KIPAC Tea Talk: Universal Features of Magnetic Fields Near Kerr Black Holes / Backlighting the Cosmic Web with Fast Radio Bursts: First Measurements and Constraints

Gelles Abstract: Supermassive black holes launch luminous, relativistic jets that are observed at parsec-level scales. While black hole spin is believed to play an important role in powering these jets, the spin parameter has proven difficult to measure directly. In this talk, I will discuss how black hole spin imprints itself on surrounding magnetic fields, yielding a new observable signature of spin. Specifically, I will show how black hole spin “winds up” the magnetic field at critical surfaces called light cylinders, where plasma begins to accelerate into an inflow or an outflow. Using a combination of semi-analytic models and numerical simulations, I will then demonstrate that this wind-up translates into spatial polarization swings in the emission, which can be observed through radio interferometry. This technique would allow us to constrain supermassive black hole spins and jet Lorentz factors with forthcoming observations of several nearby AGN.

Sharma Abstract: The dispersion measures (DMs) of fast radio bursts (FRBs) have emerged as a powerful new probe of cosmic baryons, offering a key advantage over traditional methods in being largely unbiased with respect to gas density, metallicity, and temperature. In this talk, I will present a unified view of recent advances in FRB cosmology, including the DM-redshift relation inference and the cross-correlation measurements with large-scale structure and other baryon tracers. Using a sample of localized FRBs with robust host associations, we infer the baryon distribution across galaxy groups and cluster-scale halos in the local Universe, providing insights into the impact of feedback physics on matter clustering. Complementing this approach, we perform a comprehensive set of cross-correlations of FRB DMs with a wide range of tracers of large-scale structure and baryons. These correlations directly map the baryon over- and under-densities along FRB sight-lines, establishing FRBs as effective backlights of the cosmic web. Looking ahead, next-generation FRB experiments will significantly enhance both DM-z inference and cross-correlation measurements, enabling precise constraints on baryonic feedback and its redshift evolution. These measurements will be complementary to other probes of baryon distribution, and will demonstrate powerful synergies with upcoming large-scale structure surveys.