Topology, parity-time (PT) symmetry, and nonlinearity are at the origin of many fundamental phenomena in complex systems across the natural sciences, but their mutual interplay remains unexplored. We established a nonlinear non-Hermitian topological platform for active tuning of PT symmetry and topological states. We found that the loss in a topological defect potential in a non-Hermitian photonic lattice can be tuned solely by nonlinearity, enabling the transition between PT-symmetric and non–PT-symmetric regimes and the maneuvering of topological zero modes. The interaction between two apparently antagonistic effects is revealed: the sensitivity close to exceptional points and the robustness of non-Hermitian topological states. Our scheme using single-channel control of global PT symmetry and topology via local nonlinearity may provide opportunities for unconventional light manipulation and device applications.

Very little information exists concerning the properties of the interstellar medium (ISM)-induced starlight polarization at high Galactic latitudes. Future optopolarimetric surveys promise to fill this gap. We conduct a small-scale pathfinding survey designed to identify the average polarization properties of the diffuse ISM locally, at regions with the lowest dust content. We perform deep optopolarimetric surveys within three ∼ 15 × 15 regions located at b > 48 • using the RoboPol polarimeter. The observed samples of stars are photometrically complete to ∼16 mag in the R-band. The selected regions exhibit low total reddening compared to the majority of high-latitude sightlines. We measure the level of systematic uncertainty for all observing epochs and find it to be 0.1% in fractional linear polarization, p. The majority of individual stellar measurements have low signal-to-noise ratios. However, our survey strategy enables us to locate the mean fractional linear polarization p mean in each of the three regions. The region with lowest dust content yields p mean = (0.054 ± 0.038)%, not significantly different from zero. We find significant detections for the remaining two regions of: p mean = (0.113 ± 0.036)% and p mean = (0.208 ± 0.044)%. Using a Bayesian approach, we provide upper limits on the intrinsic spread of the small-scale distributions of q and u. At the detected p mean levels, the determination of the systematic uncertainty is critical for the reliability of the measurements. We verify the significance of our detections with statistical tests, accounting for all sources of uncertainty. Using publicly available HI emission data, we identify the velocity components that most likely account for the observed p mean and find their morphologies to be misaligned with the orientation of the mean polarization at a spatial resolution of 10 . We find indications that the standard upper envelope of p with reddening underestimates the maximum p at very low E(B-V) (≤ 0.01 mag).

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Despite the influx of unprecedented-quality data from the Fermi Gamma-Ray Space Telescope that have been collected over nine years of operation, the contribution of normal star-forming galaxies to the extragalactic gamma-ray background (EGRB) remains poorly constrained. Different estimates are discrepant both their underlying physical assumptions and their results. With several detections and many upper limits for the gamma-ray fluxes of nearby star-forming galaxies now available, estimates that rely on empirical scalings between gamma-ray and longer-wavelength luminosities have become possible and increasingly popular. In this paper we examine factors that can bias such estimates, including: a) possible sources of nontrivial redshift dependence; b) dependence on the choice of star-formation tracer; c) uncertainties in the slope and normalisation of empirical scalings. We find that such biases can be significant, pointing towards the need for more sophisticated models for the star-forming galaxy contribution to the gamma-ray background, implementing more, and more confident, physics in their buildup. Finally, we show that there are large regions of acceptable parameter space in observational inputs that significantly overproduce the gamma-ray background, implying that the observed level of the background can yield significant constraints on models of the average cosmic gamma-ray emissivity associated with star formation.(4) We use a standard ΛCDM cosmology with ΩM = 0.3, ΩΛ = 0.7, H0 = 73 km sec −1 Mpc −1 . c 0000 RAS, MNRAS 000, 000-000

We demonstrate that optical nonlinearity can effectively modulate the loss of a topological defect waveguide in a non-Hermitian photonic lattice, leading to switching between PT and non-PT-symmetric regimes and control of topological zero modes.