Dylan Jow- Stanford University

Title: Coherent map making of the nanohertz gravitational wave sky with pulsar timing arrays

Abstract: Recent evidence for a gravitational wave background from pulsar timing arrays has energized the field of nanohertz gravitational wave astronomy. However, many of the science cases for nanohertz GW astronomy require going beyond a simple detection of the background amplitude towards detecting individual sources. Constructing maps of the GW power on the sky is a natural step towards this goal and mirrors the historical development of CMB science. However, basic questions remain. In particular, what is the look-elsewhere factor that must be taken into account for map-based searches for coherent point sources? This relates to basic questions of the resolution of PTA maps and the statistics of noise in map space. In this talk, I will motivate the turn to map-based approaches to single-source searches and, thereby, the urgency of these questions. While the answers will ultimately depend on the precise configuration of the PTA (i.e. which particular puslars we will be able to get good timing and distance information from), I will discuss some general effects that may guide PTA strategies.

Jonatan Jacquemin-Ide- University of Colorado, Boulder

Title: Demystifying flux eruptions: Magnetic flux transport in magnetically arrested disks

Abstract: Flux eruptions in magnetically arrested disks (MADs) are a leading candidate for explaining variability in low-luminosity active galactic nuclei, including dramatic horizon-scale outbursts; however, what sets the timing of these events remains unclear, especially since their recurrence times are much longer than the dynamical timescale of the accretion flow. I show that magnetic field transport, inward advection and outward diffusion, near black holes sets the recurrence timescale of flux eruptions. I further show that MADs settle into a quasi-steady state where advection and diffusion nearly cancel; this balance naturally explains the long recurrence times. Small imbalances in this equilibrium then drive flux eruptions. I compute an analytic recurrence time, t_rec = 1500 r_g/c , in excellent agreement with simulations. I also present the first direct measurement of turbulent resistivity in MADs, showing that magnetic diffusion is dominated by small-scale turbulence and is consistent with magnetorotational instability turbulence. Finally, I highlight similar variability in other accreting systems and show that such events offer a direct probe of magnetic dissipation and flux transport in astrophysical accretion flows.