A Factor Graph Approach to Automated Design of Bayesian Signal Processing Algorithms

The benefits of automating design cycles for Bayesian inference-based algorithms are becoming increasingly recognized by the machine learning community. As a result, interest in probabilistic programming frameworks has much increased over the past few years. This paper explores a specific probabilistic programming paradigm, namely message passing in Forney-style factor graphs (FFGs), in the context of automated design of efficient Bayesian signal processing algorithms. To this end, we developed "ForneyLab" (https://github.com/biaslab/ForneyLab.jl) as a Julia toolbox for message passing-based inference in FFGs. We show by example how ForneyLab enables automatic derivation of Bayesian signal processing algorithms, including algorithms for parameter estimation and model comparison. Crucially, due to the modular makeup of the FFG framework, both the model specification and inference methods are readily extensible in ForneyLab. In order to test this framework, we compared variational message passing as implemented by ForneyLab with automatic differentiation variational inference (ADVI) and Monte Carlo methods as implemented by state-of-the-art tools "Edward" and "Stan". In terms of performance, extensibility and stability issues, ForneyLab appears to enjoy an edge relative to its competitors for automated inference in state-space models.Comment: Accepted for publication in the International Journal of Approximate Reasonin

Entropy and entanglement in a bipartite quasi-Hermitian system and its Hermitian counterparts

We consider a quantum oscillator coupled to a bath of $N$ other oscillators. The total system evolves with a quasi-Hermitian Hamiltonian. Associated to it is a family of Hermitian systems, parameterized by a unitary map $W$. Our main goal is to find the influence of $W$ on the entropy and the entanglement in the Hermitian systems. We calculate explicitly the reduced density matrix of the single oscillator for all Hermitian systems and show that, regardless of $W$, their von Neumann entropy oscillates with a common period which is twice that of the non-Hermitian system. We show that generically, the oscillator and the bath are entangled for almost all times. While the amount of entanglement depends on the choice of $W$, it is independent of $W$ when averaged over a period. These results describe some universality in the physical properties of all Hermitian systems associated to a given non-Hermitian one

DASCH Discovery of Large Amplitude ~10-100 Year Variability in K Giants

Here we present the discovery of three unusual long-term variables found in the Digital Access to a Sky Century at Harvard (DASCH) project, with ~1 magnitude variations in their lightcurves on ~10-100 yr timescales. They are all spectroscopically identified as K2III giant stars, probably in the thick disk. Their lightcurves do not match any previously measured for known types of variable stars, or any theoretical model reported for red giants, and instead suggest a new dust formation mechanism or the first direct observation of "short" timescale evolution-driven variability. More theoretical work on the lithium flash near the Red Giant Branch (RGB) bump and the helium shell ignition in the lower Asymptotic Giant Branch (AGB), as well as long term monitoring of K2III thick disk stars is needed.Comment: 6 pages, 3 figures. Accepted for publication in ApJ Letters. Typo correcte

Numerical simulation of spreading drops

We consider a liquid drop that spreads on a wettable surface. Different time evolutions have been observed for the base radius r depending of the relative role played by inertia, viscosity, surface tension and the wetting condition. Numerical simulations were performed to discuss the relative effect of these parameters on the spreading described by the evolution of the base radius r(t) and the spreading time tS. Different power law evolutions r(t) ∝ tⁿ have been observed when varying the parameters. At the early stage of the spreading, the power law t½ (n = 1/2) is observed as long as capillarity is balanced by inertia at the contact line. When increasing the viscosity contribution, the exponent n is found to increase despite the increase of the spreading time. The effect of the surface wettability is observed for liquids more viscous than water. For a small contact angle, the power law t½ is then followed by the famous Tanner law t1/10 once the drop shape has reached a spherical cap

Refining Obstacle Perception Safety Zones via Maneuver-Based Decomposition

A critical task for developing safe autonomous driving stacks is to determine whether an obstacle is safety-critical, i.e., poses an imminent threat to the autonomous vehicle. Our previous work showed that Hamilton Jacobi reachability theory can be applied to compute interaction-dynamics-aware perception safety zones that better inform an ego vehicle's perception module which obstacles are considered safety-critical. For completeness, these zones are typically larger than absolutely necessary, forcing the perception module to pay attention to a larger collection of objects for the sake of conservatism. As an improvement, we propose a maneuver-based decomposition of our safety zones that leverages information about the ego maneuver to reduce the zone volume. In particular, we propose a "temporal convolution" operation that produces safety zones for specific ego maneuvers, thus limiting the ego's behavior to reduce the size of the safety zones. We show with numerical experiments that maneuver-based zones are significantly smaller (up to 76% size reduction) than the baseline while maintaining completeness.Comment: * indicates equal contribution. Accepted into the IEEE Intelligent Vehicles Symposium 202

A New Mid-Infrared and X-ray Machine Learning Algorithm to Discover Compton-thick AGN

We present a new method to predict the line-of-sight column density (NH) values of active galactic nuclei (AGN) based on mid-infrared (MIR), soft, and hard X-ray data. We developed a multiple linear regression machine learning algorithm trained with WISE colors, Swift-BAT count rates, soft X-ray hardness ratios, and an MIR-soft X-ray flux ratio. Our algorithm was trained off 451 AGN from the Swift-BAT sample with known NH and has the ability to accurately predict NH values for AGN of all levels of obscuration, as evidenced by its Spearman correlation coefficient value of 0.86 and its 75% classification accuracy. This is significant as few other methods can be reliably applied to AGN with Log(NH <) 22.5. It was determined that the two soft X-ray hardness ratios and the MIR-soft X-ray flux ratio were the largest contributors towards accurate NH determination. This algorithm will contribute significantly to finding Compton-thick (CT-) AGN (NH >= 10^24 cm^-2), thus enabling us to determine the true intrinsic fraction of CT-AGN in the local universe and their contribution to the Cosmic X-ray Background

The born again (VLTP) scenario revisited: The mass of the remnants and implications for V4334 Sgr

We present 1-D numerical simulations of the very late thermal pulse (VLTP) scenario for a wide range of remnant masses. We show that by taking into account the different possible remnant masses, the observed evolution of V4334 Sgr (a.k.a. Sakurai's Object) can be reproduced within the standard 1D-MLT stellar evolutionary models without the inclusion of any $ad-hoc$ reduced mixing efficiency. Our simulations hint at a consistent picture with present observations of V4334 Sgr. From energetics, and within the standard MLT approach, we show that low mass remnants \hbox{($M\lesssim0.6$\msun)} are expected to behave markedly different than higher mass remnants \hbox{($M\gtrsim0.6$\msun)} in the sense that the latter are not expected to expand significantly as a result of the violent H-burning that takes place during the VLTP. We also assess the discrepancy in the born again times obtained by different authors by comparing the energy that can be liberated by H-burning during the VLTP event.Comment: Submitted to MNRAS. In includes an appendix regarding the treatment of reduced convective motions within the Mixing Length Theor