Conference

The standard way to do this is to use the chain rule to backpropagate gradients through layers of neurons. I shall briefly review a few of the engineering successes of backpropagation and then describe a very different way of getting the gradients that, for a while, seemed a lot more plausible as a model of how the brain gets gradients. Consider a system composed of binary neurons that can be active or inactive with weighted pairwise couplings between pairs of neurons, including long range couplings. If the neurons represent pixels in a binary image, we can store a set of binary training images by adjusting the coupling weights so that the images are local minima of a Hopfield energy function which is minus the sum over all pairs of active neurons of their coupling weights. But this energy function can only capture pairwise correlations. It cannot represent the kinds of complicated higher-order correlations that occur in images. Now suppose that in addition to the "visible" neurons that represent the pixel intensities, we also have a large set of hidden neurons that have weighted couplings with each other and with the visible neurons. Suppose also that all of the neurons are asynchronous and stochastic: They adopt the active state with a log odds that is equal to the difference in the energy function when the neuron is inactive versus active. Given a set of training images, is there a simple way to set the weights on all of the couplings so that the training images are local minima of the free energy function obtained by integrating out the states of the hidden neurons? The Boltzmann machine learning algorithm solved this problem in an elegant way. It was proof of principle that learning in neural networks with hidden neurons was possible using only locally available information, contrary to what was generally believed at the time.

Building on recent advancements in understanding particle transport in magnetized media, we present a first-principles scaling law for the formation of non-thermal tails in particle spectra within mildly and strongly magnetized turbulent plasmas. This scaling is validated using results from kinetic Particle-In-Cell simulations, which show excellent agreement with our theoretical predictions. Finally, we discuss the astrophysical implications of these findings, particularly for the proton spectra in the coronae of supermassive black holes.

It has been proposed recently that the breaking of MHD waves in the inner magnetosphere of strongly magnetized neutron stars can power different types of high-energy transients. We have studied the steepening and dissipation of a strongly magnetized fast magnetosonic wave propagating in a declining background magnetic field, by means of particle-in-cell simulations that encompass MHD scales. Our analysis confirms the formation of a monster shock as $B^2-E^2$ approaches zero, that dissipates about half of the fast magnetosonic wave energy, and reveals, for the first time, the generation of a high-frequency precursor wave at the monster shock, carrying a fraction of 0.001 of the total energy dissipated at the shock. The spectrum of the precursor wave exhibits several sharp harmonic peaks, with frequencies in the GHz band under conditions anticipated in magnetars. Such signals may appear as fast radio bursts.

Neutron star magnetospheres are a source of abundant X-ray activity. They have transients observed in different bands, like the fast radio burst (FRB) and associated hard X-ray flare from the Galactic magnetar SGR 1935+2154. We present global models for magnetar X-ray emission, including a landmark first-principle radiative particle-in-cell simulation of the twisted magnetar magnetosphere with the GPU-PIC code Entity. In one scenario, plasma particles accelerated by surface-motion-induced discharges interact resonantly with thermal background photons. Our GPU-accelerated particle-in-cell simulations track up-scattered high-energy photons that drive secondary pair production and ignite a magnetospheric circuit that persistently generates X-rays. We divulge the plasma properties of such a magnetospheric circuit, including densities and velocities, and give an outlook on alternative ignition scenarios for persistent magnetar X-ray emission.

Black hole X-ray binaries (BHXRBs) and Active Galactic Nuclei (AGN) transition through a series of accretion states in a well-defined order. The accretion states, each associated with different luminosities, spectral and variability characteristics, quasi periodic oscillations (QPOs) and outflow properties, are thought to be triggered by physical changes in the accretion disk around the central black hole.The mechanisms behind state transitions, the geometry of transitional disks and the physical mechanisms driving the emission characteristics we observe remain highly debated. General relativistic magneto-hydrodynamic simulations (GRMHD) are increasingly providing crucial insights into the accretion process, the launch of outflows and the physical processes driving state transitions in BHXRBs and AGN. Using GRMHD simulations conducted with the H-AMR code I: 1) Discuss how high and low-frequency QPOs can be produced by a highly tilted, geometrically thin accretion disk. 2) Present the first GRMHD simulation showing the self-consistent formation of a truncated accretion disk– a proposed disk model for the hard intermediate accretion state, in which the accretion flow is thick and hot close to the black hole, while the outer regions of the flow are thin and cool. 3) Describe how QPOs can be generated at the truncation radius (the radius at which the disk transitions from thick to thin) in a truncated accretion disk.

Fast magnetic dissipation powers the bursting activity of isolated neutron stars, merging neutron stars, and flares from black holes. Four mechanisms of fast dissipation can operate in the magnetospheres of compact objects: Alfvenic turbulent cascade, magnetic reconnection, collision of Alfvén waves, and monsters shocks. Radiation produced by these dissipation modes is controlled by the compactness parameter. The main radiative output is usually in X-rays; in some cases, radio bursts are produced.

Radio pulsars display a range of puzzling long-term variability in their magnetospheric emission and spin-down. In this talk I will discuss the mechanisms for internal magnetic field evolution of pulsars, how it differs from the MHD of classical conducting fluids, and how the neutron star interior couples to the magnetosphere and its radiation.

Numerical modeling of accreting millisecond X-ray pulsars (AMXPs) allows us to understand the physical origin of different observational signatures detected from these systems. Since the birth sites of these signals are strongly influenced by the gravitational potential of the star, magnetohydrodynamic (MHD) simulations in full GR (GRMHD) are essential to accurately capture space-time curvature effects and inherent variations in the X-ray spectra. In this talk, I will present results from 3D GRMHD simulations of accreting neutron stars with oblique magnetospheres. I will discuss the pulse profiles generated from the GRMHD simulations and their implications for mass-radius inference in the accreting sources. Apart from the surface features, AMXPs are also good candidates for studying neutron star jet formation mechanisms. Though there have been extensive investigations into black hole jets, neutron star jets remain highly unexplored. Our 2D axisymmetric study in the quiescent regime suggests that the thick disk collimates the initial open stellar flux, leading to jets like the Blandford-Znajek mechanism proposed for black holes. However, much remains to be done before we can draw a complete picture of jets launched from neutron stars. The global 3D GRMHD simulations of the accreting neutron stars allow us to explore the jet formation mechanisms in these systems in detail for the first time.

Accreting supermassive binary black holes (SMBBHs), which are the expected outcome of galaxy mergers, are potential powerful multimessenger sources of gravitational waves (GWs) and electromagnetic (EM) radiation. The latter may be periodically modulated by an asymmetric density distribution in the circumbinary disk (CBD), typically referred to as the “lump”. Possible enhancement of this modulation is predicted to occur when considering unequal mass binaries. In that scenario the less massive black hole (often called the "secondary") is expected to consume a majority of the inflowing gas as it gets closer to the inner edge of the CBD possibly dominating the overall EM luminosity. We perform the first set of full 3D general relativistic magnetohydrodynamic (GRMHD) simulations of astrophysically realistic unequal mass (q=1:2) black hole binaries, both spinning and non spinning, embedded in a CBD, adopting the IllinoisGRMHD code. We use relaxed initial data for the CBD retrieved from a previous long-term simulation, performed with the SphericalNR code, which employs curvilinear coordinates and a post-Newtonian (PN) metric with a cutout excising the central region containing the binary. We study the dependence of multiple diagnostics, including the mass accretion rate, the Poynting flux and the mass enclosed at different radii, on the spins of the black holes and their mass ratio. Additionally, we analyze the dynamics and structure of the minidisks surrounding each black hole and the evolution of the jets ejected by them.