive observations of synchrotron emission from galaxies will provide complimentary information on their magnetic field strengths in the sky plane. The core shift measurements of AGNs can provide more precise measurements of magnetic field in the sub parsec region near the black hole and its evolution. The low band of SKA-I will also be useful to study circularly polarized emission from Sun and comparing various models of field configurations with observations.

Core shift effect in blazars

“…Radio continuum surveys with the SKA can provide AGN imaging and timing (flux density, polarization) data across a range of frequencies simultaneously, ideally suited for the application of the time delay method. The SKA-VLBI synergy can provide key and unique contributions to the current study and additionally, in the context of studies of magnetic fields in AGN jets (Roy et al 2016). Some of these include (1) verification of the SSA model using high resolution sub-pc-scale images, as the SSA opacity is related to the magnetic field strength and core distance (e.g.…”

Diffuse Radio Emission from Galaxy Clusters

“…In the more distant future, the SKA will provide even larger samples. This will enable the detailed characterization of magnetic fields in some individual (nearby) clusters, employing background and cluster sources (Krause et al 2009;Bonafede et al 2015b;Johnston-Hollitt et al 2015b;Roy et al 2016).…”

Faraday rotation signatures of fluctuation dynamos in young galaxies

“…This process referred to as Fluctuation or Small-scale dynamo, amplifies seed magnetic fields exponentially fast (on eddy-turnover time-scales) by random stretching of field lines by the turbulent eddies, until some saturation process sets in (Kazantsev 1968;Zeldovich et al 1990;Subramanian 1999;Haugen et al 2004a;Schekochihin et al 2004;Brandenburg & Subramanian 2005;Federrath et al 2011;Brandenburg et al 2012;Bhat & Subramanian 2013;Federrath 2016). Apart from generating and maintaining fields in galaxy clusters, fluctuation dynamo can also be a suitable candidate for generating magnetic fields in young galaxies at high redshifts (Schober et al 2013;Roy et al 2016;Rieder & Teyssier 2017). This is mainly due to the following reasons.…”